Recent Advances in Sn-Based Lead-Free Solders for Electronic Packaging: A Review

Recent Advances in Sn-Based Lead-Free Solders for Electronic Packaging: A Review | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Recent Advances in Sn-Based Lead-Free Solders for Electronic Packaging: A Review Qinrong Sun, Xuehai Liao, Jinhong Dai, Wei Feng, Long Zhang, Limeng Yin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8100930/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract With the global enforcement of environmental regulations restricting the use of lead-based solders, the development of lead-free solder alloys for electronic packaging has become a central topic in ensuring both device reliability and sustainable manufacturing. This review summarizes the current research progress on lead-free solders for electronic packaging, focusing on the performance characteristics and modification strategies of major alloy systems such as Sn-Cu, Sn-Zn, Sn-Bi, Sn-Ag, and Sn-Ag-Cu. Although each system exhibits unique advantages, challenges such as high melting temperature, poor wettability, oxidation susceptibility, and high material cost remain unresolved. Performance optimization can be effectively achieved through alloying element doping and nanoparticle reinforcement. Alloying elements refine the microstructure via solid-solution strengthening, grain refinement, and secondary-phase strengthening while suppressing the coarsening of intermetallic compounds (IMCs). Meanwhile, nanoparticles enhance interfacial stability through dispersion strengthening and diffusion barrier effects, thereby improving wettability, mechanical strength, and long-term reliability. Finally, this paper proposes potential research directions for the next generation of lead-free solders and provides an outlook on their future applications in advanced electronic packaging. Lead-free solder Alloying elements Nanoparticles Microstructure and properties Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1 Introduction In the context of the rapidly evolving global electronics industry, electronic packaging technology, which serves as a critical bridge ensuring efficient interconnection between electronic components and substrates, has undergone a paradigm shift. Its performance requirements have advanced beyond basic interconnection functions to include high reliability, enhanced integration, and prolonged service life. These developments have imposed increasingly stringent demands on the comprehensive properties of packaging materials [ 1 ],[ 2 ] .As a key functional material within electronic packaging systems, solder plays a crucial role in electrical signal transmission, thermal dissipation, and mechanical bonding. Its physicochemical properties and mechanical behavior directly determine the reliability and service life of electronic devices operating under complex environmental conditions [ 3 ],[ 4 ] .However, traditional tin-lead (Sn-Pb) solders contain highly toxic lead [ 5 ] , which not only causes persistent environmental contamination during manufacturing, operation, and recycling, but also poses serious risks to human health through bioaccumulation, particularly affecting the nervous and hematopoietic systems [ 6 ] . In response, a series of international environmental regulations, most notably the European Union’s Restriction of Hazardous Substances (RoHS) directive, have explicitly restricted the use of lead in electronic manufacturing [ 7 ] . This regulatory framework has fundamentally driven the research, application, and industrialization of lead-free solder alloys, establishing them as a focal point and inevitable trend in the development of modern electronic materials [ 8 ] . During the systematic research and engineering application of lead-free solders, extensive studies have been conducted focusing on key performance indicators such as microstructural characteristics, wettability [ 9 ] and spreadability [ 10 ] , mechanical strength [ 11 ] , oxidation resistance [ 12 ] , and long-term reliability [ 13 ] . Through continuous exploration, researchers have successively developed a series of multicomponent solder systems, including Sn-Cu [ 14 ], Sn-Ag [ 15 ] , Sn-Zn [ 16 ] , and Sn-Bi [ 17 ] alloys, each tailored to optimize specific performance requirements. Among these systems, Sn-Ag-Cu (SAC) solders have emerged as the predominant choice in the current market owing to their well-balanced combination of mechanical, thermal, and wetting properties, while Sn-Zn and Sn-Bi systems, characterized by their relatively low melting points, exhibit distinct advantages in low-temperature packaging applications [ 18 ] . Despite significant progress, the existing lead-free solder systems still exhibit inherent limitations. SAC-type solders suffer from the high cost of Ag, which restricts their large-scale industrial application, while their relatively high processing temperatures may induce thermal damage to chips and substrates [ 19 ] . Sn-Cu solders, on the other hand, are constrained by their high melting points and poor wettability [ 20 ] . In Sn-Zn systems, the high chemical activity of Zn leads to insufficient oxidation resistance and unstable wetting behavior, thereby compromising the reliability of the soldering process [ 21 ] . Meanwhile, Sn-Bi solders are limited by the inherent brittleness of Bi, resulting in poor fatigue resistance and a tendency to develop cracks during service [ 22 ] . Therefore, this review systematically summarizes recent research progress on lead-free solders for electronic packaging, with particular emphasis on the evolution of their microstructures, performance regulation mechanisms, and application limitations. The key challenges, including high melting temperatures, insufficient wettability, oxidation susceptibility, and increased material costs, are comprehensively analyzed. Accordingly, this paper highlights modification strategies based on alloying element doping, nanoparticle reinforcement, and multi-component alloying, and elucidates their effects on microstructural refinement and macroscopic performance enhancement of solder alloys. These insights are of significant theoretical and practical importance for addressing existing technological bottlenecks and advancing the development of high-performance, low-cost lead-free solders. Finally, potential future research directions are discussed in the context of the continuing evolution of electronic packaging technologies. 2 Solder Alloy Systems 2.1Sn–Cu Solder Alloys The Sn-Cu-based lead-free solder, owing to the low cost of copper, has been widely used in electronic packaging, particularly in wave soldering applications [ 23 ] . This solder system exhibits a suitable melting temperature, good fluidity, and low susceptibility to thermal cracking and segregation, making it a practical alternative for large-scale industrial applications [ 24 ] . However, during the soldering process, the molten solder tends to dissolve copper atoms from the substrate, leading to an increased Cu concentration on the solder side of the joint interface. As the Cu content rises at the interface, the local melting point of the solder correspondingly increases, which may induce interfacial defects and compromise the integrity of the solder joint. At room temperature, the stable intermetallic phases are Cu₃Sn and Cu₆Sn₅ [ 25 ] . With the increasing Cu concentration at the interface, the Cu₆Sn₅ phase tends to transform into the brittle Cu₃Sn phase. This phase transformation significantly affects the mechanical properties and service reliability of the solder joint [ 26 ] . To address the drawbacks of the Sn-Cu lead-free solder system, such as its relatively high melting point and poor wettability, researchers have introduced trace alloying elements including Bi, Ni, and In to enhance its overall performance and soldering reliability. The addition of these elements effectively improves the thermal properties, refines the solder microstructure, and increases the tensile strength of the joints. Huang [ 27 ] et al. investigated the effects of adding different proportions of Bi to Sn-Cu solder alloys to improve their overall performance. High-purity Sn, Cu, and Bi (99.9%) were used as raw materials. According to the designed compositions of Sn-2Cu-xBi (x = 0, 1, 3, and 5 wt.%), the elements were accurately weighed and placed into a graphite crucible, which was then loaded into an electric resistance furnace. Nitrogen gas was introduced into the furnace to suppress the oxidation of Bi at elevated temperatures. The alloy mixture was heated to 400°C and held for 1 h to ensure complete melting and homogenization. Prior to casting, the molten alloy was cooled to 250°C, and the mold was preheated to the same temperature to minimize thermal gradients. Finally, the melt was allowed to cool naturally to room temperature for solidification, completing the preparation of the Sn-2Cu-xBi alloy series. Studies have shown that the addition of Bi can significantly reduce the melting point and undercooling of Sn-Cu solder alloys, while effectively refining the intermetallic compounds (IMCs) [ 28 ] . The microstructural morphology of the samples was examined using scanning electron microscopy (SEM), as shown in Figure. 1. The addition of Bi resulted in the refinement of the Cu₆Sn₅ intermetallic phase, and with increasing Bi content, a larger number of Bi-rich precipitates were observed. In addition, Fan [ 29 ] et al. improved the intermetallic compound (IMC) layer characteristics by incorporating Ni into the Sn-Cu solder alloy. When Sn-based solders are joined with Cu substrates, interfacial reactions occur to form a brittle η′-Cu₆Sn₅ intermetallic compound (IMC). The excessive growth of this IMC layer often deteriorates the mechanical integrity of the solder joint, leading to a reduction in both tensile and shear strength. The incorporation of Ni has been shown to stabilize the η′-Cu₆Sn₅ phase thermodynamically, refine its morphology, and enhance its fracture toughness, thereby improving the overall mechanical reliability of the solder joint [ 30 ] . With the increase in Ni content, the alloy microstructure becomes progressively refined, while the (Cu,Ni)₆Sn₅ intermetallic compound (IMC) tends to coarsen and aggregate [ 31 ] . Consequently, the microhardness of the solder exhibits an initial increase followed by a gradual decrease. Moreover, both the wetting behavior and spreading performance improve with rising soldering temperature and extended dwell time. Among the tested compositions, the Sn-0.7Cu-0.2Ni solder demonstrates the optimal wettability at 280°C after 10 min of reflow [ 32 ] . Kelly [ 33 ] et al. reported that the addition of indium (In) can effectively reduce the melting point of Sn-based solders, refine their microstructure, and enhance mechanical properties. Moreover, In incorporation suppresses the formation of Cu₆Sn₅ intermetallic compounds (IMCs) during the electromigration process. Therefore, introducing a small amount of In is considered an effective strategy to improve the overall performance of Sn-based solders. Sn-0.7 wt.% Cu and Sn-0.7 wt.% Cu-<1 wt.% In solders were selected for the study. Copper wires with a diameter of 250 µm were used as substrates, and the electromigration (EM) tests were conducted under a current density of 1×10⁴ A/cm². Cross-sectional observations of the solder joints using scanning electron microscopy (SEM) revealed that some intermetallic compounds (IMCs) in the Sn-In/Cu joints grew into large needle-like structures. The incorporation of In partially substituted Sn in Cu₆Sn₅, resulting in the transformation of the IMC phase from Cu₆Sn₅ to Cu₆(Sn,In)₅. Furthermore, analysis of the Sn-Cu-In samples after 200 hours of electromigration indicated the presence of indium-rich clusters near the anode interface, which gradually migrated toward the anode with increasing electromigration duration. The inhibitory mechanism of indium (In) on the growth of intermetallic compounds (IMCs) lies in its substitution of Sn atoms in both η′-Cu₆Sn₅ and Cu₃Sn phases, which increases the migration energy barrier of Cu and Sn atoms and consequently reduces the IMC growth rate [ 34 ] . In addition to alloying elements, the performance of solders can also be improved by incorporating nanoparticles. However, due to the significant density mismatch between most nanoparticles and Sn-0.7Cu solder, they tend to float on the surface of the molten solder, leading to uneven mixing. Liu [ 35 ] et al. addressed this issue by adding Nb nanoparticles, which have a density similar to that of Sn-0.7Cu solder. This modification effectively enhances wettability, refines the microstructure, and improves the mechanical properties of the solder. Sn-0.7Cu-xNb composite solders with varying Nb content were prepared using a mechanical mixing method. After reflow soldering, the wettability was assessed by measuring the spreading area of the solder on a Cu substrate, and the best wettability was observed at 0.12 wt.% Nb. The composite solder was then etched, and its microstructure was examined using scanning electron microscopy (SEM). The microstructure consists of β-Sn phase and β-Sn/Cu₆Sn₅ eutectic. The addition of 0.12 wt.% Nb refines the β-Sn grain size to 8.9 µm. The refinement effect of Nb nanoparticles, along with the proliferation of dislocations and the hindrance of dislocation motion, results in the maximum ultimate tensile strength and elongation of the composite solder. Lu [ 36 ] et al. synthesized carbon framework nanosheets loaded with cobalt nanoparticles (Co&C) through a metal-organic framework (MOF)-derived method and incorporated them into Sn-0.7Cu solder. nglish SCI-polished version: This approach avoids the shortcomings associated with the sole addition of Co, which promotes IMC growth [ 37 ] , or the use of carbon materials alone, which may result in weak bonding with the solder [ 38 ] The melting point and contact angle of the solder were determined using differential scanning calorimetry (DSC) and a contact angle goniometer. The results indicate that the addition of trace amounts of Co&C has a negligible effect on the solder's melting point, but significantly improves its wettability by reducing the contact angle. Reflow testing was conducted with the peak temperature set to 250°C in the reflow furnace. The solder balls and Cu substrates were held at this temperature for four different durations: 450 s, 900 s, 1500 s, and 2100 s. The results, shown in Fig. 2 , illustrate the variation in the intermetallic compound (IMC) layer thickness at the Sn-0.7Cu-xCo&C/Cu interface with different reflow times. As the reflow time increased from 450 s to 900 s, the IMC thickness at the interface grew from 5.20 µm to 7.97 µm, with a growth rate significantly higher than that observed for other composite solders. The mechanism behind this effect lies in the ability of Co to occupy the Cu sites in Cu₆Sn₅, forming (Cu, Co)₆Sn₅, which suppresses the growth of Cu₃Sn and transforms the scallop-shaped IMCs into a more planar structure. Additionally, the carbon framework in the Co&C nanosheets acts as an inert physical barrier, hindering the diffusion of Sn and Cu atoms, thereby preventing the further growth of IMCs [ 39 ] . In recent years, due to the high tin content inherent in Sn-Cu-based solders, the enhancement of overall properties through the addition of a single minor alloying element has proven to be limited. To meet the performance requirements under diverse service environments, future research should focus on addressing the intrinsic drawbacks of this system, particularly its high melting point and insufficient wettability. The development of multi-element composite lead-free solders through synergistic alloying design is expected to offer a promising pathway toward achieving improved comprehensive performance [ 40 ] . The addition of Bi to Sn-0.7Cu solder can effectively reduce its melting point; however, further optimization of its overall performance is still required [ 41 ] . Han et al. reported that the addition of Cr significantly improves the wettability of the solder, refines its microstructure, and effectively suppresses the growth of intermetallic compound (IMC) layers [ 42 ] . The underlying mechanism is that with the increasing Cr content, the aggregation of Bi atoms and the size of β-Sn grains are gradually refined, which effectively hinders the diffusion of Sn and Cu atoms. However, excessive Cr addition leads to the precipitation of brittle intermetallic compounds, resulting in reduced ductility. When the Cr content reaches 0.2 wt.