Absorption Kinetics
Transport of solvent molecules into a polymer matrix occurs at a rate proportional to the concentration gradient of the penetrating species. Analyzing Fickian solvent diffusion allows fabricators to predict how printed circuit board substrates absorb moisture or industrial solvents during wet chemical processing and assembly. The rate of absorption depends heavily on the temperature of the solvent bath and the polymer free volume.
This behavior is characterized by a linear increase in mass relative to the square root of time, and it applies strictly before the polymer reaches saturation. When a composite laminate is submerged in a stripping bath, the liquid molecules migrate into the epoxy resin, filling the gaps between polymer chains and reducing the overall structural density of the board.
Concentration Gradient
Solvent concentration differences between the outer surface and the inner core of the resin drive the penetration depth over time. In a typical fabrication environment, boards exposed to stripping solvents absorb the fluid at their exposed edges. This absorption profile creates localized zones of low glass transition temperature.
The presence of solvent-swollen resin next to dry resin generates internal stress.
Substrate Expansion
Swelling of the resin matrix occurs as solvent molecules occupy the intermolecular spaces between polymer chains. This physical expansion degrades the adhesion between the fiberglass fabric and the epoxy resin, leading to micro-cracking and eventual delamination during thermal cycling. Halting the diffusion process requires immediate rinsing and baking of the substrate to drive out volatile chemicals before they can disrupt the structural integrity of the composite board.
