Aging induced evolution of the cyclic stress-strain behavior of lead free solders

Solder joints in electronic assemblies are often subjected to cyclic (positive/negative) mechanical strains and stresses. Such exposures can occur in variable temperature application environments or during accelerated life thermal cycling tests used for qualification. Cyclic loading leads to damage accumulation, crack initiation, crack propagation, and eventually to fatigue failure. On the microscopic level, aging causes both grain and phase coarsening, and leads to recrystallization at Sn grain boundaries. These changes of the solder microstructure are closely tied to the damage that occurs during cyclic mechanical loading. In this investigation, we have explored the effects of aging on the cyclic stress-strain and fatigue behavior of lead free solders. At the same time, changes of the solder microstructure caused by aging have been studied. Cylindrical uniaxial lead free solder test specimens (SAC305 and SAC405) have been prepared and subjected to cyclic stress/strain loading for different aging conditions. Prior to testing, the specimens were aged (preconditioned) at 125 °C for various aging times, and then the samples were subjected to cyclic loading at room temperature (25 °C). It has been observed that aging leads to the microstructural coarsening and degrades the mechanical fatigue properties, and those degradations are much more significant at the first few days of aging. From the recorded cyclic stress-strain curves, the evolution of the solder hysteresis loop, plastic strain range, and peak load with aging have been characterized and empirically modeled. Either the loop area or the plastic strain range is often considered to be the fatigue damage driving force and used in fatigue life prediction models. Similar to solder stress-strain and creep behavior, there is a strong effect of aging on the cyclic stress-strain and fatigue behavior of the solder specimens.