A reduced weakly-coupled thermo-mechanical model based on the proper generalized decomposition method was developed for the numerical analysis of power modules. The employed model reduction method enabled us to obtain, in a preliminary offline phase, the solution of the thermo-mechanical problem over a large range of design parameters, with much time saving compared to a classical (brute force) multi-resolution finite element method. In an online postprocessing phase, the power module lifetime, modeled with a strain-life law, was then computed in a straightforward manner by rapidly evaluating the solution for any value of the parameters. Sensitivity analysis was conducted to select parameters values leading to acceptable module lifetimes with respect to given criteria. A robust design study was also performed to illustrate the performance of the proposed approach.