%, the Sn-0.7Cu-10Bi solder exhibits the optimal overall performance in terms of melting temperature, wettability, microstructural refinement, and interfacial reliability. In addition, the incorporation of indium (In) can enhance the electrical conductivity of the solder and refine its grain structure. The underlying mechanism lies in the formation of the intermetallic compound Cu₂In₂O₅, which is a transparent conductive oxide semiconductor generated from the reaction between In and Cu oxides. This compound effectively improves the electrical performance of the alloy. Furthermore, the addition of In significantly reduces the average grain size of the solder alloy, thereby enhancing its hardness, tensile strength, and overall mechanical properties [ 43 ] . 2.2Sn-Zn Solder Alloys Sn-Zn solder alloys have emerged as one of the core alternatives to traditional Sn-Pb solders due to their low cost, environmental friendliness, excellent mechanical properties, and a eutectic melting point close to that of Sn-Pb alloys [ 44 ] . Owing to these advantages, Sn-Zn solders have been widely applied in consumer electronics, automotive electronics, and other industrial fields [ 45 ] . The eutectic composition of the Sn-Zn alloy is Sn-9Zn, with a eutectic temperature of 198.5°C [ 46 ] . Its microstructure consists of coarse primary β-Sn dendrites and a fine β-Sn/Zn eutectic matrix [ 47 ] . Numerous researchers and institutions worldwide have devoted extensive efforts to improving the performance of Sn-Zn solder alloys by incorporating various alloying elements [ 48 ] . To address the issue of oxidation susceptibility, Ag is often incorporated into the Sn-Zn solder system to improve its oxidation resistance and overall performance [ 49 ] . Because Ag exhibits a stronger chemical affinity for Zn, which is more chemically active in the solder matrix, Ag preferentially reacts with Zn to form the intermetallic compound AgZn₃ [ 50 ] . This reaction alters the microstructure of the solder matrix, effectively suppressing the precipitation and growth of needle-like Zn-rich phases while simultaneously refining the grain size and homogenizing the microstructure, thereby enhancing the fundamental mechanical properties of the solder alloy [ 51 ] . However, the poor wettability of Sn-Zn binary solder alloys is primarily attributed to their inadequate oxidation resistance, which can be effectively improved by the addition of Bi [ 52 ] . The addition of an appropriate amount of Bi can effectively reduce the surface tension of the solder [ 53 ] . This phenomenon arises from the fact that the surface tension of liquid Sn-Bi eutectic alloys is significantly lower than that of liquid Sn-Zn eutectic solders under the same conditions. The reduction in surface tension directly enhances the spreading kinetics of the solder on the Cu substrate surface and accelerates interfacial diffusion between the solder and the substrate. Consequently, the wettability of the solder is synergistically improved from both thermodynamic and kinetic perspectives. In addition to alloying element additions, the incorporation of nanoparticles can also be employed to enhance the overall performance of solder alloys [ 54 ] . To address the inherent drawbacks of Sn-Zn solders, such as limited electrical conductivity, low melting point, and high susceptibility to oxidation. Li [ 55 ] et al. introduced aluminum (Al) nanoparticles into the Sn-Zn solder matrix to mitigate the inherent drawbacks of Sn-Zn-based solders. Composite solder joints with varying Al nanoparticle contents were fabricated using transient liquid-phase bonding (TLPB) technology. The interfacial reaction layers of the joints were primarily composed of Cu₃(Sn, Zn) and Cu₆(Sn, Zn)₅ intermetallic compounds (IMCs) [ 56 ] . The microstructures of solder joints containing varying amounts of Al nanoparticles were examined using scanning electron microscopy (SEM). The average intermetallic compound (IMC) thickness is shown in Fig. 3 . As the Al nanoparticle content increased, the IMC layer thickness initially decreased and then increased, reaching its minimum value at 0.6 wt% Al. Similarly, the porosity of the solder joints exhibited a decreasing-increasing trend, reaching the lowest level at 0.6 wt% Al, which was 57.56% lower than that of the solder joints without Al addition. In terms of mechanical properties, the shear strength of the solder joints increased initially and then decreased with rising Al nanoparticle content, reaching a maximum at 0.6 wt%, which represents a 54.20% improvement compared with the joints without Al addition. Meanwhile, the fracture mode gradually shifted from ductile fracture to brittle fracture, and the fracture location transitioned from the in-situ reaction zone to the interfacial reaction zone [ 55 ] . In addition, the incorporation of TiO₂ nanoparticles can effectively optimize the microstructural morphology and enhance the creep resistance of the solder alloy [ 57 ] . After the incorporation of TiO₂ nanoparticles into the solder alloy, the SEM micrographs reveal uniformly dispersed flower-like and eye-shaped morphologies within the alloy matrix. These nanoparticles effectively stabilize the grain structure, suppress grain coarsening, and act as pinning centers for dislocation motion, thereby increasing resistance to plastic deformation and significantly enhancing the creep resistance of the solder [ 58 ] . The microstructure of Sn-Zn solder primarily consists of a β-Sn matrix phase interspersed with Zn-rich phases. The Zn phase typically appears as irregular particles or dendritic structures, and due to the limited solid solubility between Zn and Sn, Zn tends to segregate at grain boundaries or within the matrix [ 59 ] . The addition of Cu to Sn-Zn solder promotes the formation of Cu-Zn intermetallic compounds (IMCs), which effectively reduce the chemical activity of Zn and thereby enhance the wettability of the solder [ 60 ] . As the Cu content increases, Cu reacts with Zn or Sn to promote the formation of intermetallic compounds (IMCs). These IMCs are uniformly distributed between the β-Sn matrix and the Zn-rich phase, contributing to microstructural refinement and suppressing the coarsening of Zn phases. However, the improvement in overall properties achieved by adding a single minor alloying element remains limited. Therefore, Bi is introduced into the Sn-Zn-Cu solder system to further enhance performance through the synergistic effects of multiple alloying elements [ 61 ] . The microstructure of Sn-Zn-Cu-Bi solder primarily consists of eutectic structures, β-Sn phases, Cu₅Zn₈ intermetallic compounds, and precipitated Bi particles. As the Bi content increases, the segregation of Bi particles promotes heterogeneous nucleation within the matrix, resulting in refined solder microstructures. Meanwhile, with increasing Bi addition, the tensile strength of the solder improves while its ductility decreases, and the fracture mode transitions from ductile to brittle. This behavior is attributed to the solid-solution strengthening effect of fine Bi particles; however, excessive Bi at grain boundaries introduces brittleness, significantly reducing ductility.In addition to Bi modification, further enhancements to Sn-Zn-based solders can be achieved by incorporating Ag, Al, or Li elements, which contribute to optimizing the microstructure and improving overall mechanical and thermal properties [ 62 ] . In terms of thermal properties, the addition of Ag and Al elements increases the melting point of the Sn-Zn eutectic alloy, whereas the incorporation of Li results in a decrease in melting temperature. Regarding mechanical performance, the addition of Al and Li enhances both tensile strength and plasticity of the solder. From the perspective of wettability, the inclusion of Li and Ag reduces the alloy’s coefficient of thermal expansion. Moreover, Li significantly decreases the surface and interfacial tensions of the solder, thereby improving its wettability and promoting superior interfacial bonding behavior. The SEM microstructures of Sn-Zn solders with varying metal element contents are shown in Fig. 4 (a-e). The eutectic Sn-Zn alloy exhibits a dual-phase microstructure composed of bright Sn-rich regions and dark, needle-like Zn-rich phases. Upon the addition of Ag, the AgZn₃ intermetallic compound (IMC) forms, featuring a higher Zn concentration in its outer layer than in its core, and a refined eutectic microstructure. When Li is introduced, the overall microstructure becomes finer, with the eutectic region consisting of Sn-rich and α-Zn precipitates; partial dezincification may occur. Since Sn has a stronger solubility for Li than Zn, Li tends to dissolve preferentially into the Sn matrix, further refining the structure. The addition of Al results in Al-rich spots and relatively coarse α-Zn and Ag-rich precipitates, with Al segregating preferentially within the Zn-rich phase. The Sn-Zn-Ag-Al-Li alloy combines these microstructural characteristics, exhibiting a composite structure comprising Sn-rich phases, rod-like α-Zn phases, AgZn₃ IMCs, and dispersed Al-rich regions. The tensile test results are shown in Fig. 4 (f), illustrating the relationship between tensile strength and elongation for Sn-Zn-based solders containing varying amounts of metallic elements. The addition of Ag promotes the formation of AgZn₃ intermetallic compounds, which strengthen the solder matrix and enhance tensile strength. Furthermore, the incorporation of Al and Li elements leads to a simultaneous improvement in both strength and elongation, primarily attributed to grain refinement and the inhibition of crack propagation by the finely dispersed precipitates. 2.3Sn-Bi Solder Alloys The Sn-Bi solder system, characterized by its low eutectic melting point of 139°C, has emerged as an ideal candidate for the packaging of temperature-sensitive electronic components. Its yield strength, fracture resistance, wettability, and solderability are comparable to those of the traditional eutectic Sn-Pb solder, making it a promising alternative for low-temperature interconnection applications [ 63 ] . During practical soldering processes, the Sn-Bi system is capable of forming a more uniform and homogeneous joint region, effectively reducing void formation and ensuring gas-tight reliability under low-temperature conditions. These advantages make it one of the most prominent and practical lead-free solder systems for advanced electronic packaging applications [ 64 ] . The eutectic composition of Sn-Bi solder is Sn-57Bi, corresponding to a eutectic temperature of 139°C [ 46 ] . Under equilibrium solidification conditions, no intermetallic compounds (IMCs) are formed in this system. The microstructure consists of alternating lamellar distributions of Sn-rich and Bi-rich phases. During alloy solidification and cooling, Bi atoms precipitate as fine particles within the Sn-rich matrix [ 65 ] . In terms of performance advantages, Sn-Bi solder exhibits a low melting point along with excellent wettability, oxidation resistance, and room-temperature tensile strength. These characteristics make it highly suitable for temperature-sensitive electronic components and low-temperature flip-chip interconnection applications, where both reliability and thermal compatibility are critical. Although Sn-Bi solder possesses numerous advantages, it still suffers from several inherent drawbacks [ 66 ] . These include poor thermal fatigue resistance, limited ductility, and suboptimal processability, as well as a high intrinsic brittleness of the solder matrix, which leads to insufficient creep resistance and long-term reliability of the solder joints [ 67 ] . The root causes of these deficiencies are closely associated with the intrinsic properties of the bismuth (Bi) element, which can be summarized in two main aspects.First, Bi exhibits a high solid solubility in the Sn matrix, leading to the precipitation of a large number of Bi particles during solidification. This process is often accompanied by a significant volumetric expansion effect, which introduces internal stress and microcracks.Second, Bi crystallizes in a rhombohedral (trigonal) crystal structure with a limited number of active slip systems, resulting in its inherent hardness and brittleness, as well as relatively poor electrical and thermal conductivity [ 68 ] . The combined influence of these factors renders the performance of Sn-Bi solder alloys highly sensitive to Bi content. Consequently, the current Sn-Bi system still fails to meet the stringent service requirements of applications involving high temperatures and cyclic mechanical stresses, such as automotive circuit boards and military-grade electronic devices [ 69 ] . To address the inherent brittleness and insufficient mechanical performance of Sn-Bi solder alloys, extensive studies have been conducted by researchers worldwide focusing on alloying modification strategies. These investigations have confirmed that the addition of elements such as Ni, Ag, In, and Cu can effectively enhance the overall properties of Sn-Bi solders by refining the microstructure and introducing strengthening phases through various metallurgical mechanisms [ 70 ] . Zhang [ 71 ] et al. reported that the addition of Ni to Sn-58Bi solder results in the formation of the Ni₃Sn₄ intermetallic phase, which serves as a strengthening phase and contributes to microstructural refinement. This modification significantly enhances both the ultimate tensile strength and the elastic modulus of the solder alloy. However, excessive Ni addition (1.0 wt%) leads to significant grain coarsening, resulting in a marked reduction in ductility. In contrast, an addition of 0.5 wt% Ni achieves a better balance among strength, ductility, and elastic modulus, exhibiting superior overall tensile performance. When the Ni content reaches 1.0 wt%, the hardness of the solder increases sharply, even exceeding the typical empirical ratio where hardness is approximately one-third of the ultimate tensile strength. This deviation is attributed to the formation of hard intermetallic compounds (IMCs) promoted by Ni, which enhances the alloy’s resistance to plastic deformation. Nevertheless, the shear strength of the solder with Ni addition is slightly lower than that of plain Sn-58Bi solder, primarily because Ni promotes an increase in both the volume fraction and coarsening of the Bi-rich phase, which in turn adversely affects shear performance to some extent [ 72 ] . In addition, Hu [ 73 ] et al. investigated the effects of Ag and In additions on improving the mechanical properties of Sn-Bi solder alloys. The addition of Ag can enhance the strength of Sn-Bi solder alloys by forming the Ag₃Sn intermetallic compound, while simultaneously suppressing the coarsening of the Bi phase. However, excessive Ag content leads to an increased formation of brittle phases, resulting in a sharp decline in ductility. Under an appropriate cooling rate, Ag addition not only improves the mechanical strength and wettability of the solder but also effectively inhibits Bi segregation within the microstructure. The addition of In significantly enhances the ductility of Sn-Bi solder alloys and reduces Bi phase precipitation. This improvement arises because In atoms suppress the coarsening of the Bi phase, thereby refining the microstructure and promoting a more uniform distribution of Bi. However, the tensile strength exhibits a slight decrease, while the hardness first increases and then decreases with increasing In content [ 74 ] .Junpei Umeyama [ 75 ] et al. demonstrated that the addition of Cu to Sn-Bi solder can effectively enhance its strength and hardness. This improvement is primarily attributed to the formation of hard intermetallic compounds (IMCs) and the refinement of the microstructure, which collectively strengthen the solder matrix and improve its overall mechanical performance. The mechanical properties of the solder were evaluated under varying temperature and strain rate conditions. Tensile tests conducted on solders with different Cu contents revealed that the optimal Cu addition is 0.1 wt.