The effects of aging on the fatigue life of lead free solders

Solder joints in electronic assemblies are typically subjected to thermal cycling, either in actual application or in accelerated life testing used for qualification. Mismatches in the thermal expansion coefficients of the assembly materials leads to the solder joints being subjected to cyclic (positive/negative) mechanical strains and stresses. This cyclic loading leads to thermomechanical fatigue damage that involves damage accumulation, crack initiation, crack propagation, and failure. While the effects of isothermal aging on solder constitutive behavior (stress-strain and creep) have been examined in some detail, there have been few prior studies on the effects of aging on solder failure and 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. In this investigation, we have examined the effects of aging on the cyclic stress-strain and fatigue behaviors of lead free solders. Solder test specimens (SAC105 and SAC305) have been prepared and subjected to cyclic stress/strain loading at different aging conditions. Both uniaxial specimens subjected to cyclic tension/compression and Iosipescu lap shear samples subjected to cyclic positive/negative shear have been studied. A four-parameter hyperbolic tangent empirical model has been used to fit the entire cyclic stress-strain curve and the hysteresis loop size (area) was calculated using definite integration for a given strain limit. This area represents the energy dissipated per cycle, which is correlated to the damage accumulation in the joint. Samples were subjected to cyclic loading over a particular strain range until fatigue failure occurred, and then various popular empirical failure criteria such as the Coffin-Manson model and the Morrow model have been used to estimate the fatigue life. Fatigue failure was defined to occur when there was a 50% peak load - rop during mechanical cycling. 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 prior aging dramatically decreases the mechanical fatigue life. It was also found that degradations in the fatigue/failure behavior of the lead free solders with aging are highly accelerated for lower silver content alloys (e.g., SAC105). Comparisons have been made between the fatigue lives under both uniaxial tension/compression and shear loadings, and good agreement was found. A microstructural adaptive fatigue model including aging effects has been proposed, and shown to accurately predict the measured fatigue data for all aging conditions.