Modeling and Optimization of Thermal Cycling Performance to Reduce Ratcheting-Induced Passivation Cracking in High-Voltage Power Modules

Passivation thin film overlaying a metal layer is a typical structure in power electronics. When a package is subject to thermal cycling conditions, plastic strains could accumulate in the metals after each cycle, eventually causing stress relaxation and collapse of metals. The overlaying passivation could crack because of the metal failure, which is a phenomenon known as metal ratcheting induced cracking. In this work, three-dimensional finite element analysis is used to investigate the ratcheting risk of a power electronic package. To select a proper criterion, we first studied the effect of a buffer layer that is added between passivation and epoxy mold compound to lower the interface stress and thus metal ratcheting. While the buffer layer does not reduce the passivation stress, it gives lower plastic strain in the metals based on the simulation. In addition, Cu thin film is found to produce higher passivation stress but much lower plastic strains than Al thin film. Therefore, the incremental plastic work per thermal cycle is selected as the criterion, with which we further optimized the package's performance in ANSYS (R) optiSLang. Sensitivity analysis was run using a total of five parameters and 100 DoEs. An approximation model with high accuracy is obtained from the sensitivity analysis. Optimization is then performed upon the model without running the actual finite element simulation. An optimized solution is obtained and validated with finite element analysis. The best design indicates that larger buffer layer thickness and greater metal-edge-to-die distance would give a lower risk of metal ratcheting, which is consistent with the internal test data.