Considerations in Solution Stabilization for Thermal Fatigue Modeling of Lead-free Solder Joints

The acquisition of a "stabilized" solution is necessary for the consistent evaluation of the solder joint thermal fatigue damage by the common finite element (FE) based modeling approaches. Yet, there are no universal guidelines developed for this purpose. The present study aims to provide insight into this subject. A 1-D analytical model is firstly developed for this study. The balance between the "forward creep strain" and the "backward creep strain" is observed to be the driving force that leads to solution stabilization of the 1-D model. Considerations are then made for the 2-D and 3-D cases. It is found that there is discrepancy in the solution behavior between the 1-D and the multi-dimensional situations. Accordingly, the von Mises creep strain may not appropriately reflect the stabilization status of the multidimensional models. The von Mises stress, therefore, is proposed to serve the purpose instead. A set of stabilization criteria is developed eventually. These criteria are expected to be universally applicable to all 2-D and 3-D solder joint thermal fatigue problems. As a demonstration, the criteria are implemented on a 2-D model for which the solder joint thermal fatigue damage of a custom-made assembly is evaluated. All analyses presented in this paper are based on the SnAgCu (SAC) lead-free solder.