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Thermal analysis of power cycling effects on high power IGBT modules by the boundary element method

The technology of high power IGBT modules has been significantly improved in the last few years against thermal fatigue. The most frequently observed failure mode, due to thermal fatigue, is solder cracks between the copper base plate and the DCB (direct copper bonding) substrate. Specific simulation tools are needed to carry out reliability research and to develop device lifetime models. In other respects, accurate temperature and flux distributions are essential when computing thermomechanical 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 submitted to power cycling constraints. 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 shows the effect of the IGBT silicon chip position on the DCB substrate on the thermal constraints. In particular, a slight shifting of the silicon chips may be sufficient to delay significantly the initiation and propagation of the cracks, allowing a higher device lifetime for the module.