%. The tensile strength increases with temperature but decreases with lower strain rates, while the elongation exhibits the opposite trend. When the testing conditions exceed 333 K in temperature and fall below a strain rate of 5.25×10⁻⁴ s⁻¹, the solder exhibits superplastic behavior. The dominant mechanisms of this superplastic deformation are recovery-induced recrystallization, which refines the grains, and grain boundary sliding within the Sn-Bi eutectic phases.SEM microstructural observations show that an appropriate amount of Cu addition leads to a more uniform distribution of the Bi phase and a fracture surface characterized by ductile features. However, excessive Cu addition can result in performance fluctuations, attributed to the formation of the hard Cu₆Sn₅ intermetallic compound (IMC) and grain boundary pinning effects. Therefore, precise control of Cu content is essential to maintain an optimal balance between strength and ductility. In addition, the performance of Sn-Bi solders can be further improved by introducing nanoparticle reinforcements. The incorporation of Co nanoparticles at varying concentrations has been shown to refine the microstructure of the solder and enhance its mechanical properties [ 76 ] . The microstructure of the pure Sn-58Bi solder consists of two distinct phases: a dark Sn-rich matrix and bright Bi-rich regions, where the Bi phase exhibits significant coarsening. After the addition of Co nanoparticles, the solder’s ductility is markedly improved, and the grain structure becomes significantly refined. As the concentration of Co nanoparticles increases, the grain refinement effect becomes more pronounced. The uniformly distributed Co nanoparticles act as heterogeneous nucleation sites during solidification and effectively inhibit grain growth by pinning grain boundaries. Consequently, the addition of Co nanoparticles not only enhances the mechanical properties of the Sn-58Bi solder but also mitigates the inherent brittleness caused by Bi phase aggregation. This phenomenon can be attributed to the ability of Co nanoparticles to suppress the coarsening of the Bi phase and to promote the formation of more thermodynamically stable intermetallic compounds (IMCs). The mechanical properties of Sn-Bi solders containing Co nanoparticles were evaluated before and after aging, as shown in Fig. 5 (a). In the unaged condition, tensile tests revealed that the addition of Co nanoparticles significantly enhanced the ultimate tensile strength of the Sn-58Bi solder, and the strength increased progressively with rising Co concentration. After aging for different durations, a decrease in ultimate tensile strength was observed in all samples; however, the reduction was notably smaller in the Co-containing solders. This improvement is primarily attributed to the grain-refinement effect induced by Co nanoparticles, which contributes to the suppression of strength degradation during aging. Moreover, the formation of thermodynamically stable IMCs helps to maintain microstructural stability and consequently enhances the overall mechanical performance of the solder. Kim [ 77 ] et al. enhanced the wettability and mechanical properties of solder alloys and suppressed the growth of intermetallic compounds (IMCs) by incorporating TiC nanoparticles at various concentrations. Differential scanning calorimetry (DSC) analysis revealed that the addition of TiC nanoparticles had a negligible effect on the melting point of Sn-58Bi solder. The spreading ratio of the solder initially increased and then decreased with the addition of TiC nanoparticles, reaching its maximum value at 0.1 wt% TiC. However, excessive TiC addition led to a reduction in spreading performance, primarily because an appropriate amount of TiC can decrease surface tension and improve wettability, whereas excessive TiC tends to agglomerate and increase the viscosity of the molten solder due to its high melting point, thereby hindering spreading behavior.The mechanism by which TiC nanoparticles suppress IMC growth lies in their function as heterogeneous nucleation sites, promoting grain refinement. Additionally, TiC possesses high chemical stability, making it resistant to coarsening or chemical reactions. As a result, the thickness of interfacial IMCs decreases with TiC addition. This effect arises because TiC particles adsorb at the Bi/Cu₆Sn₅ interface, lowering interfacial energy and obstructing the diffusion of Sn and Cu atoms, which effectively retards IMC growth. Sn-Bi alloys, owing to their non-toxic nature and low melting point, have emerged as ideal candidates for low-temperature lead-free solders. However, they inherently suffer from brittleness, poor wettability, and interfacial Bi segregation, which can lead to joint embrittlement and compromised mechanical reliability. The Ag element, known for its excellent electrical conductivity and wettability, is commonly incorporated into Sn-Bi solders to enhance their performance [ 78 ] . However, the improvement achieved through a single minor alloying addition remains limited. Therefore, the co-addition of Si has been explored to further enhance tensile strength and hardness, as well as to optimize the overall wettability of the solder alloy [ 79 ] . After introducing a trace amount of Si into the Sn-Bi-Ag solder, the microstructure of the alloy became significantly refined. The morphology of the Bi phase transformed from coarse block-like structures into an interconnected network, accompanied by an increased number of needle-shaped Bi phases embedded within the Sn matrix. Moreover, the wettability of the solder was notably improved, and the thickness of the diffusion layer formed at the solder/Cu substrate interface was substantially reduced, while the melting point and melting range remained essentially unchanged.These improvements can be attributed to the inhibitory effect of Si on Bi phase coarsening and segregation, as well as its barrier effect on Cu atomic diffusion across the interface. The tensile test results for alloys containing varying Ag and Si contents, as shown in Fig. 5 (b), demonstrate that the addition of Si effectively enhances both the tensile strength and microhardness of the solder. Furthermore, as the Ag content increases, the mechanical properties of the Sn-Bi-Ag-Si solder alloys are further improved, indicating a synergistic strengthening effect between Ag and Si. 2.4Sn-Ag Solder Alloys Given the environmental toxicity of lead, global environmental protection regulations have imposed strict restrictions on its use in the electronics manufacturing industry. Against this backdrop, tin-based lead-free solder alloys have emerged as a key alternative to traditional tin-lead (Sn-Pb) systems, offering a more environmentally sustainable and regulatory-compliant solution [ 80 ] . Among them, the Sn-Ag solder system has attracted considerable attention due to its combination of excellent mechanical strength and superior wettability. Its eutectic composition is Sn-3.5Ag, with a corresponding eutectic temperature of 221°C [ 81 ] . Under equilibrium solidification conditions, its microstructure consists of primary β-Sn dendritic phases and a Sn/Ag₃Sn eutectic matrix. The dispersed Ag₃Sn intermetallic compounds (IMCs) within the tin matrix significantly enhance the overall mechanical and physical properties of the solder alloy [ 82 ] . Owing to its outstanding mechanical strength, oxidation resistance, soldering reliability, and service stability, the Sn-Ag system has been widely utilized in electronic packaging and interconnection applications. However, it still suffers from inferior wettability compared with Sn-Pb solders and a relatively high melting point. In the Sn-Ag alloy system, a higher Ag content is typically required to achieve desirable performance; however, the incorporation of Cu can effectively reduce this dependence on Ag—for instance, in the widely used SAC305 (Sn-3.0Ag-0.5Cu) alloy—thereby maintaining mechanical integrity while lowering material costs [ 83 ] . In addition, SAC solders exhibit superior wettability and a soldering temperature comparable to that of conventional Sn-Pb solders, making them compatible with existing soldering equipment and industrial processes. However, Sn-Ag solders alone possess relatively poor wettability, which necessitates a higher soldering temperature, thereby increasing the risk of thermal damage to electronic components [ 84 ] . Sn-3.0Ag-0.5Cu (SAC305) has become the mainstream lead-free solder in modern electronic packaging owing to its well-balanced comprehensive performance. Its mechanical properties including yield strength, tensile strength, and elongation are notably superior, while solder joints fabricated with SAC305 exhibit enhanced mechanical strength, thermal fatigue resistance, and corrosion resistance compared to traditional Sn-37Pb solders [ 85 ] . However, with the large-scale adoption of SAC305 solder in the electronics industry, several inherent challenges have emerged. The high cost of silver (Ag) significantly increases the overall material expense; the melting point of 217°C remains too high for temperature-sensitive electronic components, posing risks of thermal damage during assembly; and the poor drop-impact resistance limits its reliability in consumer electronics, where devices are often subjected to frequent mechanical shocks and harsh service conditions [ 15 ] . The Sn-Ag-Cu (SAC) lead-free solder exhibits excellent solderability, wettability, and mechanical properties, making it one of the most reliable interconnection materials for modern electronic packaging applications [ 86 ] . The ternary Sn-Ag-Cu solder alloy has a eutectic temperature of approximately 217.2°C, which is 3.8°C lower than that of the Sn-Ag binary eutectic alloy [ 87 ] . The addition of Cu not only reduces the melting point of the Sn-Ag solder but also enhances its wettability [ 88 ] . Consequently, the Sn-Ag-Cu alloy demonstrates superior solderability and reliability compared with both Sn-Cu and Sn-Ag solder systems. The eutectic microstructure of Sn-Ag-Cu solder alloys primarily consists of β-Sn, Ag₃Sn, and Cu₆Sn₅ phases. Among them, both Cu₆Sn₅ and Ag₃Sn often exhibit coarse lamellar (plate-like) morphologies, where the Ag₃Sn plates are identified as a key factor contributing to the failure of Sn-Ag-Cu/Cu solder joints. This is because the presence of coarse intermetallic compounds (IMCs) tends to increase the brittleness of the solder matrix, thereby degrading its mechanical performance. Consequently, it has become a common industrial strategy to reduce the Ag and Cu contents in the solder composition in order to suppress the excessive growth of IMC phases and improve the overall joint reliability. Peng [ 89 ] et al. investigated the morphological evolution of IMCs in solder joints with varying Ag contents under different thermal shock durations using SEM analysis, as shown in Fig. 6 . The results revealed that higher Ag content led to coarser IMC grains, and for all solder joints, the Cu₆Sn₅ grains coarsened while grain boundary spacing decreased with prolonged thermal exposure.For solder joints with low Ag content, the IMC grains were initially fine but coarsened rapidly under thermal cycling, accompanied by numerous grain boundary cracks and poor oxidation and crack resistance. Conversely, high-Ag-content joints exhibited coarse grains, where Ag₃Sn phases enhanced grain boundary cohesion, but excessive coarsening and brittleness caused crack concentration within IMC layers.In contrast, the medium-Ag-content solder joints achieved a balanced grain size and boundary stability, showing the slowest crack propagation rate and optimal overall reliability and mechanical performance. The modification of Ag and Cu contents alone offers limited improvement in the overall properties of the solder. Therefore, introducing additional alloying elements has become an effective strategy to enhance the comprehensive performance of SAC solders, particularly in optimizing their mechanical properties [ 90 ] . Li [ 91 ] et al . investigated the effect of Bi addition on the microstructure of SAC solder by introducing 0.5 wt% and 5 wt% Bi. The corresponding SEM microstructures are shown in Fig. 7 (a) and (b). The addition of 5.0 wt% Bi exhibited the most pronounced refinement effect, as the Bi content exceeded its solubility limit in Sn (approximately 4 wt%), resulting in Bi phase precipitation that refined the β-Sn dendrite size and expanded the eutectic region. At 0.5 wt% Bi, most Bi atoms formed a solid solution within the Sn matrix, whereas at 5.0 wt% Bi, the precipitation of fine Bi particles further refined the microstructure and promoted a more uniform distribution of Ag₃Sn and Cu₆Sn₅ phases. As shown in Figure. 7(c), which presents the mechanical property test results, increasing the Bi content to 5.0 wt% significantly enhanced the hardness, shear strength, and tensile strength of the solder alloy, while the elongation decreased with higher Bi content. This improvement can be attributed to the combined effects of solid-solution strengthening, [ 92 ] grain-boundary strengthening [ 93 ] and precipitation strengthening. The dominant deformation mechanism is dislocation climb controlled by self-diffusion within the Sn lattice [ 94 ] . The addition of Ni can further refine the microstructure of the solder alloy and effectively enhance its mechanical properties, thermal stability, and tensile creep resistance [ 95 ] .The addition of Ni significantly refines the alloy microstructure by promoting the formation of intermetallic compounds (IMCs) such as (Cu,Ni)₆Sn₅. This refinement results in finer β-Sn grains and transforms the eutectic regions into a fibrous morphology, thereby enhancing resistance to dislocation motion [ 96 ] . After the addition of Ni, the solidus temperature slightly increases, and the melting point stabilizes within the range of 228.98-230.62°C, which meets the process requirements for electronic packaging. The creep rate increases with rising temperature and applied stress but decreases with Ni addition. When the Ni content reaches 0.05 wt.