With an increasing demand for high-power electronics, the need to meet stringent automotive norms and better understand the critical failure mechanisms are crucial in order to improve their reliablity. To that end, we developed an in-situ reliability monitoring setup capable of actively measuring the thermal performance of the package during lifetime testing. A Thermal Test Chip (TTC) assembled into a Power Quad Flat No-lead (PQFN) package was employed as a test vehicle for non-destructive reliability assessment. The TTC comprises resistive heaters as a heat source and resistive temperature elements for measuring the thermal response. The transient thermal behavior was evaluated based on the contribution of heat source to a temperature field, and the temperature distribution was measured at multiple spatial positions. The experimental results provide insights into the thermal properties' influence on the thermal behavior of the package. A compact electro-thermal model based on analogies was developed to deconvolute and analyze the transient thermal measurements. The results of the compact model correlate with the experimental measurements, and the model's accuracy was verified using finite element simulations. The development of such thermal characterization experiments and computationally inexpensive models assist in further understanding the impact of failures in advancing high-power density electronics.