Evolution of the cyclic stress-strain and constitutive behaviors of SAC305 lead free solder during fatigue testing

When exposed to a temperature changing environment, solder joints in electronic assemblies are subjected to cyclic thermal-mechanical loading due to the mismatches in coefficients of thermal expansion (CTE) of the different assembly materials. Eventually, the cyclic loading can result in fatigue failure of solder joints, which is one of the common failure modes in electronic packaging. While it has been known that the reversal of inelastic strain can change the stress-strain behavior of materials (Bauschinger effect), there have been few prior studies on how the cycling changes the microstructure and degrades the mechanical properties of lead free solders during fatigue testing. In this investigation, we have explored the effects of mechanical cycling on the cyclic stress-strain behavior (hysteresis loop area, plastic strain range, and peak stress) and on the constitutive behavior (stress-strain and creep) of SAC305 lead free solder in fatigue testing. At the same time, effects of cycling on solder microstructure have been studied. The goal of the study was to explore the damage accumulation that occurs during fatigue testing.