A Stress-Relieved Method Based on Bottom Pattern Design Considering Thermal and Mechanical Behavior of DBC Substrate

Abstract

Traditionally, reliability-improved methods like etched dimples are set at the edge of the copper pattern to relieve the thermal-mechanical stress on direct bonded copper (DBC). When the copper ratio of the top copper pattern is less than the copper ratio of the bottom copper pattern, these methods are less effective at the inner edges of the top ceramic interface where the initial crack appears. Currently, no explanation is found for this phenomenon and other solutions should be taken to this problem. Hence, this paper analyzed the mechanism of the phenomenon and proposed a bottom pattern design to solve the problem. Based on a bi-material model and FE analysis, it can be found that the stress concentration on the top ceramic interface is caused by the opposite bending actions on the two ceramic interfaces. To eliminate the geometrical difference between two copper layers, Parallel dimple traces are used on the bottom instead of mirror-copied gap on the consideration of thermal resistance and mechanical strength. The parameters of dimple trace were chosen by simulations on stress-strain results under thermal cycling and thermal resistance calculations in a module packaging. FE results show that the bottom dimple trace with a diameter of 0.4mm and a gap of 2mm can adjust the thermal-induced strain behavior of the singularities at the top copper to be even and sacrifices only a 2% thermal resistance rise of the module packaging. Life prediction model based on strain shows that the bottom design can improve the life time of the substrate. The thermal cycling test result on sample DBC and thermal resistance measurement on module packaged by DBC with bottom pattern design verify the FE analysis. This method can be widely used on the packaging design of power module using stand-alone DBC substrate.