Effects of Aging on the Cyclic Stress Strain and Fatigue Behaviors of Lead-free Solders

Abstract

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. While the effects of aging on solder constitutive behavior (stress-strain and creep) have been examined in some detail, there have been no 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, the effects of aging on the cyclic stress-strain and fatigue behaviors of lead free solders have been investigated. Solder test specimens (SAC105 and SAC305) have been prepared and subjected to cyclic stress/strain loading for various prior 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 can be 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 drop 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 aging decreases the mechanical fatigue life, and the effects of aging on the peak load drop have been studied. It has also been observed 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). From the recorded cyclic stress-strain curves, the evolution of the solder hysteresis loops with aging have been characterized and empirically modeled. Similar to solder stress-strain and creep behavior, there were strong effects of aging on the hysteresis loop size. Comparisons have been made between isothermal uniaxial fatigue life and shear fatigue life and good agreement has been found. A microstructural adaptive fatigue model has been proposed that includes aging effects via consideration of grain/sub-grain coarsening, recrystallization, and IMC particle growth.