Fickian Diffusion Kinetics and Desorption Times for Molded Packages
Fickian desorption calculations use temperature-dependent diffusivity to determine exact package bake times, preventing reflow popcorning and structural delamination.
Dual-Mode Sorption
Penetrant Distribution
Penetrant gas accumulation within a glassy polymer involves two distinct population sites. In the context of electronic encapsulants, dual-mode sorption accounts for molecules dissolving into the polymer matrix alongside those filling pre-existing micro-voids. This model describes how water or other contaminants distribute throughout a substrate during storage.
Validity of this model ends when the temperature exceeds the glass transition point of the resin.
Hole Filling
Voids within the rigid structure of a thermoset resin act as traps for moisture during the initial phase of exposure. The dual-mode sorption behavior transitions as these holes saturate, shifting the rate of uptake toward a different mechanism. Because polymers used in circuit boards exist below their glass transition temperature, these stationary sites remain available until occupied by external molecules.
The process differs from simple diffusion by recognizing that not all absorbed matter contributes to the swell or plasticization of the material.
Solubility Boundary
Saturation limits depend on the specific density of the micro-voids and the partial pressure of the surrounding gas. While the dissolved component follows a linear relationship, the hole-filling part of dual-mode sorption follows a Langmuir isotherm. High humidity environments trigger the rapid filling of these sites before the bulk matrix reaches equilibrium.
The model fails when the polymer undergoes structural relaxation or enters a rubbery state where the fixed holes disappear.