Micromechanical Model for Describing Intergranular Fatigue Cracking in an Innovative Solder Alloy

Fatigue of solder joints remains one of the critical concerns in thermo-mechanical reliability of high-power electronic systems. Several semi-empirical fatigue models based on effective material properties at macro-scale already exist, but have shown some limitations for providing accurate lifetime prediction of solder joints in the scale of microelectronic packages. Therefore, there is a need to enrich the existing approaches by a description of the failure mechanisms at the microstructure scale, taking into account some important features of the alloy. In this study, a 3D microstructure-informed model for reproducing the intergranular fatigue crack in the solder joint is developed. The submodeling technique has been applied in order to only investigate the critical zone of the solder joint. A global model of the whole module is first simulated to obtain the inputs for a submodel focused on the zone of interest where failure is expected to develop. The submodel simultaneously makes use of the cohesive zone and the crystal plasticity theories to represent decohesion at grain boundaries and plastic slips in the grains of the solder joint, respectively. Simulations of repeated thermo-mechanical loading on the package demonstrate how cracking occurs at grain boundaries in the solder joint of the submodel. In addition, it is shown that the crack propagation rate is almost constant during the whole loading time. This suggests an ability of the present approach to give a fatigue lifetime estimate for the entire solder joint by extrapolating some specific computed quantities from the local model.