Being void-free is an important criterion for the quality control of photovoltaic (PV) modules after encapsulation. The encapsulation process of PV modules with Ethylene Vinyl Acetate (EVA) polymer is complex, due to the time-dependent processing conditions and the material properties caused by continuous heat transfer and chemical reactions such as crosslinking. Hence, an in-depth understanding on the void evolution mechanism is challenging yet critical for obtaining high-quality, durable void-free PV modules and improving their production throughput. Towards this goal, a void evolution model is proposed in order to reveal the most appropriate processing parameter windows for eliminating voids during the EVA encapsulation process. Firstly, the types and origins of the voids in PV modules are discussed. Then the pressure balance in-/outside the void is considered, to evaluate the void stability as a function of temperature and pressure in the actual encapsulation cycle. Finally, a diffusion-controlled model for the evolution of void of wet air is adapted and implemented to predict the void evolution and interpret the experimental observations throughout the encapsulation process. In the conclusion, the critical processing parameters affecting the void evolution during the encapsulation process of PV modules are highlighted.