Creep behavior of Sn-Bi solder alloys at elevated temperatures studied by nanoindentation

The creep mechanisms of eutectic Sn-Bi alloy were evaluated with indentation constant strain rate (CSR) method at elevated temperatures. The activation energy (Q) and creep stress exponent (n) of eutectic Sn-Bi alloy and other alloy compositions were measured in the temperature range from 25 to 100 A degrees C. Prior to this, the indentation CSR testing protocol for evaluation of Q and n was validated through evaluating the pure Sn (grain size > 100 A mu m) at various temperatures. The creep mechanism of large grain-sized Sn was found to be dislocation climb controlled by core diffusion in bulk. Dislocation climb though core diffusion and power-law breakdown were suggested to be the deformation mechanism for pure Bi and Sn-3%Bi alloy, respectively. For the two-phased eutectic Sn-Bi alloy, the creep mechanism was found to be strain rate and temperature dependent. Individual constituent phases were found to take turns to dominate the creep rate at different strain rates.