Design Optimization for Passivation Crack Improvement in Power Devices

The design of power packages with both high efficiency and high-power density performance while maintaining the highest possible reliability is a challenge. Silicon nitride is normally coated on aluminum for protection from corrosion and moisture penetration during handling process. However, high mismatch of thermal expansion between nitride and aluminum can lead to nitride crack for power devices during Thermal Cycling Test (TCT), which can further penetrate to function area(s) to trigger device failure. Therefore, a well understanding of factors’ effect on nitride crack will help to drive improvements on design to eliminate nitride crack.In this study, nitride crack was studied by simulation and experimental observation for a power device during TCT. Different factors’ effect on nitride stress, including distance of aluminum edge to die edge, die size, die thickness and CTE of Epoxy Molding Compound (EMC) are investigated. Simulation data show nitride stress can be reduced by long distance of aluminum edge to die edge, thin die and low CTE of EMC during TCT. Based on simulation data, directions for nitride crack improvement were provided for robust power device design Experiments were thereby designed with different die structures. and different levels of nitride cracks are found during TCT, which validated the simulation data.