Influence of interfacial interaction on the reliability of the bond between encapsulation epoxy and copper substrate

The need for power modules has been promoted by the emergence of electric vehicles. The requirements of small volume, high integrity, and high reliability for the next-generation power module lead to the change of encapsulation method from previous gel encapsulation to epoxy encapsulation. Efforts have been made to enhance epoxy materials, and commercialized high-end epoxy has shown excellent properties, but power module failures still exist. The possible reason may lie in the interfacial interaction between encapsulation epoxy and substrate, which is less noticed previously. This paper seeks to unveil the influence of interfacial interaction on the bonding reliability between encapsulation epoxy and copper substrate. Three reliability tests were performed: high-temperature storage test (HST), temperature cycling test (TCT), and pressure cooker test (PCT). The bonding strength after these reliability tests was evaluated, and the bonding interface and fracture surface were analyzed in a nanoscale. Our study discovered that interfacial interaction plays a vital role in encapsulation reliability. Copper can diffuse into the epoxy and catalyze epoxy thermal oxidative degradation, which results in bonding strength reduction after HST and PCT. The surface structure change caused by copper oxidation after HST and PCT also reduced the bonding strength. These findings will greatly benefit future power module design.