Effect of trace Ge on interfacial reaction and shear strength of Sn-0.7Cu solder joints during isothermal aging and thermal cycling

To improve the interfacial and mechanical properties of Sn-0.7Cu solder, we tried to add minor Ge into it and investigated the interfacial behavior and mechanical properties using ball solder joints during high-temperature isothermal aging and thermal cycling. After soldering, Ge addition promoted the formation of SnGe solid solution, improved the shear strength of solder joints, and also decreased the thickness of interfacial intermetallic compounds (IMCs) in solder joints. After the following isothermal aging, the evolution on IMC thickness showed that the IMC growth followed linear with square root of aging time, and Ge addition slowed down the diffusion rates of Sn and Cu atoms and the growth of interfacial Cu 6 Sn 5 and Cu 3 Sn IMCs. During thermal cycling, the addition of Ge also improved the shear strength and reliability of solder joints by inhibiting the growth of interfacial IMCs. Furthermore, the growth rate of IMCs during thermal cycling was much slower than that in isothermal aging. We defined the difference between them with factor K because it can be attributed to the fact that the activation of Cu and Sn atoms was difficult enough to make the diffusion impossible in the low-temperature stage of thermal cycling.