Diffusion Rate
Mass transfer analysis in solid polymer substrates models concentration-driven moisture absorption through a constant diffusion coefficient governed by Fick’s second law. Fickian moisture kinetics describe the predictable weight gain rate of circuit board laminates exposed to humid environmental conditions prior to reflow soldering. The absorbed moisture concentration increases linearly with the square root of exposure time during early absorption phases before approaching saturation equilibrium.
This mass transfer model applies strictly to homogeneous polymer matrices where moisture does not induce swelling or micro-cracking within the resin structure.
Absorption Mechanism
Water molecules penetrate epoxy and polyimide networks by diffusing into free volume voids between polymer chains. When printed circuit board panels undergo high-temperature assembly, entrapped moisture vaporizes rapidly, exerting internal hydrostatic pressure against internal laminate interfaces and copper foil boundaries. Delamination and internal voiding occur when internal vapor pressure exceeds the interlaminar shear strength of the resin-matrix interface.
Fabrication and assembly facilities track weight gain using precision analytical balances to determine safe moisture limits before subjecting populated boards to lead-free reflow profiles exceeding 260 degrees Celsius. Fickian moisture kinetics allow process engineers to calculate required bake-out durations based on substrate thickness and ambient humidity.
Desorption Behavior
Thermal baking drives water vapor out of the board at rates determined by the reverse diffusion profile. Elevated temperature accelerates molecular motion, increasing the effective diffusion coefficient and reducing bake times. Non-Fickian deviations appear if prolonged thermal exposure triggers resin degradation, glass transition shift or permanent micro-void formation within the laminate.
