In avionic area, the trend is to a growing use of power electronics systems. In this context, we will present results concerning test vehicles (power modules) in order to improve a device assembly technology dedicated to be located "on the engine", in harsh operating conditions with thermal cycling from -50°C to 200°C in the worst case. The paper focuses on failure modes observed under high operating temperature and high temperature cycles, especially ceramic cracks, conchoidal fracture under the copper lead-frame and solder crack initiation. Numerical thermo-mechanical simulations of the assembly allow to understand the origin of the failures and to improve the assembly lifetime under such severe operating conditions. The objectives of the study are to assess the lifetime behaviour of these modules towards both high temperature and thermal cycling effects.