Electronic power devices used for transportation applications (automotive and avionics) experience severe temperature variations which promote their thermal fatigue and failure. For example, for power modules mounted on the engine of an aircraft, temperature variations range from -55°C (in the worst case of storage before takeoff) to +200°C (flight). The studied modules are composed of direct bonded copper (DBC) substrates which allow isolating the active parts of the module (silicon dies) from their base plates. The failure occurs in DBC substrates, which are copper/ceramic/copper sandwiches. The Weibull approach was used to model the brittle fracture of the ceramic layer from a natural defect. Besides, geometric singularities in the upper ceramic/copper interface are at the origin of cracks, which grow by fatigue and finally bifurcate and break the ceramic layer. Using the finite element method, it was possible to analyse how a thermal loading history may modify the risk of failure of the DBC substrate. It was shown, in particular, that three overcooling cycles should produce an "overload retardation effect". Experimentally, applying 3 "overload cycles" (-70°C,+180°C), before applying usual thermal fatigue cycles (-30°C, +180°C). increased very significantly the fatigue life of a set power modules. This result shows that the fatigue life and the reliability of power electronic devices could be optimized using a thermo-mechanical approach of the problem and suitable failure.