%, the alloy exhibits the best creep resistance. This improvement is attributed to the formation of fibrous eutectic regions and fine intermetallic compounds (Ag₃Sn and (Cu,Ni)₆Sn₅), which effectively hinder dislocation motion [ 97 ] . El-Taher [ 98 ] et al. introduced Ge as an alloying element to enhance the strength and ductility of Sn-based solder alloys. Experimental results show that with increasing Ge content, both the eutectic temperature and undercooling decrease, while the pasty range remains stable within 6.3–6.8°C. The yield strength, ultimate tensile strength, and elongation of the alloy all increase with Ge addition, reaching optimal values at 0.5 wt%. This improvement is primarily attributed to grain boundary, solid-solution, and precipitation strengthening mechanisms.In the microstructure of the pure SAC solder, β-Sn serves as the matrix phase with dispersed Ag₃Sn and Cu₆Sn₅ intermetallic compounds. After Ge addition, the formation of Ge-rich phases promotes significant grain refinement β-Sn dendrites become smaller and Ag₃Sn particles are fragmented. At a Ge content of 0.5 wt%, fine Ge particles are uniformly distributed, the eutectic region expands, and Ge preferentially enriches within the β-Sn matrix, thereby enhancing both the strength and ductility of the solder alloy. In addition to the incorporation of multi-element alloying, numerous researchers and institutions have explored the addition of nanoparticles as an effective approach to enhance the overall performance of solder alloys. Ma [ 99 ] et al . investigated the effects of adding varying concentrations of Mo nanoparticles to SAC solder alloys. The addition of Mo nanoparticles refined the microstructure, increased grain boundary length and dislocation density, thereby significantly enhancing the tensile strength of the solder joints. The pure SAC solder primarily consists of a eutectic structure composed of β-Sn, Ag₃Sn, and Cu₆Sn₅ phases. Upon the incorporation of Mo nanoparticles, the composite solder exhibits a significantly refined and more homogeneous microstructure, with uniformly dispersed Mo nanoparticles and markedly smaller β-Sn grains. This refinement is attributed to the nanoparticles’ ability to impede grain boundary migration, thereby suppressing grain growth and enhancing the overall microstructural stability. As the Mo nanoparticle content increases, the tensile strength of the solder joints correspondingly improves, reaching an optimal level at 0.5-1.0 wt%. The enhancement in mechanical properties is primarily governed by the synergistic effects of grain boundary strengthening—where refined grains provide more barriers to dislocation motion dislocation strengthening induced by dislocation accumulation around nanoparticles, and second-phase strengthening as Mo nanoparticles act as effective obstacles to dislocation glide. Consequently, the addition of Mo nanoparticles not only strengthens the SAC solder alloy but also contributes to improved structural uniformity and reliability, making it a promising modification strategy for advanced electronic interconnection applications. Yin [ 100 ] et al . enhanced the wettability, interfacial intermetallic compound (IMC) growth behavior, and mechanical properties of SAC0307 solder by incorporating SiC nanoparticles. After the addition of SiC nanoparticles, the SiC particles were adsorbed onto the surfaces of Sn and Cu atoms and accumulated at the interface, thereby impeding atomic interdiffusion and reducing the growth rate of intermetallic compounds (IMCs) [ 101 ] . The wettability and mechanical properties of the solder gradually improved with the increasing content of SiC nanoparticles [ 102 ] . The strengthening mechanism lies in the fact that SiC nanoparticles reduce the surface tension between the flux and the molten solder, thereby improving wettability. Additionally, the SiC nanoparticles refine the IMC grains, and the refined IMCs together with SiC particles act as second-phase pinning sites at the grain boundaries. This pinning effect impedes grain boundary sliding, increases dislocation density, and suppresses dislocation motion, thereby strengthening the solder matrix and enhancing its mechanical properties [ 103 ] . 3 Conclusion and future work Mainstream solder systems such as Sn-Cu, Sn-Zn, Sn-Bi, Sn-Ag, and SAC have developed relatively mature performance optimization pathways. Through alloying element doping and nanoparticle reinforcement strategies, these systems have effectively overcome their intrinsic limitations, providing diversified and feasible alternatives to traditional Sn-Pb solders. In Sn-Cu solder systems, the addition of elements such as Bi and Cr can effectively refine β-Sn grains and Cu₆Sn₅ intermetallic compounds (IMCs), thereby enhancing tensile strength and ductility. The incorporation of nanoparticles such as Co&C and Nb serves as a physical barrier that hinders atomic diffusion, reduces IMC layer thickness, and improves interfacial stability—enabling the solder to maintain cost advantages while ensuring basic connection reliability. In Sn-Zn solder systems, alloying with elements such as Ag, Cu, and Bi leads to the formation of AgZn₃ and Cu₅Zn₈ phases, which refine the microstructure and reduce Zn activity, achieving a balanced combination of wettability, strength, and ductility. Moreover, the introduction of nanoparticles such as Al and TiO₂ further suppresses oxidation and IMC coarsening, demonstrating significant practical potential in low-temperature packaging applications. In Sn-Bi solder systems, the addition of elements such as Ni and In enhances both strength and ductility through the formation of reinforcing phases or by suppressing Bi phase coarsening. Meanwhile, nanoparticles such as Co and TiC contribute to fine-grain strengthening and act as diffusion barriers, significantly reducing IMC growth rates and improving long-term reliability—making them promising candidates for low-temperature chip packaging applications. In Sn-Ag solder systems, SAC solders are the most widely applied in electronic packaging. By reducing Ag content and introducing elements such as Bi and Ge, the microstructure can be refined and solid-solution strengthening can be achieved, thereby improving strength, ductility, and reliability while reducing production costs. Furthermore, nanoparticles such as Mo and SiC enhance grain boundary density and dislocation strengthening, further optimizing mechanical performance and achieving a favorable balance between cost and performance. Overall, research on lead free solders has evolved from improving individual properties to achieving the coordinated optimization of comprehensive performance. Alloying regulates the microstructure through the synergistic effects of multiple elements, such as solid solution strengthening and grain refinement, while nanocomposite reinforcement enhances interfacial stability through particle dispersion strengthening and diffusion barrier effects. The combination of these approaches provides an effective pathway to overcome the inherent limitations of different alloy systems. Future research should focus on the quantitative analysis of multielement synergistic mechanisms, long term reliability assessment under extreme service environments such as high temperature and high humidity, and the customized development of low cost, high performance solders designed for advanced electronic packaging with high integration, wide temperature range, and extended service life. Declarations Data Availability Statement: The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. All data included in this study are available upon request by contact with the corresponding author. Author contributions: Qinrong Sun: Methodology, investigation, writing - review and editing, supervision, project management, funding acquisition. Xuehai Liao: Methodology, investigation, data compilation, writing - review and editing. Jinhong Dai: survey, data compilation. Wei Feng: Investigation, data organization, review and editing, supervision. Limeng Yin: review & editing, Supervision, Project administration, Funding acquisition. Funding: This work was supported by the National Natural Science Foundation of China (U24A20117,52175288), Natural Science Foundation of Chongqing (CSTB2023NSCQ-LZX0002),Jiangxi Key Laboratory of Forming and Joining Technology for Aerospace Components, Nanchang Hangkong University (EL202380301). Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJZD-M202401503). 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J Mater Res Technol 22:403–412. https://doi.org/10.1016/j.jmrt.2022.11.126 Wang J, Xue S, Zhang P et al (2019) The reliability of lead-free solder joint subjected to special environment: a review. J Mater Sci: Mater Electron 30:9065–9086. https://doi.org/10.1007/s10854-019-01333-w Choi H, Illés B, Hurtony T et al (2023) Corrosion problems of SAC-SiC composite solder alloys. Corros Sci 224:111488. https://doi.org/10.1016/j.corsci.2023.111488 Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 30 Nov, 2025 Reviewers invited by journal 28 Nov, 2025 Editor invited by journal 21 Nov, 2025 Editor assigned by journal 14 Nov, 2025 First submitted to journal 12 Nov, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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10:17:38","extension":"html","order_by":20,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":222670,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8100930/v1/8f2cf32fadbb2c243fa23cfa.html"},{"id":97148283,"identity":"73260578-c4c3-4408-b79d-95f7a93069c7","added_by":"auto","created_at":"2025-12-01 10:17:34","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":122988,"visible":true,"origin":"","legend":"\u003cp\u003eSEM microstructures of (a) Sn-2Cu, (b) Sn-2Cu-1Bi, (c) Sn-2Cu-3Bi, and (d) Sn-2Cu-5Bi\u003csup\u003e[27]\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8100930/v1/460311732b19890602ac55d1.png"},{"id":97148360,"identity":"b063ccda-4c29-4f60-86f3-52ed913396b3","added_by":"auto","created_at":"2025-12-01 10:17:47","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":52786,"visible":true,"origin":"","legend":"\u003cp\u003eVariation in IMC layer thickness at the Sn-0.7Cu-xCo\u0026amp;C/Cu interface under different reflow times: (a) total IMC layer, (b) Cu₆Sn₅ or (Cu, Co)₆Sn₅ layer, and (c) Cu₃Sn layer\u003csup\u003e[36]\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8100930/v1/8072e465c94f7ef6ccc25467.png"},{"id":97248427,"identity":"1442316a-98f4-4efb-afc4-fa1b88b3ac27","added_by":"auto","created_at":"2025-12-02 12:58:17","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":66245,"visible":true,"origin":"","legend":"\u003cp\u003eAverage thickness of intermetallic compounds (IMCs) in solder joints with varying Al nanoparticle contents\u003csup\u003e[55]\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8100930/v1/b280432d91c5c1ea5246532c.png"},{"id":97248339,"identity":"1205a3c1-bf5f-44e6-b9bc-a17fd5b606e3","added_by":"auto","created_at":"2025-12-02 12:54:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":628685,"visible":true,"origin":"","legend":"\u003cp\u003eSEM microstructures of Sn-Zn-based solders and their corresponding tensile properties:(a) Sn-14 at.% Zn solder;(b) Sn-Zn + 1 at.% Ag solder;(c) Sn-Zn + 1 at.% Ag + 0.5 at.% Li solder;(d) Sn-Zn + 1 at.% Ag + 1 at.% Al solder;(e) SZAAL solder;(f) Tensile strength-elongation curves of Sn-Zn eutectic solders with different alloying element additions\u003csup\u003e[62]\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8100930/v1/9b1163b39651cba6d998c8c7.png"},{"id":97148277,"identity":"3a14fdb4-00f9-4b1b-9e45-9c53da7f0f65","added_by":"auto","created_at":"2025-12-01 10:17:33","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":98673,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Ultimate tensile strength of the solder alloys under different aging durations; (b) Tensile strength of the solder alloys with varying Ag and Si contents\u003csup\u003e[76],[79]\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8100930/v1/adec8a96039fb37546c10646.png"},{"id":97148384,"identity":"c365a6a7-eec3-4aec-895e-b67a9c2591f8","added_by":"auto","created_at":"2025-12-01 10:17:48","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1447967,"visible":true,"origin":"","legend":"\u003cp\u003eMorphological evolution of intermetallic compounds (IMCs) in solder joints with different Ag contents under various thermal shock durations\u003csup\u003e[89]\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8100930/v1/d72f414c4c86d8a2211220d3.png"},{"id":97148331,"identity":"e6d48f82-5367-4e1f-a8a6-f9ee96279acb","added_by":"auto","created_at":"2025-12-01 10:17:46","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":477465,"visible":true,"origin":"","legend":"\u003cp\u003eSEM microstructures of (a) SAC157-0.5Bi and (b) SAC157-5.0Bi solders; (c) comparison of tensile stress-strain curves for solders with different Bi contents\u003csup\u003e[91]\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8100930/v1/2ab77d52d841597b3063cbbe.png"},{"id":97366706,"identity":"acf8f081-ff11-4a6d-b2da-2d09a8ec2b81","added_by":"auto","created_at":"2025-12-03 16:02:22","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3500240,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8100930/v1/d0edb615-fe71-4228-9c78-a2e40fe7f99f.pdf"}],"financialInterests":"","formattedTitle":"Recent Advances in Sn-Based Lead-Free Solders for Electronic Packaging: A Review","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eIn the context of the rapidly evolving global electronics industry, electronic packaging technology, which serves as a critical bridge ensuring efficient interconnection between electronic components and substrates, has undergone a paradigm shift. Its performance requirements have advanced beyond basic interconnection functions to include high reliability, enhanced integration, and prolonged service life. These developments have imposed increasingly stringent demands on the comprehensive properties of packaging materials\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e],[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e.As a key functional material within electronic packaging systems, solder plays a crucial role in electrical signal transmission, thermal dissipation, and mechanical bonding. Its physicochemical properties and mechanical behavior directly determine the reliability and service life of electronic devices operating under complex environmental conditions\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e],[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e.However, traditional tin-lead (Sn-Pb) solders contain highly toxic lead\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e, which not only causes persistent environmental contamination during manufacturing, operation, and recycling, but also poses serious risks to human health through bioaccumulation, particularly affecting the nervous and hematopoietic systems\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. In response, a series of international environmental regulations, most notably the European Union\u0026rsquo;s Restriction of Hazardous Substances (RoHS) directive, have explicitly restricted the use of lead in electronic manufacturing\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. This regulatory framework has fundamentally driven the research, application, and industrialization of lead-free solder alloys, establishing them as a focal point and inevitable trend in the development of modern electronic materials\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eDuring the systematic research and engineering application of lead-free solders, extensive studies have been conducted focusing on key performance indicators such as microstructural characteristics, wettability\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e and spreadability\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e, mechanical strength\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e, oxidation resistance\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e, and long-term reliability\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. Through continuous exploration, researchers have successively developed a series of multicomponent solder systems, including Sn-Cu\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e],\u003c/sup\u003e Sn-Ag\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e, Sn-Zn\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e, and Sn-Bi\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e alloys, each tailored to optimize specific