Simulating moisture diffusion in polymers using thermal-moisture analogy

The present paper illustrates moisture diffusion using advanced thermal-moisture analogy approach. Experimental procedure to measure the diffusivity and solubility of epoxy based encapsulation material to be implemented in simulation has been discussed. The moisture weight gain of the sample is measured till its saturated state over time at 5 different relative humidity (RH%) and temperature combination. Then %weight gain as a function of time results are fitted with the analytical solution of 3-D diffusion equation using nonlinear regression analysis to obtain the diffusivity property. Since the saturated concentration at the boundary of the polymer can be described by Henry's law i.e. Csat = SPvp, where S is solubility (kg /m3/ Pa) and Pvp is the ambient vapor pressure (Pa), which is related to RH% and the saturated vapor pressure Pvp, sat as Pvp=RH% Pvp, sat; this relation is used to obtain modified solubility and solubility at each temperature. The temperature dependence of solubility and diffusivity are described by Arrhenius relation. The results are utilized in Finite Element (FE) analysis of the moisture diffusion model. Only Fickian moisture absorption has been attempted to be modeled. It has been shown that the moisture diffusion properties obtained from such characterization technique fits satisfactorily with the experimental results. Further the analogy scheme's degree of simplicity, boundary conditions and applicable situations have been highlighted.