Confrontation of failure mechanisms observed during Active Power Cycling tests with finite element analyze performed on a MOSFET power module

Abstract

A new type of assembly and interconnection technology in power modules has been developed to connect MOSFETs. These power modules, used as frequency inverters for electric feature, have an innovative design. They avoid using aluminum wire bond, often to be blamed for device failure, by using a copper clip soldered on the top side of the chip. The successful design for increased reliability of this electronic package depends on better understanding and modeling its fatigue behavior and its failure mechanisms. During Active Power Cycling tests, the chip acts as a heat source and temperature gradients develop in the package causing expansion of the different materials. To assess the reliability of those devices under thermal power cycling loads, both experiments and simulations have to be performed. Some failures were already observed after Active Power Cycling tests, but they do not correspond to failures usually observed in standard MOSFET packages, and thus are not thoroughly understood. For instance, the formation of a wave in the aluminum metallization layer, on top of the chip, caused by a high deformation grade, was never described. High operating life of more than 1 million cycles can be achieved by optimizing clip geometry and thicknesses of metal layers. Such packages are then clearly more robust compared to those using wire bond technology. In this paper, failures observed via testing are confronted with thermal and mechanical stresses distribution computed by Finite Element Analysis in order to improve the understanding of failure formation mechanisms. A 2D Finite Element model of MOSFET packages is used to analyze mechanical stresses induced by thermal loads. Simulations help in determining critical areas and then in improving the design of modules.