Aluminum Pad Plasticity-Related Bump Failure During Temperature Cycling

Temperature cycling has been used as a reliability test for electronic products. Due to large thermal mismatch between silicon chip and substrate, stress is developed at the bump interconnection during temperature cycling. Multiple bump failure modes have been observed during temperature cycling tests, such as aluminum pad (AP) crack, polyimide (PI) crack and delamination, silicon passivation crack and delamination, AP/under bump metallization (UBM) interfacial delamination, etc. It is important to understand the failure mechanisms underlying these failure modes. In this study, finite element models have been developed to understand the potential failure mechanisms of these failure modes. It is shown that the failure modes are related to aluminum plasticity. A mechanistic model is proposed in this study to expose the impact of aluminum plasticity on the stress increase in the overlaying layers on the aluminum pad. It demonstrates that the stress increase within the overlaying layer, such as silicon passivation, was caused due to plasticity of aluminum pad. Thus, the plasticity effect of aluminum pad must be considered to capture the failure mode related to the overlaying layers. A three-dimensional model was further developed to evaluate the failure metrics associated to different failure modes. Simulation analysis also evaluates the impact of copper pillar height, AP size, and PI opening (PIO) on these failure modes. The results help to optimize design factors to mitigate the bump failure risk.