Moisture ingress into nonhermetic enclosures and packages. A quasi-steady state model for diffusion and attenuation of ambient humidity variations

In this paper the rate of moisture ingress into enclosures was studied theoretically and experimentally. A simple quasi-steady state (QSS) model was developed enabling one to calculate easily time constants for moisture diffusion through plastic walls, rubber gaskets and openings. The characteristic time constant is τ=VL/AP+L² /2D where P is permeability, D diffusion constant, V volume of the enclosure, L wall thickness and A surface area. The model clarifies the relative importance of moisture permeability vs. diffusion constant of wall materials and is applicable to both large enclosures and microelectronic packages. For thin and/or nonabsorbing walls the first term (a function of P) predominates while with thick and/or absorbing walls the second term (which depends on D) prevails. For openings, τ=VL/AD, it was shown that the QSS model is practically equivalent to but simpler than full transient solutions of the Fick´s second law. The attenuation of variations of atmospheric humidity by packaging was also modeled with the QSS model. The inside air humidity changes with the same frequency as the ambient but its amplitude is attenuated by a factor f=cos φ=1/(1+ω²τ²)^1/2 and is phase shifted by an angle φ=arccos [1/(1+ω² τ²)^1/2] where ω is the angular frequency of humidity changes. Therefore the protective value of packaging will be different in different geographical areas