| Abstract Scope |
Using CALPHAD-assisted phase fraction calculations, Sn-1Ag-0.7Cu-5Bi-xIn (x = 4, 8, 15, 17 wt.%, denoted as xIn) alloys were designed to meet long-term high-temperature service requirements in solder joints. Bi and In were added to offset performance losses from reducing Ag content. With Bi fixed at 5 wt.%, this study investigated the effects of varying In content on melting behavior, microstructure, mechanical properties, and fracture mechanisms. Results showed that increasing In eliminated Bi-rich particles, and at ≥15 wt.% In, InSn₄ phases appeared. The interfacial IMC transformed from Cu₆Sn₅ to Cu₆(Sn,In)₅, with InSn₄ and secondary phases increasing with In content. During isothermal aging at 170 °C, 4In joints formed Cu-rich layers causing stress concentration and interfacial cracking. In 8In joints, Cu and In diffusion converted Cu₆(Sn,In)₅ to Cu₃(Sn,In), generating residual stresses. Higher In contents lowered melting points, accelerated diffusion, and stabilized Cu₆(Sn,In)₅, improving reliability. These results inform interfacial IMC control and joint design. |