The role of stress state and stress triaxiality in lifetime prediction of solder joints in different packages utilized in automotive electronics

This work presents an overview on the role of the stress state and stress Triaxiality Factor (TF, see Eq.1) in lifetime prediction of solder connections. According to various literature sources, the TF is one of the most important factors influencing initiation of ductile fracture [5, 16]. It is widely reported that lifetime of the ductile materials decreases under hydrostatic tension when combined with high TF-values. Recent investigations report that the compressive hydrostatic stress state combined with a high shearing load and low (or even zero) TF-values also contributes to failure [4,5]. Two package types, the Loss Free Packaging (LFPAK) and the Plastic Ball Grid Array (PBGA), were investigated by means of FE-simulation on Board- and System-Level, and presented damage prediction will be compared with experimental data. In the LFPAK and BGA solder joints the regimes of hydrostatic tension and compression during temperature cycles are evaluated and compared with distribution of equivalent von Mises stress, stress intensity (maximum shear stress) and triaxiality. The increments of damage related variables, inelastic strain and energy density, were modified in a postprocessing routine according to the current state of hydrostatic stress and TF for each time increment. Further, using a simplified simulation approach, the path of the crack propagation was calculated according to the distribution of the modified and non-modified inelastic strain. It is shown that when including multiaxial effects by modification of damage related variables a better correlation between calculated and experimentally observed crack path is achieved.