Solder joints in electronic packages often experience fatigue failures due to cyclic mechanical stresses and strains in fluctuating temperature environments. These stresses and strains are induced by mismatches in coefficients of thermal expansion, and lead to damage accumulation that contributes to crack initiation, crack propagation, and eventually to failure. In this study, we have tried to compare the effects of elevated mechanical cycling on SAC305 and SAC+Bi (SAC_Q). Initially, small uniaxial cylindrical samples of both alloys were prepared and reflowed in a reflow oven. These samples were then mechanically cycled for various durations at testing temperatures of 100 °C. The measured cyclic stress-strain curves were used to characterize the evolution of the hysteresis loop properties (peak stress, hysteresis loop area, and plastic strain range) with high temperature mechanical cycling. In addition, uniaxial tensile tests and creep tests were also conducted on specimens that had been previously mechanically cycled for various durations (e.g 0, 50, 100, 200, and 300 cycles) at an elevated temperature. This allowed us to study the evolution of the constitutive behavior of the solder alloys that occurred during the high temperature mechanical cycling due to the fatigue damage that builds up in the specimens. The reductions in the properties that occur during high temperature mechanical cycling were also correlated with the corresponding changes in the microstructure of the specimens. Rectangular cross-sectioned samples of the two lead free solder alloys were polished and selected regions indented to track the changes in the microstructure of a fixed region with mechanical cycling at T = 100 °C. Using the results of this study, we are working to develop better fatigue criteria for lead free solders which are subjected to variable temperature applications.