Boundary element analysis of thermal fatigue effects on high power IGBT modules

The technology of high power IGBT modules has been significantly improved these last years against thermal fatigue. The most frequently observed failure modes, due to thermal fatigue, are the solder cracks between the copper base plate and the direct copper bonding (DCB) substrate and bond wire lift-off. Specific simulation tools are needed to carry out reliability researches and to develop device lifetime models. In other respects, accurate temperature and flux distributions are essential when computing thermo-mechanical stresses in order to assess the lifetime of high power modules in real operating conditions. This study presents an analysis method based on the boundary element method (BEM) to investigate thermal behavior of high power semiconductor packages subjected to power cycling loads. The paper describes the boundary integral equation which has been solved using the BEM and applied to the case of a high power IGBT module package (3.3 kV–1.2 kA). A validation of the numerical tool is presented by comparison with experimental measurements. Finally, the paper points out the effect on the thermal stress of the IGBT chips position on the DCB substrate. In particular, a light shifting of the silicon chips may be sufficient to delay significantly the initiation and the propagation of the cracks, allowing a higher device lifetime of the studied module.