Critical evaluation of creep behavior of Sn-Ag-Cu solder alloys over wide range of temperatures

A critical survey of the current literature on creep behavior of Pb-free Sn based Sn-Ag-Cu (SAC) alloys show that there is considerable discrepancy in the creep parameters, such as activation energy, Qc, and stress exponent, n. Accordingly, this study addresses the underlying reasons for such variability in the existing data by studying the creep behavior in compression of two most commonly used SAC alloys, namely Sn-1 wt.%Ag-0.5 wt.%Cu and Sn-3 wt.% Ag-0.5 wt.% Cu, over wide range of temperatures, from 60 to 200 °C, and stresses from 7 to 14 MPa. Analysis of microstructure of SAC alloys reveals that both temperature and stress have pronounced effects on coarsening of precipitates during creep. The steady state strain rate increases with both temperature and stress as well as with decrease in Ag content of the alloys. Irrespective of the composition, SAC alloys show a transition in the creep behavior around 150 °C, with Qc and n changing from 55 kJ/mol and 7, respectively, at low temperatures to 100 kJ/mol and 5, respectively, at higher temperatures. Such a simultaneous change in both Qc and n suggests that creep mechanism in SAC alloys is dislocation climb controlled by the core diffusion at low temperatures and the lattice diffusion at high temperatures.