The technology of high power IGBT modules has been improved significantly these last years against thermal fatigue and the first weaknesses related to the bonding die attach have been well enough corrected. Nowadays, the most frequently observed failure mode is the solder layer cracks between copper base plate material and the ceramic. Experimental tests must be completed by numerical simulation tools in order to analyze this type of failure related to power cycling constraints. Thermal simulations of high power IGBT modules based on the boundary element method are described in this paper. A validation of the numerical tool is shown in steady-state and dynamic operations during a power cycle by comparison with experimental measurements. Finally, using the software, a model of solder layer cracks between copper base plate and the DCB ceramic is applied in order to investigate its effect on the thermal constraints.