Fracture mechanical modeling for the stress analysis of DBC ceramics

Nowadays, the progress in power electronics requires the improvement of the reliability of DBC ceramics. The well-documented phenomenon of conchoidal cracking initiates failures at the metallization-ceramic interface. It is a result of the CTE mismatch between metallization and ceramics. Thermal cycling stresses lead to crack propagation which can consequently lead to failure in power devices due to diminished heat dissipation. In this paper, a novel concept was used in order to analyze the thermo-mechanical stresses in DBC ceramics under passive thermal cycling conditions by combining the Finite Element Method and fracture mechanics. Fracture mechanical parameters such as stress intensity factors and the J-integral were calculated with regard to the variation of the dimple depth, the topology of the etched metal edge and the ceramic thickness. Furthermore, this concept was applied to optimize the edge geometry of the metallization with the criterion of stress reduction at the metal-ceramic interface. The concept to minimize local stresses as a basis for reliability improvement will have to be validated experimentally. By this methodology, improvements in substrate technology for future power electronic assembly are made possible. The principle of this study presented here is the basis for a future lifetime prediction.