performance requirements. Among these systems, Sn-Ag-Cu (SAC) solders have emerged as the predominant choice in the current market owing to their well-balanced combination of mechanical, thermal, and wetting properties, while Sn-Zn and Sn-Bi systems, characterized by their relatively low melting points, exhibit distinct advantages in low-temperature packaging applications\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. Despite significant progress, the existing lead-free solder systems still exhibit inherent limitations. SAC-type solders suffer from the high cost of Ag, which restricts their large-scale industrial application, while their relatively high processing temperatures may induce thermal damage to chips and substrates\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e. Sn-Cu solders, on the other hand, are constrained by their high melting points and poor wettability\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. In Sn-Zn systems, the high chemical activity of Zn leads to insufficient oxidation resistance and unstable wetting behavior, thereby compromising the reliability of the soldering process\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. Meanwhile, Sn-Bi solders are limited by the inherent brittleness of Bi, resulting in poor fatigue resistance and a tendency to develop cracks during service\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eTherefore, this review systematically summarizes recent research progress on lead-free solders for electronic packaging, with particular emphasis on the evolution of their microstructures, performance regulation mechanisms, and application limitations. The key challenges, including high melting temperatures, insufficient wettability, oxidation susceptibility, and increased material costs, are comprehensively analyzed. Accordingly, this paper highlights modification strategies based on alloying element doping, nanoparticle reinforcement, and multi-component alloying, and elucidates their effects on microstructural refinement and macroscopic performance enhancement of solder alloys. These insights are of significant theoretical and practical importance for addressing existing technological bottlenecks and advancing the development of high-performance, low-cost lead-free solders. Finally, potential future research directions are discussed in the context of the continuing evolution of electronic packaging technologies.\u003c/p\u003e"},{"header":"2 Solder Alloy Systems","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1Sn\u0026ndash;Cu Solder Alloys\u003c/h2\u003e\u003cp\u003eThe Sn-Cu-based lead-free solder, owing to the low cost of copper, has been widely used in electronic packaging, particularly in wave soldering applications\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. This solder system exhibits a suitable melting temperature, good fluidity, and low susceptibility to thermal cracking and segregation, making it a practical alternative for large-scale industrial applications\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e. However, during the soldering process, the molten solder tends to dissolve copper atoms from the substrate, leading to an increased Cu concentration on the solder side of the joint interface. As the Cu content rises at the interface, the local melting point of the solder correspondingly increases, which may induce interfacial defects and compromise the integrity of the solder joint. At room temperature, the stable intermetallic phases are Cu₃Sn and Cu₆Sn₅\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. With the increasing Cu concentration at the interface, the Cu₆Sn₅ phase tends to transform into the brittle Cu₃Sn phase. This phase transformation significantly affects the mechanical properties and service reliability of the solder joint\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eTo address the drawbacks of the Sn-Cu lead-free solder system, such as its relatively high melting point and poor wettability, researchers have introduced trace alloying elements including Bi, Ni, and In to enhance its overall performance and soldering reliability. The addition of these elements effectively improves the thermal properties, refines the solder microstructure, and increases the tensile strength of the joints. Huang\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e et al. investigated the effects of adding different proportions of Bi to Sn-Cu solder alloys to improve their overall performance. High-purity Sn, Cu, and Bi (99.9%) were used as raw materials. According to the designed compositions of Sn-2Cu-xBi (x\u0026thinsp;=\u0026thinsp;0, 1, 3, and 5 wt.%), the elements were accurately weighed and placed into a graphite crucible, which was then loaded into an electric resistance furnace. Nitrogen gas was introduced into the furnace to suppress the oxidation of Bi at elevated temperatures. The alloy mixture was heated to 400\u0026deg;C and held for 1 h to ensure complete melting and homogenization. Prior to casting, the molten alloy was cooled to 250\u0026deg;C, and the mold was preheated to the same temperature to minimize thermal gradients. Finally, the melt was allowed to cool naturally to room temperature for solidification, completing the preparation of the Sn-2Cu-xBi alloy series. Studies have shown that the addition of Bi can significantly reduce the melting point and undercooling of Sn-Cu solder alloys, while effectively refining the intermetallic compounds (IMCs) \u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. The microstructural morphology of the samples was examined using scanning electron microscopy (SEM), as shown in Figure. 1. The addition of Bi resulted in the refinement of the Cu₆Sn₅ intermetallic phase, and with increasing Bi content, a larger number of Bi-rich precipitates were observed. In addition, Fan\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e \u003cem\u003eet al.\u003c/em\u003e improved the intermetallic compound (IMC) layer characteristics by incorporating Ni into the Sn-Cu solder alloy. When Sn-based solders are joined with Cu substrates, interfacial reactions occur to form a brittle η\u0026prime;-Cu₆Sn₅ intermetallic compound (IMC). The excessive growth of this IMC layer often deteriorates the mechanical integrity of the solder joint, leading to a reduction in both tensile and shear strength. The incorporation of Ni has been shown to stabilize the η\u0026prime;-Cu₆Sn₅ phase thermodynamically, refine its morphology, and enhance its fracture toughness, thereby improving the overall mechanical reliability of the solder joint\u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e. With the increase in Ni content, the alloy microstructure becomes progressively refined, while the (Cu,Ni)₆Sn₅ intermetallic compound (IMC) tends to coarsen and aggregate\u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. Consequently, the microhardness of the solder exhibits an initial increase followed by a gradual decrease. Moreover, both the wetting behavior and spreading performance improve with rising soldering temperature and extended dwell time. Among the tested compositions, the Sn-0.7Cu-0.2Ni solder demonstrates the optimal wettability at 280\u0026deg;C after 10 min of reflow\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. Kelly\u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e et al. reported that the addition of indium (In) can effectively reduce the melting point of Sn-based solders, refine their microstructure, and enhance mechanical properties. Moreover, In incorporation suppresses the formation of Cu₆Sn₅ intermetallic compounds (IMCs) during the electromigration process. Therefore, introducing a small amount of In is considered an effective strategy to improve the overall performance of Sn-based solders. Sn-0.7 wt.% Cu and Sn-0.7 wt.% Cu-\u0026lt;1 wt.% In solders were selected for the study. Copper wires with a diameter of 250 \u0026micro;m were used as substrates, and the electromigration (EM) tests were conducted under a current density of 1\u0026times;10⁴ A/cm\u0026sup2;. Cross-sectional observations of the solder joints using scanning electron microscopy (SEM) revealed that some intermetallic compounds (IMCs) in the Sn-In/Cu joints grew into large needle-like structures. The incorporation of In partially substituted Sn in Cu₆Sn₅, resulting in the transformation of the IMC phase from Cu₆Sn₅ to Cu₆(Sn,In)₅. Furthermore, analysis of the Sn-Cu-In samples after 200 hours of electromigration indicated the presence of indium-rich clusters near the anode interface, which gradually migrated toward the anode with increasing electromigration duration. The inhibitory mechanism of indium (In) on the growth of intermetallic compounds (IMCs) lies in its substitution of Sn atoms in both η\u0026prime;-Cu₆Sn₅ and Cu₃Sn phases, which increases the migration energy barrier of Cu and Sn atoms and consequently reduces the IMC growth rate\u003csup\u003e[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn addition to alloying elements, the performance of solders can also be improved by incorporating nanoparticles. However, due to the significant density mismatch between most nanoparticles and Sn-0.7Cu solder, they tend to float on the surface of the molten solder, leading to uneven mixing. Liu\u003csup\u003e[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e et al. addressed this issue by adding Nb nanoparticles, which have a density similar to that of Sn-0.7Cu solder. This modification effectively enhances wettability, refines the microstructure, and improves the mechanical properties of the solder. Sn-0.7Cu-xNb composite solders with varying Nb content were prepared using a mechanical mixing method. After reflow soldering, the wettability was assessed by measuring the spreading area of the solder on a Cu substrate, and the best wettability was observed at 0.12 wt.% Nb. The composite solder was then etched, and its microstructure was examined using scanning electron microscopy (SEM). The microstructure consists of β-Sn phase and β-Sn/Cu₆Sn₅ eutectic. The addition of 0.12 wt.% Nb refines the β-Sn grain size to 8.9 \u0026micro;m. The refinement effect of Nb nanoparticles, along with the proliferation of dislocations and the hindrance of dislocation motion, results in the maximum ultimate tensile strength and elongation of the composite solder. Lu\u003csup\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e et al. synthesized carbon framework nanosheets loaded with cobalt nanoparticles (Co\u0026amp;C) through a metal-organic framework (MOF)-derived method and incorporated them into Sn-0.7Cu solder. nglish SCI-polished version:\u003c/p\u003e\u003cp\u003eThis approach avoids the shortcomings associated with the sole addition of Co, which promotes IMC growth\u003csup\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e, or the use of carbon materials alone, which may result in weak bonding with the solder\u003csup\u003e[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e The melting point and contact angle of the solder were determined using differential scanning calorimetry (DSC) and a contact angle goniometer. The results indicate that the addition of trace amounts of Co\u0026amp;C has a negligible effect on the solder's melting point, but significantly improves its wettability by reducing the contact angle. Reflow testing was conducted with the peak temperature set to 250\u0026deg;C in the reflow furnace. The solder balls and Cu substrates were held at this temperature for four different durations: 450 s, 900 s, 1500 s, and 2100 s. The results, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, illustrate the variation in the intermetallic compound (IMC) layer thickness at the Sn-0.7Cu-xCo\u0026amp;C/Cu interface with different reflow times. As the reflow time increased from 450 s to 900 s, the IMC thickness at the interface grew from 5.20 \u0026micro;m to 7.97 \u0026micro;m, with a growth rate significantly higher than that observed for other composite solders. The mechanism behind this effect lies in the ability of Co to occupy the Cu sites in Cu₆Sn₅, forming (Cu, Co)₆Sn₅, which suppresses the growth of Cu₃Sn and transforms the scallop-shaped IMCs into a more planar structure. Additionally, the carbon framework in the Co\u0026amp;C nanosheets acts as an inert physical barrier, hindering the diffusion of Sn and Cu atoms, thereby preventing the further growth of IMCs\u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn recent years, due to the high tin content inherent in Sn-Cu-based solders, the enhancement of overall properties through the addition of a single minor alloying element has proven to be limited. To meet the performance requirements under diverse service environments, future research should focus on addressing the intrinsic drawbacks of this system, particularly its high melting point and insufficient wettability. The development of multi-element composite lead-free solders through synergistic alloying design is expected to offer a promising pathway toward achieving improved comprehensive performance\u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e. The addition of Bi to Sn-0.7Cu solder can effectively reduce its melting point; however, further optimization of its overall performance is still required\u003csup\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e. Han et al. reported that the addition of Cr significantly improves the wettability of the solder, refines its microstructure, and effectively suppresses the growth of intermetallic compound (IMC) layers\u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e. The underlying mechanism is that with the increasing Cr content, the aggregation of Bi atoms and the size of β-Sn grains are gradually refined, which effectively hinders the diffusion of Sn and Cu atoms. However, excessive Cr addition leads to the precipitation of brittle intermetallic compounds, resulting in reduced ductility. When the Cr content reaches 0.2 wt.