This study examines the effect of cyclic damage on the constitutive response and microstructural evolution of SAC305 solder. Cyclic damage is induced through isothermal, mechanical cycling tests at high strain rate and room temperature, using modified lap shear microscale specimens (180μm wide solder joint). The properties of interest are elastic, plastic, yield, and viscoplastic material constitutive behavior. In the current study, creep strain accumulation is accommodated when determining the constants, unlike those reported in prior studies . Insights into the evolution of the measured properties are provided by correlating previously reported microstructural grain evolution of microscale SAC305 solder as a function of cyclic damage [2, 3].
The hysteresis response and the elastic, plastic and yield measurements from the initial cycles show significant piece-to-piece variability (similar to prior virgin state viscoplastic measurements ). The scatter arises since as-reflowed SAC solder joints at length scales of 200μm consist of only a few anisotropic Sn grains that make the joint mechanically inhomogeneous. However, when subject to mechanical cycling fatigue at room temperature these joints undergo grain homogenization due to recrystallization, which is a possible explanation to the drop in scatter with progressing damage. The observed grain evolution is similar to that seen in solder joints under life-cycle loading.
The elastic-plastic response and yield strength of SAC305 solder do not show significant contribution from creep deformations at the chosen load levels. The properties degrade with increasing accumulated cyclic damage, at a rate that is proportional to the severity of the cyclic load. The yield stress measurements suggest that SAC305 obeys an independent hardening rule, rather than isotropic or kinematic hardening. The performance of a continuum damage mechanics based model from prior studies in representing the measured degradation in elastic, plastic and yield properties is discussed [4,5].
Comparison of the creep behavior of cycled SAC305 specimens (at 50% load drop) with that of uncycled specimens shows that the effective creep compliance and effective secondary creep strain rate increase significantly. As a point of comparison, the creep resistance of cycled SAC305 specimens is even lower than that of as-reflowed Sn37Pb specimens. Similar changes are seen in the stress relaxation behavior. Challenges and limitations of the current studies are included.