Effects of aging on shear cyclic stress strain and fatigue behaviors of lead free solder joints

Solder materials in electronic packages are often subjected to thermal cycling, either during their application in products or during accelerated life qualification testing. Cyclic temperatures cause the solder joints to be subjected to cyclic mechanical stresses and strains due to the mismatches in the thermal expansion coefficients of the assembly materials. Such loads lead to thermomechanical fatigue that results in damage accumulation, crack initiation and propagation, and eventual failure. The authors have extensively examined the effects of isothermal aging on solder stress-strain and creep (constitutive) behavior. However, there have been no prior investigations on aging effects on solder fatigue behavior. Aging leads to both grain and phase coarsening, and can cause recrystallization at Sn grain boundaries. Such changes are closely tied to the damage that occurs during cyclic mechanical loading.