Influence of Ag and Zn on the Microstructure and Properties of Sn-40 Bi Alloy for Improved Soldering Applications

In this study, the effect of adding 2 wt % of Ag and Zn on the microstructural characteristics and properties of Sn–40 Bi alloy (SB40), focusing on mechanical, thermal, and soldering behaviors, was investigated. Microstructural analyses revealed Sn- and Bi-rich phases in SB40, SBA402, and SBZ402. Notably, SBZ402 exhibited a needle-like Zn-rich phase, while SBA402 contained a small amount of Ag3Sn. SBA402 exhibited the highest tensile strength of 78.1 MPa, which can be attributed to its finer microstructure and solid solution strengthening. Ag and Zn additives can lower the liquidus onset temperature of SB40, thereby influencing its soldering characteristics. Interfacial reactions showed that SBA402/Cu had a uniform and very thin interlayer of Cu6Sn5(Ag) ideal for soldering, in contrast to the interfacial voids in SBZ402/Cu. Phase diagram calculations elucidated the microstructural and thermal properties and the formation of interfacial intermetallic compound (IMC). These findings suggest that SBA402 can be favorably used in soldering applications.