%, the Sn-0.7Cu-10Bi solder exhibits the optimal overall performance in terms of melting temperature, wettability, microstructural refinement, and interfacial reliability. In addition, the incorporation of indium (In) can enhance the electrical conductivity of the solder and refine its grain structure. The underlying mechanism lies in the formation of the intermetallic compound Cu₂In₂O₅, which is a transparent conductive oxide semiconductor generated from the reaction between In and Cu oxides. This compound effectively improves the electrical performance of the alloy. Furthermore, the addition of In significantly reduces the average grain size of the solder alloy, thereby enhancing its hardness, tensile strength, and overall mechanical properties\u003csup\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2Sn-Zn Solder Alloys\u003c/h2\u003e\u003cp\u003eSn-Zn solder alloys have emerged as one of the core alternatives to traditional Sn-Pb solders due to their low cost, environmental friendliness, excellent mechanical properties, and a eutectic melting point close to that of Sn-Pb alloys\u003csup\u003e[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e. Owing to these advantages, Sn-Zn solders have been widely applied in consumer electronics, automotive electronics, and other industrial fields\u003csup\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/sup\u003e. The eutectic composition of the Sn-Zn alloy is Sn-9Zn, with a eutectic temperature of 198.5\u0026deg;C\u003csup\u003e[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]\u003c/sup\u003e. Its microstructure consists of coarse primary β-Sn dendrites and a fine β-Sn/Zn eutectic matrix\u003csup\u003e[\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]\u003c/sup\u003e. Numerous researchers and institutions worldwide have devoted extensive efforts to improving the performance of Sn-Zn solder alloys by incorporating various alloying elements\u003csup\u003e[\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]\u003c/sup\u003e. To address the issue of oxidation susceptibility, Ag is often incorporated into the Sn-Zn solder system to improve its oxidation resistance and overall performance\u003csup\u003e[\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]\u003c/sup\u003e. Because Ag exhibits a stronger chemical affinity for Zn, which is more chemically active in the solder matrix, Ag preferentially reacts with Zn to form the intermetallic compound AgZn₃\u003csup\u003e[\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]\u003c/sup\u003e. This reaction alters the microstructure of the solder matrix, effectively suppressing the precipitation and growth of needle-like Zn-rich phases while simultaneously refining the grain size and homogenizing the microstructure, thereby enhancing the fundamental mechanical properties of the solder alloy\u003csup\u003e[\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]\u003c/sup\u003e. However, the poor wettability of Sn-Zn binary solder alloys is primarily attributed to their inadequate oxidation resistance, which can be effectively improved by the addition of Bi\u003csup\u003e[\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]\u003c/sup\u003e. The addition of an appropriate amount of Bi can effectively reduce the surface tension of the solder\u003csup\u003e[\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]\u003c/sup\u003e. This phenomenon arises from the fact that the surface tension of liquid Sn-Bi eutectic alloys is significantly lower than that of liquid Sn-Zn eutectic solders under the same conditions. The reduction in surface tension directly enhances the spreading kinetics of the solder on the Cu substrate surface and accelerates interfacial diffusion between the solder and the substrate. Consequently, the wettability of the solder is synergistically improved from both thermodynamic and kinetic perspectives.\u003c/p\u003e\u003cp\u003eIn addition to alloying element additions, the incorporation of nanoparticles can also be employed to enhance the overall performance of solder alloys\u003csup\u003e[\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]\u003c/sup\u003e. To address the inherent drawbacks of Sn-Zn solders, such as limited electrical conductivity, low melting point, and high susceptibility to oxidation. Li\u003csup\u003e[\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]\u003c/sup\u003e et al. introduced aluminum (Al) nanoparticles into the Sn-Zn solder matrix to mitigate the inherent drawbacks of Sn-Zn-based solders. Composite solder joints with varying Al nanoparticle contents were fabricated using transient liquid-phase bonding (TLPB) technology. The interfacial reaction layers of the joints were primarily composed of Cu₃(Sn, Zn) and Cu₆(Sn, Zn)₅ intermetallic compounds (IMCs) \u003csup\u003e[\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]\u003c/sup\u003e. The microstructures of solder joints containing varying amounts of Al nanoparticles were examined using scanning electron microscopy (SEM). The average intermetallic compound (IMC) thickness is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. As the Al nanoparticle content increased, the IMC layer thickness initially decreased and then increased, reaching its minimum value at 0.6 wt% Al. Similarly, the porosity of the solder joints exhibited a decreasing-increasing trend, reaching the lowest level at 0.6 wt% Al, which was 57.56% lower than that of the solder joints without Al addition. In terms of mechanical properties, the shear strength of the solder joints increased initially and then decreased with rising Al nanoparticle content, reaching a maximum at 0.6 wt%, which represents a 54.20% improvement compared with the joints without Al addition. Meanwhile, the fracture mode gradually shifted from ductile fracture to brittle fracture, and the fracture location transitioned from the \u003cem\u003ein-situ reaction zone\u003c/em\u003e to the \u003cem\u003einterfacial reaction zone\u003c/em\u003e\u003csup\u003e[\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]\u003c/sup\u003e. In addition, the incorporation of TiO₂ nanoparticles can effectively optimize the microstructural morphology and enhance the creep resistance of the solder alloy\u003csup\u003e[\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]\u003c/sup\u003e. After the incorporation of TiO₂ nanoparticles into the solder alloy, the SEM micrographs reveal uniformly dispersed flower-like and eye-shaped morphologies within the alloy matrix. These nanoparticles effectively stabilize the grain structure, suppress grain coarsening, and act as pinning centers for dislocation motion, thereby increasing resistance to plastic deformation and significantly enhancing the creep resistance of the solder\u003csup\u003e[\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe microstructure of Sn-Zn solder primarily consists of a β-Sn matrix phase interspersed with Zn-rich phases. The Zn phase typically appears as irregular particles or dendritic structures, and due to the limited solid solubility between Zn and Sn, Zn tends to segregate at grain boundaries or within the matrix\u003csup\u003e[\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e]\u003c/sup\u003e. The addition of Cu to Sn-Zn solder promotes the formation of Cu-Zn intermetallic compounds (IMCs), which effectively reduce the chemical activity of Zn and thereby enhance the wettability of the solder\u003csup\u003e[\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e]\u003c/sup\u003e. As the Cu content increases, Cu reacts with Zn or Sn to promote the formation of intermetallic compounds (IMCs). These IMCs are uniformly distributed between the β-Sn matrix and the Zn-rich phase, contributing to microstructural refinement and suppressing the coarsening of Zn phases. However, the improvement in overall properties achieved by adding a single minor alloying element remains limited. Therefore, Bi is introduced into the Sn-Zn-Cu solder system to further enhance performance through the synergistic effects of multiple alloying elements\u003csup\u003e[\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e]\u003c/sup\u003e. The microstructure of Sn-Zn-Cu-Bi solder primarily consists of eutectic structures, β-Sn phases, Cu₅Zn₈ intermetallic compounds, and precipitated Bi particles. As the Bi content increases, the segregation of Bi particles promotes heterogeneous nucleation within the matrix, resulting in refined solder microstructures. Meanwhile, with increasing Bi addition, the tensile strength of the solder improves while its ductility decreases, and the fracture mode transitions from ductile to brittle. This behavior is attributed to the solid-solution strengthening effect of fine Bi particles; however, excessive Bi at grain boundaries introduces brittleness, significantly reducing ductility.In addition to Bi modification, further enhancements to Sn-Zn-based solders can be achieved by incorporating Ag, Al, or Li elements, which contribute to optimizing the microstructure and improving overall mechanical and thermal properties\u003csup\u003e[\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e]\u003c/sup\u003e. In terms of thermal properties, the addition of Ag and Al elements increases the melting point of the Sn-Zn eutectic alloy, whereas the incorporation of Li results in a decrease in melting temperature. Regarding mechanical performance, the addition of Al and Li enhances both tensile strength and plasticity of the solder. From the perspective of wettability, the inclusion of Li and Ag reduces the alloy\u0026rsquo;s coefficient of thermal expansion. Moreover, Li significantly decreases the surface and interfacial tensions of the solder, thereby improving its wettability and promoting superior interfacial bonding behavior. The SEM microstructures of Sn-Zn solders with varying metal element contents are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a-e). The eutectic Sn-Zn alloy exhibits a dual-phase microstructure composed of bright Sn-rich regions and dark, needle-like Zn-rich phases. Upon the addition of Ag, the AgZn₃ intermetallic compound (IMC) forms, featuring a higher Zn concentration in its outer layer than in its core, and a refined eutectic microstructure. When Li is introduced, the overall microstructure becomes finer, with the eutectic region consisting of Sn-rich and α-Zn precipitates; partial dezincification may occur. Since Sn has a stronger solubility for Li than Zn, Li tends to dissolve preferentially into the Sn matrix, further refining the structure. The addition of Al results in Al-rich spots and relatively coarse α-Zn and Ag-rich precipitates, with Al segregating preferentially within the Zn-rich phase. The Sn-Zn-Ag-Al-Li alloy combines these microstructural characteristics, exhibiting a composite structure comprising Sn-rich phases, rod-like α-Zn phases, AgZn₃ IMCs, and dispersed Al-rich regions. The tensile test results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(f), illustrating the relationship between tensile strength and elongation for Sn-Zn-based solders containing varying amounts of metallic elements. The addition of Ag promotes the formation of AgZn₃ intermetallic compounds, which strengthen the solder matrix and enhance tensile strength. Furthermore, the incorporation of Al and Li elements leads to a simultaneous improvement in both strength and elongation, primarily attributed to grain refinement and the inhibition of crack propagation by the finely dispersed precipitates.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3Sn-Bi Solder Alloys\u003c/h2\u003e\u003cp\u003eThe Sn-Bi solder system, characterized by its low eutectic melting point of 139\u0026deg;C, has emerged as an ideal candidate for the packaging of temperature-sensitive electronic components. Its yield strength, fracture resistance, wettability, and solderability are comparable to those of the traditional eutectic Sn-Pb solder, making it a promising alternative for low-temperature interconnection applications\u003csup\u003e[\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e]\u003c/sup\u003e. During practical soldering processes, the Sn-Bi system is capable of forming a more uniform and homogeneous joint region, effectively reducing void formation and ensuring gas-tight reliability under low-temperature conditions. These advantages make it one of the most prominent and practical lead-free solder systems for advanced electronic packaging applications\u003csup\u003e[\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e]\u003c/sup\u003e. The eutectic composition of Sn-Bi solder is Sn-57Bi, corresponding to a eutectic temperature of 139\u0026deg;C\u003csup\u003e[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]\u003c/sup\u003e. Under equilibrium solidification conditions, no intermetallic compounds (IMCs) are formed in this system. The microstructure consists of alternating lamellar distributions of Sn-rich and Bi-rich phases. During alloy solidification and cooling, Bi atoms precipitate as fine particles within the Sn-rich matrix\u003csup\u003e[\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e]\u003c/sup\u003e. In terms of performance advantages, Sn-Bi solder exhibits a low melting point along with excellent wettability, oxidation resistance, and room-temperature tensile strength. These characteristics make it highly suitable for temperature-sensitive electronic components and low-temperature flip-chip interconnection applications, where both reliability and thermal compatibility are critical.\u003c/p\u003e\u003cp\u003eAlthough Sn-Bi solder possesses numerous advantages, it still suffers from several inherent drawbacks\u003csup\u003e[\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e]\u003c/sup\u003e. These include poor thermal fatigue resistance, limited ductility, and suboptimal processability, as well as a high intrinsic brittleness of the solder matrix, which leads to insufficient creep resistance and long-term reliability of the solder joints\u003csup\u003e[\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e]\u003c/sup\u003e. The root causes of these deficiencies are closely associated with the intrinsic properties of the bismuth (Bi) element, which can be summarized in two main aspects.First, Bi exhibits a high solid solubility in the Sn matrix, leading to the precipitation of a large number of Bi particles during solidification. This process is often accompanied by a significant volumetric expansion effect, which introduces internal stress and microcracks.Second, Bi crystallizes in a rhombohedral (trigonal) crystal structure with a limited number of active slip systems, resulting in its inherent hardness and brittleness, as well as relatively poor electrical and thermal conductivity\u003csup\u003e[\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e]\u003c/sup\u003e. The combined influence of these factors renders the performance of Sn-Bi solder alloys highly sensitive to Bi content. Consequently, the current Sn-Bi system still fails to meet the stringent service requirements of applications involving high temperatures and cyclic mechanical stresses, such as automotive circuit boards and military-grade electronic devices\u003csup\u003e[\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e]\u003c/sup\u003e. To address the inherent brittleness and insufficient mechanical performance of Sn-Bi solder alloys, extensive studies have been conducted by researchers worldwide focusing on alloying modification strategies. These investigations have confirmed that the addition of elements such as Ni, Ag, In, and Cu can effectively enhance the overall properties of Sn-Bi solders by refining the microstructure and introducing strengthening phases through various metallurgical mechanisms\u003csup\u003e[\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e]\u003c/sup\u003e. Zhang\u003csup\u003e[\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e]\u003c/sup\u003e \u003cem\u003eet al.\u003c/em\u003e reported that the addition of Ni to Sn-58Bi solder results in the formation of the Ni₃Sn₄ intermetallic phase, which serves as a strengthening phase and contributes to microstructural refinement. This modification significantly enhances both the ultimate tensile strength and the elastic modulus of the solder alloy. However, excessive Ni addition (1.0 wt%) leads to significant grain coarsening, resulting in a marked reduction in ductility. In contrast, an addition of 0.5 wt% Ni achieves a better balance among strength, ductility, and elastic modulus, exhibiting superior overall tensile performance. When the Ni content reaches 1.0 wt%, the hardness of the solder increases sharply, even exceeding the typical empirical ratio where hardness is approximately one-third of the ultimate tensile strength. This deviation is attributed to the formation of hard intermetallic compounds (IMCs) promoted by Ni, which enhances the alloy\u0026rsquo;s resistance to plastic deformation.