Modeling of board level solder joint reliability under mechanical drop test with the consideration of plastic strain hardening of lead-free solder

In the present study, a novel computational model is proposed to investigate the plastic strain hardening effect of lead-free solder joints in PCB assembly subject to mechanical drops. The test condition is based on JESD22-B111. The dynamic loading is applied using the popular input-G method. The strain hardening properties of various lead-free solders are experimentally characterized and input to the computational model. The simulation results show that the peeling force between the bulk solder and the intermetallic compound (IMC) increases incrementally with repetitive drops. This elevated peeling force is crucial to the solder joint reliability because the gradual increases in the transmitted loading will eventually exceed the fracture strength of the IMC layer. This may explain why the brittle fracture of solder joints occurs after a number of repetitive drops. In addition to the peeling force, the different solder joint mechanical behaviors under drop test such as relative displacement time history, stress/strain distribution are also included in this study. The effects of different parameters such as drop condition and IMC thickness (thermal aging effect) are also evaluated. The results of this study do not only provide a better understanding in the failure mechanism, but also give a fundamental insight of the root cause of failure under repetitive mechanical drop loading.