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The on-state resistance of power MOSFET devices tasked to perform repetitive avalanche operations is subject to modifications caused by the growth of voids and cracks in the source metallization. Endurance tests are the traditional way to monitor these changes in order to assess device reliability. However, they are very time intensive, requiring even months of uninterrupted operations. An interesting alternative is assessment of reliability through a suitable model, but no standard techniques have been developed up until now to accomplish this task. A possible approach, dynamic analysis of the temperature distribution over the source metal, is presented in this paper. Coupling the results of the thermodynamic analysis with a reliability model, and based on the Coffin-Manson law, device degradation over time can be estimated and the level of reliability can be evaluated. The consistence of the obtained reliability prediction is confirmed by comparison with endurance test results. The described approach can be usefully applied to assess the reliability of MOSFETs in a large set of applications in the automotive field.