Effects of strain rate and aging on deformation and fracture of Sn-Ag-Cu solder joints

With the proliferation of mobile electronics in everyday life, the ability of electronic packages to sustain impact loading under drop conditions has become a paramount reliability concern. Since solder joints, which serve as mechanical and electrical interconnects in a package, are particularly prone to failure during a drop, the fracture behavior of solders at high strain rates is a critically important design parameter. In the present study, tests were conducted on Sn3.5AgO.7Cu (SAC 387) alloy at various strain rates in the range of 0.1 - 100 s^-1 and in various modes. Yield strength and work hardening were observed to increase substantially with strain rate. The yield strength and work hardening for aged samples were lower than as reflowed samples. Associated with the alteration of flow behavior, transitions in deformation and fracture behavior were noted. Fracture toughness (Gc) decreased with increasing strain rate, and also with increasing mode-mixity (Psi) values between 0-23deg. For low Psi, severe aging appeared to increase fracture toughness, although at higher Psi values, the distinction appeared to disappear. Correlations between joint microstructure and the observed fracture mechanism are highlighted.