In recent years, a defect on the surface of photovoltaic (PV) modules called 'snail trails' has become a widespread reliability issue, affecting many modules installed in the field. In this work, silver acetate is identified as one component of snail trails through non-destructive Raman analysis. The chemical reaction between the silver grid line, oxygen and acetic acid on top of the micro-crack is proposed as the mechanism. The generation and/or diffusion of acetic acid released from ethylene vinyl acetate (EVA) encapsulant, oxygen and moisture are modeled using finite element method to predict the formation of silver acetate. The simulation results indicate that the existence of micro-crack plus cell gap are necessary for snail trail formation and act as the pathway for transport. As the source of acetic acid, encapsulant plays an important role in snail trail formation. Oxygen transmission rate of backsheet also has significant influence on snail trail formation. Water vapor transmission rate is shown to have no effect on snail trials over a wide rate of transmission rates. An accelerated aging snail trail test has been developed that can simulate snail trails within days, and is compared to modeling and field results.