Moisture Ingress Into Packages With Walls of Varying Thickness And/Or Properties: A Simple Calculation Method

For electronic devices that work at high humidity (such as implants), but for which only a moderate lifetime is required, it is possible to use polymer packages for protection. However, these materials are porous and allow moisture to diffuse through the package. Fick´s laws applied to water diffusion have a solution which can either be solved numerically, or approximated accurately with the single exponential “quasi-steady-state (QSS) model.” This paper adapts the so-called QSS model to the case of moisture ingress into a package which has walls or elements of different thicknesses or properties. Using an electrical analogy, we propose a model which allows estimating the change in relative humidity with time inside the enclosure cavity, using a simple calculation method.