Power devices have to withstand fast thermal cycling in automotive applications. In order to guarantee the reliability in these applications, a detailed understanding of the degradation mechanisms is required. One of these mechanisms is inter-layer-dielectric cracking, caused by progressive plastic deformation of metal lines. In a previous publication we have shown that DMOS transistors, with different active areas, which operate in similar conditions, have dissimilar reliabilities. This cannot be explained by state-of-the-art methods, which estimate reliability from the peak junction temperature. In this paper, extended measurement results of a DMOS transistor will be analyzed with the aid of electro-thermal and thermo-mechanical simulations and a new approach for reliability estimation will be proposed.