Sodium induced shunting under an electric field is a challenging reliability issue in crystalline Si solar modules. THe source of this Potential-Induced Degradation of the Shunting type (PID-s) is well understood  and its influence on power loss has been intensively studied based on phenomenological models on cell or module level relating the experimental power-loss to stressing parameters (time, temperature, voltage) . However, little is known about the Na ion migration kinetics, responsible for PID on a microscopic level, and its quantitative relation to the efficiency degradation. In this paper we present our investigations of sodium ion migration in Ethylene-Vinyl Acetate (EVA) and silicon through Dynamic Secondary Ion Mass Spectroscopy (D-SIMS). Each sample was annealed at field relevant temperatures from 60-90 °C to address typical migration mechanisms of common PV installations. Analysis of the SIMS migration profiles revealed a diffusivity constant D0,EVA = 0.09 ± 0.14 cm2/s and an activation energy EA,EVA = 0.85 ±.04 eV for Na in EVA and diffusivities higher than extrapolated literature values in silicon (D0,Si = (3.03 ± 2.42)x10-5 cm2/s, and EA,Si = 0.98 ± 0.02 eV). The new insight will be included in a drift-diffusion based degradation model accounting for the partition coefficient across all relevant interfaces. This model can assist in predicting PID-failure in the field based on the given mudle stack and the diffusion of Na+ through each material. This tool can be used for process optimization as well as material selection significantly reducing the cost and time to validate a technology.