Moisture Transport under Power Cycling in Silicone-Gel Encapsulated Power Modules

Humidity is a critical reliability concern in power electronic modules, particularly for wide-bandgap devices operating under high electric fields. Silicone gels, widely used for encapsulation, offer mechanical and dielectric protection but remain permeable to moisture. This paper investigates moisture transport in silicone-gel subjected to power cycling. A physically consistent one-dimensional thermo-hygroscopic model is developed, combining heat diffusion and Fickian moisture transport with temperature-dependent diffusion and solubility. A reference case without power cycling, corresponding to conditions close to condensation at the chip/gel interface, is first considered as a limiting baseline. Power cycling is then applied under identical ambient and cooling conditions to isolate its specific impact on moisture redistribution. The results show that cyclic junction temperature variations induce alternating moisture absorption and desorption phases, leading to a moisture pumping/depumping effect that can significantly reduce the average moisture content in the gel compared to the static case. An analytical formulation is further introduced to quantify the influence of key cycling parameters on maximum relative humidity levels at chip/gel and air/gel interfaces. The study demonstrates that moisture accumulation and condensation risks are governed not only by ambient conditions but also by the temporal structure of thermal loading, with important implications for power cycling tests and reliability assessment of power modules.