\u003c/p\u003e\u003cp\u003eNevertheless, the shear strength of the solder with Ni addition is slightly lower than that of plain Sn-58Bi solder, primarily because Ni promotes an increase in both the volume fraction and coarsening of the Bi-rich phase, which in turn adversely affects shear performance to some extent\u003csup\u003e[\u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e]\u003c/sup\u003e. In addition, Hu\u003csup\u003e[\u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e]\u003c/sup\u003e et al. investigated the effects of Ag and In additions on improving the mechanical properties of Sn-Bi solder alloys. The addition of Ag can enhance the strength of Sn-Bi solder alloys by forming the Ag₃Sn intermetallic compound, while simultaneously suppressing the coarsening of the Bi phase. However, excessive Ag content leads to an increased formation of brittle phases, resulting in a sharp decline in ductility. Under an appropriate cooling rate, Ag addition not only improves the mechanical strength and wettability of the solder but also effectively inhibits Bi segregation within the microstructure. The addition of In significantly enhances the ductility of Sn-Bi solder alloys and reduces Bi phase precipitation. This improvement arises because In atoms suppress the coarsening of the Bi phase, thereby refining the microstructure and promoting a more uniform distribution of Bi. However, the tensile strength exhibits a slight decrease, while the hardness first increases and then decreases with increasing In content\u003csup\u003e[\u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e]\u003c/sup\u003e.Junpei Umeyama\u003csup\u003e[\u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e75\u003c/span\u003e]\u003c/sup\u003e \u003cem\u003eet al.\u003c/em\u003e demonstrated that the addition of Cu to Sn-Bi solder can effectively enhance its strength and hardness. This improvement is primarily attributed to the formation of hard intermetallic compounds (IMCs) and the refinement of the microstructure, which collectively strengthen the solder matrix and improve its overall mechanical performance. The mechanical properties of the solder were evaluated under varying temperature and strain rate conditions. Tensile tests conducted on solders with different Cu contents revealed that the optimal Cu addition is 0.1 wt.%. The tensile strength increases with temperature but decreases with lower strain rates, while the elongation exhibits the opposite trend. When the testing conditions exceed 333 K in temperature and fall below a strain rate of 5.25\u0026times;10⁻⁴ s⁻\u0026sup1;, the solder exhibits superplastic behavior. The dominant mechanisms of this superplastic deformation are recovery-induced recrystallization, which refines the grains, and grain boundary sliding within the Sn-Bi eutectic phases.SEM microstructural observations show that an appropriate amount of Cu addition leads to a more uniform distribution of the Bi phase and a fracture surface characterized by ductile features. However, excessive Cu addition can result in performance fluctuations, attributed to the formation of the hard Cu₆Sn₅ intermetallic compound (IMC) and grain boundary pinning effects. Therefore, precise control of Cu content is essential to maintain an optimal balance between strength and ductility.\u003c/p\u003e\u003cp\u003eIn addition, the performance of Sn-Bi solders can be further improved by introducing nanoparticle reinforcements. The incorporation of Co nanoparticles at varying concentrations has been shown to refine the microstructure of the solder and enhance its mechanical properties\u003csup\u003e[\u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e76\u003c/span\u003e]\u003c/sup\u003e. The microstructure of the pure Sn-58Bi solder consists of two distinct phases: a dark Sn-rich matrix and bright Bi-rich regions, where the Bi phase exhibits significant coarsening. After the addition of Co nanoparticles, the solder\u0026rsquo;s ductility is markedly improved, and the grain structure becomes significantly refined. As the concentration of Co nanoparticles increases, the grain refinement effect becomes more pronounced. The uniformly distributed Co nanoparticles act as heterogeneous nucleation sites during solidification and effectively inhibit grain growth by pinning grain boundaries. Consequently, the addition of Co nanoparticles not only enhances the mechanical properties of the Sn-58Bi solder but also mitigates the inherent brittleness caused by Bi phase aggregation. This phenomenon can be attributed to the ability of Co nanoparticles to suppress the coarsening of the Bi phase and to promote the formation of more thermodynamically stable intermetallic compounds (IMCs). The mechanical properties of Sn-Bi solders containing Co nanoparticles were evaluated before and after aging, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a). In the unaged condition, tensile tests revealed that the addition of Co nanoparticles significantly enhanced the ultimate tensile strength of the Sn-58Bi solder, and the strength increased progressively with rising Co concentration. After aging for different durations, a decrease in ultimate tensile strength was observed in all samples; however, the reduction was notably smaller in the Co-containing solders. This improvement is primarily attributed to the grain-refinement effect induced by Co nanoparticles, which contributes to the suppression of strength degradation during aging. Moreover, the formation of thermodynamically stable IMCs helps to maintain microstructural stability and consequently enhances the overall mechanical performance of the solder. Kim\u003csup\u003e[\u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e77\u003c/span\u003e]\u003c/sup\u003e \u003cem\u003eet al.\u003c/em\u003e enhanced the wettability and mechanical properties of solder alloys and suppressed the growth of intermetallic compounds (IMCs) by incorporating TiC nanoparticles at various concentrations. Differential scanning calorimetry (DSC) analysis revealed that the addition of TiC nanoparticles had a negligible effect on the melting point of Sn-58Bi solder. The spreading ratio of the solder initially increased and then decreased with the addition of TiC nanoparticles, reaching its maximum value at 0.1 wt% TiC. However, excessive TiC addition led to a reduction in spreading performance, primarily because an appropriate amount of TiC can decrease surface tension and improve wettability, whereas excessive TiC tends to agglomerate and increase the viscosity of the molten solder due to its high melting point, thereby hindering spreading behavior.The mechanism by which TiC nanoparticles suppress IMC growth lies in their function as heterogeneous nucleation sites, promoting grain refinement. Additionally, TiC possesses high chemical stability, making it resistant to coarsening or chemical reactions. As a result, the thickness of interfacial IMCs decreases with TiC addition. This effect arises because TiC particles adsorb at the Bi/Cu₆Sn₅ interface, lowering interfacial energy and obstructing the diffusion of Sn and Cu atoms, which effectively retards IMC growth.\u003c/p\u003e\u003cp\u003eSn-Bi alloys, owing to their non-toxic nature and low melting point, have emerged as ideal candidates for low-temperature lead-free solders. However, they inherently suffer from brittleness, poor wettability, and interfacial Bi segregation, which can lead to joint embrittlement and compromised mechanical reliability. The Ag element, known for its excellent electrical conductivity and wettability, is commonly incorporated into Sn-Bi solders to enhance their performance\u003csup\u003e[\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e]\u003c/sup\u003e. However, the improvement achieved through a single minor alloying addition remains limited. Therefore, the co-addition of Si has been explored to further enhance tensile strength and hardness, as well as to optimize the overall wettability of the solder alloy\u003csup\u003e[\u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e79\u003c/span\u003e]\u003c/sup\u003e. After introducing a trace amount of Si into the Sn-Bi-Ag solder, the microstructure of the alloy became significantly refined. The morphology of the Bi phase transformed from coarse block-like structures into an interconnected network, accompanied by an increased number of needle-shaped Bi phases embedded within the Sn matrix. Moreover, the wettability of the solder was notably improved, and the thickness of the diffusion layer formed at the solder/Cu substrate interface was substantially reduced, while the melting point and melting range remained essentially unchanged.These improvements can be attributed to the inhibitory effect of Si on Bi phase coarsening and segregation, as well as its barrier effect on Cu atomic diffusion across the interface. The tensile test results for alloys containing varying Ag and Si contents, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b), demonstrate that the addition of Si effectively enhances both the tensile strength and microhardness of the solder. Furthermore, as the Ag content increases, the mechanical properties of the Sn-Bi-Ag-Si solder alloys are further improved, indicating a synergistic strengthening effect between Ag and Si.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4Sn-Ag Solder Alloys\u003c/h2\u003e\u003cp\u003eGiven the environmental toxicity of lead, global environmental protection regulations have imposed strict restrictions on its use in the electronics manufacturing industry. Against this backdrop, tin-based lead-free solder alloys have emerged as a key alternative to traditional tin-lead (Sn-Pb) systems, offering a more environmentally sustainable and regulatory-compliant solution\u003csup\u003e[\u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e80\u003c/span\u003e]\u003c/sup\u003e. Among them, the Sn-Ag solder system has attracted considerable attention due to its combination of excellent mechanical strength and superior wettability. Its eutectic composition is Sn-3.5Ag, with a corresponding eutectic temperature of 221\u0026deg;C\u003csup\u003e[\u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e81\u003c/span\u003e]\u003c/sup\u003e. Under equilibrium solidification conditions, its microstructure consists of primary β-Sn dendritic phases and a Sn/Ag₃Sn eutectic matrix. The dispersed Ag₃Sn intermetallic compounds (IMCs) within the tin matrix significantly enhance the overall mechanical and physical properties of the solder alloy\u003csup\u003e[\u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e82\u003c/span\u003e]\u003c/sup\u003e. Owing to its outstanding mechanical strength, oxidation resistance, soldering reliability, and service stability, the Sn-Ag system has been widely utilized in electronic packaging and interconnection applications. However, it still suffers from inferior wettability compared with Sn-Pb solders and a relatively high melting point. In the Sn-Ag alloy system, a higher Ag content is typically required to achieve desirable performance; however, the incorporation of Cu can effectively reduce this dependence on Ag\u0026mdash;for instance, in the widely used SAC305 (Sn-3.0Ag-0.5Cu) alloy\u0026mdash;thereby maintaining mechanical integrity while lowering material costs\u003csup\u003e[\u003cspan citationid=\"CR83\" class=\"CitationRef\"\u003e83\u003c/span\u003e]\u003c/sup\u003e. In addition, SAC solders exhibit superior wettability and a soldering temperature comparable to that of conventional Sn-Pb solders, making them compatible with existing soldering equipment and industrial processes. However, Sn-Ag solders alone possess relatively poor wettability, which necessitates a higher soldering temperature, thereby increasing the risk of thermal damage to electronic components\u003csup\u003e[\u003cspan citationid=\"CR84\" class=\"CitationRef\"\u003e84\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eSn-3.0Ag-0.5Cu (SAC305) has become the mainstream lead-free solder in modern electronic packaging owing to its well-balanced comprehensive performance. Its mechanical properties including yield strength, tensile strength, and elongation are notably superior, while solder joints fabricated with SAC305 exhibit enhanced mechanical strength, thermal fatigue resistance, and corrosion resistance compared to traditional Sn-37Pb solders\u003csup\u003e[\u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e85\u003c/span\u003e]\u003c/sup\u003e. However, with the large-scale adoption of SAC305 solder in the electronics industry, several inherent challenges have emerged. The high cost of silver (Ag) significantly increases the overall material expense; the melting point of 217\u0026deg;C remains too high for temperature-sensitive electronic components, posing risks of thermal damage during assembly; and the poor drop-impact resistance limits its reliability in consumer electronics, where devices are often subjected to frequent mechanical shocks and harsh service conditions\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe Sn-Ag-Cu (SAC) lead-free solder exhibits excellent solderability, wettability, and mechanical properties, making it one of the most reliable interconnection materials for modern electronic packaging applications\u003csup\u003e[\u003cspan citationid=\"CR86\" class=\"CitationRef\"\u003e86\u003c/span\u003e]\u003c/sup\u003e. The ternary Sn-Ag-Cu solder alloy has a eutectic temperature of approximately 217.2\u0026deg;C, which is 3.8\u0026deg;C lower than that of the Sn-Ag binary eutectic alloy\u003csup\u003e[\u003cspan citationid=\"CR87\" class=\"CitationRef\"\u003e87\u003c/span\u003e]\u003c/sup\u003e. The addition of Cu not only reduces the melting point of the Sn-Ag solder but also enhances its wettability\u003csup\u003e[\u003cspan citationid=\"CR88\" class=\"CitationRef\"\u003e88\u003c/span\u003e]\u003c/sup\u003e. Consequently, the Sn-Ag-Cu alloy demonstrates superior solderability and reliability compared with both Sn-Cu and Sn-Ag solder systems. The eutectic microstructure of Sn-Ag-Cu solder alloys primarily consists of β-Sn, Ag₃Sn, and Cu₆Sn₅ phases. Among them, both Cu₆Sn₅ and Ag₃Sn often exhibit coarse lamellar (plate-like) morphologies, where the Ag₃Sn plates are identified as a key factor contributing to the failure of Sn-Ag-Cu/Cu solder joints. This is because the presence of coarse intermetallic compounds (IMCs) tends to increase the brittleness of the solder matrix, thereby degrading its mechanical performance. Consequently, it has become a common industrial strategy to reduce the Ag and Cu contents in the solder composition in order to suppress the excessive growth of IMC phases and improve the overall joint reliability. Peng\u003csup\u003e[\u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e89\u003c/span\u003e]\u003c/sup\u003e \u003cem\u003eet al.\u003c/em\u003e investigated the morphological evolution of IMCs in solder joints with varying Ag contents under different thermal shock durations using SEM analysis, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The results revealed that higher Ag content led to coarser IMC grains, and for all solder joints, the Cu₆Sn₅ grains coarsened while grain boundary spacing decreased with prolonged thermal exposure.For solder joints with low Ag content, the IMC grains were initially fine but coarsened rapidly under thermal cycling, accompanied by numerous grain boundary cracks and poor oxidation and crack resistance. Conversely, high-Ag-content joints exhibited coarse grains, where Ag₃Sn phases enhanced grain boundary cohesion, but excessive coarsening and brittleness caused crack concentration within IMC layers.In contrast, the medium-Ag-content solder joints achieved a balanced grain size and boundary stability, showing the slowest crack propagation rate and optimal overall reliability and mechanical performance.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe modification of Ag and Cu contents alone offers limited improvement in the overall properties of the solder. Therefore, introducing additional alloying elements has become an effective strategy to enhance the comprehensive performance of SAC solders, particularly in optimizing their mechanical properties\u003csup\u003e[\u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e90\u003c/span\u003e]\u003c/sup\u003e. Li\u003csup\u003e[\u003cspan citationid=\"CR91\" class=\"CitationRef\"\u003e91\u003c/span\u003e]\u003c/sup\u003e \u003cem\u003eet al\u003c/em\u003e. investigated the effect of Bi addition on the microstructure of SAC solder by introducing 0.5 wt% and 5 wt% Bi. The corresponding SEM microstructures are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a) and (b). The addition of 5.0 wt% Bi exhibited the most pronounced refinement effect, as the Bi content exceeded its solubility limit in Sn (approximately 4 wt%), resulting in Bi phase precipitation that refined the β-Sn dendrite size and expanded the eutectic region. At 0.5 wt% Bi, most Bi atoms formed a solid solution within the Sn matrix, whereas at 5.0 wt% Bi, the precipitation of fine Bi particles further refined the microstructure and promoted a more uniform distribution of Ag₃Sn and Cu₆Sn₅ phases. As shown in Figure. 