High-Temperature Micropillar Compression Creep Testing of Constituent Phases in Lead-Free Solder

As the size of solder interconnects used in electronics packaging decreases, methods for characterizing the creep behavior of solder at ever smaller length scales need to be developed. Long duration micropillar compression experiments at constant loads and temperatures ranging from 60 to 160°C were used to characterize the creep behavior in pure Sn dendrites and the Sn-Ag₃Sn eutectic constituent in a lead-free Sn-3.5Ag solder alloy. The stress exponent for the Sn dendtrites as well as the eutectic at low stresses was shown to be 1, suggesting a diffusion-based mechanism, while at high stresses the eutectic stress exponent was 4, suggesting a dislocation-based mechanism. Low activation energies indicative of a high diffusivity path (around 30 kJ mol^-1) were seen in the dendrites at all temperatures and stresses as well as in the eutectic at low stresses. In the high stress regime of the eutectic, the activation energy transitions from 36 to 97 kJ mol^-1 above 120°C, indicating a transition to a lattice diffusion-based mechanism.