A Non-destructive Condition Assessment Method for DBC Substrate: Dielectric Measurement

Abstract

Direct bonding copper (DBC) substrate is employed in high power density modules, improving its reliability is of critical significance for the safety of power electronic system. In this paper, thermal cycling ageing from -55°C to 150°C was conducted on alumina DBC substrate. Both the variation of interface structures and dielectric properties were obtained in the failure process. Based on the experimental results, the substrate damaged at ~70 cycles due to the thermal expansion coefficient (CTE) mismatch between copper and ceramic. After 45 cycles, copper/ceramic interface turned rougher. Equivalent capacitance (Ceq) was found to be almost unchanged at ~115.8 pF. While tanδ increased from 6.2×10−4 to 1.3×10−3 since the initiation of micro cracks. Afterwards, dramatic drop of Ceq and tanδ was noticed, with Ceq decreased from 115.8 pF to 83.0 pF and tanδ declined to 4.5×10−4. Besides, cliff shaped structure was found around the crack point. It is convinced that, propagation of micro cracks broadened the air gap between copper and ceramic, finally diminished the Ceq. Obviously, the turning point of Ceq is an important indicator in thermal cycling fatigue. Dielectric measurement might be an effective non-destructive technique for condition assessment of DBC substrate.