7(c), which presents the mechanical property test results, increasing the Bi content to 5.0 wt% significantly enhanced the hardness, shear strength, and tensile strength of the solder alloy, while the elongation decreased with higher Bi content. This improvement can be attributed to the combined effects of solid-solution strengthening, \u003csup\u003e[\u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e92\u003c/span\u003e]\u003c/sup\u003e grain-boundary strengthening\u003csup\u003e[\u003cspan citationid=\"CR93\" class=\"CitationRef\"\u003e93\u003c/span\u003e]\u003c/sup\u003e and precipitation strengthening. The dominant deformation mechanism is dislocation climb controlled by self-diffusion within the Sn lattice\u003csup\u003e[\u003cspan citationid=\"CR94\" class=\"CitationRef\"\u003e94\u003c/span\u003e]\u003c/sup\u003e. The addition of Ni can further refine the microstructure of the solder alloy and effectively enhance its mechanical properties, thermal stability, and tensile creep resistance\u003csup\u003e[\u003cspan citationid=\"CR95\" class=\"CitationRef\"\u003e95\u003c/span\u003e]\u003c/sup\u003e.The addition of Ni significantly refines the alloy microstructure by promoting the formation of intermetallic compounds (IMCs) such as (Cu,Ni)₆Sn₅. This refinement results in finer β-Sn grains and transforms the eutectic regions into a fibrous morphology, thereby enhancing resistance to dislocation motion\u003csup\u003e[\u003cspan citationid=\"CR96\" class=\"CitationRef\"\u003e96\u003c/span\u003e]\u003c/sup\u003e. After the addition of Ni, the solidus temperature slightly increases, and the melting point stabilizes within the range of 228.98-230.62\u0026deg;C, which meets the process requirements for electronic packaging. The creep rate increases with rising temperature and applied stress but decreases with Ni addition. When the Ni content reaches 0.05 wt.%, the alloy exhibits the best creep resistance. This improvement is attributed to the formation of fibrous eutectic regions and fine intermetallic compounds (Ag₃Sn and (Cu,Ni)₆Sn₅), which effectively hinder dislocation motion\u003csup\u003e[\u003cspan citationid=\"CR97\" class=\"CitationRef\"\u003e97\u003c/span\u003e]\u003c/sup\u003e. El-Taher\u003csup\u003e[\u003cspan citationid=\"CR98\" class=\"CitationRef\"\u003e98\u003c/span\u003e]\u003c/sup\u003e \u003cem\u003eet al.\u003c/em\u003e introduced Ge as an alloying element to enhance the strength and ductility of Sn-based solder alloys. Experimental results show that with increasing Ge content, both the eutectic temperature and undercooling decrease, while the pasty range remains stable within 6.3\u0026ndash;6.8\u0026deg;C. The yield strength, ultimate tensile strength, and elongation of the alloy all increase with Ge addition, reaching optimal values at 0.5 wt%. This improvement is primarily attributed to grain boundary, solid-solution, and precipitation strengthening mechanisms.In the microstructure of the pure SAC solder, β-Sn serves as the matrix phase with dispersed Ag₃Sn and Cu₆Sn₅ intermetallic compounds. After Ge addition, the formation of Ge-rich phases promotes significant grain refinement β-Sn dendrites become smaller and Ag₃Sn particles are fragmented. At a Ge content of 0.5 wt%, fine Ge particles are uniformly distributed, the eutectic region expands, and Ge preferentially enriches within the β-Sn matrix, thereby enhancing both the strength and ductility of the solder alloy.\u003c/p\u003e\u003cp\u003eIn addition to the incorporation of multi-element alloying, numerous researchers and institutions have explored the addition of nanoparticles as an effective approach to enhance the overall performance of solder alloys. Ma\u003csup\u003e[\u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e99\u003c/span\u003e]\u003c/sup\u003e \u003cem\u003eet al\u003c/em\u003e. investigated the effects of adding varying concentrations of Mo nanoparticles to SAC solder alloys. The addition of Mo nanoparticles refined the microstructure, increased grain boundary length and dislocation density, thereby significantly enhancing the tensile strength of the solder joints. The pure SAC solder primarily consists of a eutectic structure composed of β-Sn, Ag₃Sn, and Cu₆Sn₅ phases. Upon the incorporation of Mo nanoparticles, the composite solder exhibits a significantly refined and more homogeneous microstructure, with uniformly dispersed Mo nanoparticles and markedly smaller β-Sn grains. This refinement is attributed to the nanoparticles\u0026rsquo; ability to impede grain boundary migration, thereby suppressing grain growth and enhancing the overall microstructural stability. As the Mo nanoparticle content increases, the tensile strength of the solder joints correspondingly improves, reaching an optimal level at 0.5-1.0 wt%. The enhancement in mechanical properties is primarily governed by the synergistic effects of grain boundary strengthening\u0026mdash;where refined grains provide more barriers to dislocation motion dislocation strengthening induced by dislocation accumulation around nanoparticles, and second-phase strengthening as Mo nanoparticles act as effective obstacles to dislocation glide. Consequently, the addition of Mo nanoparticles not only strengthens the SAC solder alloy but also contributes to improved structural uniformity and reliability, making it a promising modification strategy for advanced electronic interconnection applications. Yin\u003csup\u003e[\u003cspan citationid=\"CR100\" class=\"CitationRef\"\u003e100\u003c/span\u003e]\u003c/sup\u003e \u003cem\u003eet al\u003c/em\u003e. enhanced the wettability, interfacial intermetallic compound (IMC) growth behavior, and mechanical properties of SAC0307 solder by incorporating SiC nanoparticles. After the addition of SiC nanoparticles, the SiC particles were adsorbed onto the surfaces of Sn and Cu atoms and accumulated at the interface, thereby impeding atomic interdiffusion and reducing the growth rate of intermetallic compounds (IMCs) \u003csup\u003e[\u003cspan citationid=\"CR101\" class=\"CitationRef\"\u003e101\u003c/span\u003e]\u003c/sup\u003e. The wettability and mechanical properties of the solder gradually improved with the increasing content of SiC nanoparticles\u003csup\u003e[\u003cspan citationid=\"CR102\" class=\"CitationRef\"\u003e102\u003c/span\u003e]\u003c/sup\u003e. The strengthening mechanism lies in the fact that SiC nanoparticles reduce the surface tension between the flux and the molten solder, thereby improving wettability. Additionally, the SiC nanoparticles refine the IMC grains, and the refined IMCs together with SiC particles act as second-phase pinning sites at the grain boundaries. This pinning effect impedes grain boundary sliding, increases dislocation density, and suppresses dislocation motion, thereby strengthening the solder matrix and enhancing its mechanical properties\u003csup\u003e[\u003cspan citationid=\"CR103\" class=\"CitationRef\"\u003e103\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Conclusion and future work","content":"\u003cp\u003eMainstream solder systems such as Sn-Cu, Sn-Zn, Sn-Bi, Sn-Ag, and SAC have developed relatively mature performance optimization pathways. Through alloying element doping and nanoparticle reinforcement strategies, these systems have effectively overcome their intrinsic limitations, providing diversified and feasible alternatives to traditional Sn-Pb solders.\u003c/p\u003e\u003cp\u003eIn Sn-Cu solder systems, the addition of elements such as Bi and Cr can effectively refine β-Sn grains and Cu₆Sn₅ intermetallic compounds (IMCs), thereby enhancing tensile strength and ductility. The incorporation of nanoparticles such as Co\u0026amp;C and Nb serves as a physical barrier that hinders atomic diffusion, reduces IMC layer thickness, and improves interfacial stability\u0026mdash;enabling the solder to maintain cost advantages while ensuring basic connection reliability.\u003c/p\u003e\u003cp\u003eIn Sn-Zn solder systems, alloying with elements such as Ag, Cu, and Bi leads to the formation of AgZn₃ and Cu₅Zn₈ phases, which refine the microstructure and reduce Zn activity, achieving a balanced combination of wettability, strength, and ductility. Moreover, the introduction of nanoparticles such as Al and TiO₂ further suppresses oxidation and IMC coarsening, demonstrating significant practical potential in low-temperature packaging applications.\u003c/p\u003e\u003cp\u003eIn Sn-Bi solder systems, the addition of elements such as Ni and In enhances both strength and ductility through the formation of reinforcing phases or by suppressing Bi phase coarsening. Meanwhile, nanoparticles such as Co and TiC contribute to fine-grain strengthening and act as diffusion barriers, significantly reducing IMC growth rates and improving long-term reliability\u0026mdash;making them promising candidates for low-temperature chip packaging applications.\u003c/p\u003e\u003cp\u003eIn Sn-Ag solder systems, SAC solders are the most widely applied in electronic packaging. By reducing Ag content and introducing elements such as Bi and Ge, the microstructure can be refined and solid-solution strengthening can be achieved, thereby improving strength, ductility, and reliability while reducing production costs. Furthermore, nanoparticles such as Mo and SiC enhance grain boundary density and dislocation strengthening, further optimizing mechanical performance and achieving a favorable balance between cost and performance.\u003c/p\u003e\u003cp\u003eOverall, research on lead free solders has evolved from improving individual properties to achieving the coordinated optimization of comprehensive performance. Alloying regulates the microstructure through the synergistic effects of multiple elements, such as solid solution strengthening and grain refinement, while nanocomposite reinforcement enhances interfacial stability through particle dispersion strengthening and diffusion barrier effects. The combination of these approaches provides an effective pathway to overcome the inherent limitations of different alloy systems. Future research should focus on the quantitative analysis of multielement synergistic mechanisms, long term reliability assessment under extreme service environments such as high temperature and high humidity, and the customized development of low cost, high performance solders designed for advanced electronic packaging with high integration, wide temperature range, and extended service life.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability Statement:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe raw/processed data required to reproduce these findings cannot be shared at\u0026nbsp;this time as the data also forms part of an ongoing study. All data included in this study\u0026nbsp;are available upon request by contact with the corresponding author.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eQinrong Sun:\u0026nbsp;\u003c/strong\u003eMethodology, investigation, writing - review and editing, supervision, project management, funding acquisition. \u003cstrong\u003eXuehai Liao:\u0026nbsp;\u003c/strong\u003eMethodology, investigation, data compilation, writing - review and editing. \u003cstrong\u003eJinhong Dai:\u003c/strong\u003e survey, data compilation. \u003cstrong\u003eWei Feng:\u003c/strong\u003e Investigation, data organization, review and editing, supervision. \u003cstrong\u003eLimeng Yin:\u0026nbsp;\u003c/strong\u003ereview \u0026amp; editing, Supervision, Project administration, Funding acquisition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding: \u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (U24A20117,52175288), Natural Science Foundation of Chongqing (CSTB2023NSCQ-LZX0002),Jiangxi Key Laboratory of Forming and Joining Technology for Aerospace Components, Nanchang Hangkong University (EL202380301). Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJZD-M202401503).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests: \u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo conflict of interest exits in this manuscript, and manuscript is approved by all \u0026nbsp;authors for publication.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSonawane PD, BRVK (2019) ADVANCES IN LEAD-FREE SOLDERS. Int J Mech Eng Technol 2:520\u0026ndash;526\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDepiver JA, Mallik S, Harmanto D (2021) Solder joint failures under thermo-mechanical loading conditions \u0026ndash; A review. 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Corros Sci 224:111488. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.corsci.2023.111488\u003c/span\u003e\u003cspan address=\"10.1016/j.corsci.2023.111488\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Lead-free solder, Alloying elements, Nanoparticles, Microstructure and properties","lastPublishedDoi":"10.21203/rs.3.rs-8100930/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8100930/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWith the global enforcement of environmental regulations restricting the use of lead-based solders, the development of lead-free solder alloys for electronic packaging has become a central topic in ensuring both device reliability and sustainable manufacturing. This review summarizes the current research progress on lead-free solders for electronic packaging, focusing on the performance characteristics and modification strategies of major alloy systems such as Sn-Cu, Sn-Zn, Sn-Bi, Sn-Ag, and Sn-Ag-Cu. Although each system exhibits unique advantages, challenges such as high melting temperature, poor wettability, oxidation susceptibility, and high material cost remain unresolved. Performance optimization can be effectively achieved through alloying element doping and nanoparticle reinforcement. Alloying elements refine the microstructure via solid-solution strengthening, grain refinement, and secondary-phase strengthening while suppressing the coarsening of intermetallic compounds (IMCs). Meanwhile, nanoparticles enhance interfacial stability through dispersion strengthening and diffusion barrier effects, thereby improving wettability, mechanical strength, and long-term reliability. Finally, this paper proposes potential research directions for the next generation of lead-free solders and provides an outlook on their future applications in advanced electronic packaging.\u003c/p\u003e","manuscriptTitle":"Recent Advances in Sn-Based Lead-Free Solders for Electronic Packaging: A Review","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-01 10:09:47","doi":"10.21203/rs.3.rs-8100930/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-12-01T03:34:30+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-28T15:39:47+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2025-11-22T00:15:21+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-14T08:24:23+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2025-11-13T04:06:59+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"5a5d7048-88a4-4cf6-9d8d-d8d75d12fbb0","owner":[],"postedDate":"December 1st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-09T08:19:29+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-01 10:09:47","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8100930","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8100930","identity":"rs-8100930","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}