Internal Force
Hydrostatic pressure generated within a polymer gel when solvent molecules diffuse into a concentrated region causes localized volume expansion. The action of swelling osmotic pressure becomes noticeable when printed circuit board laminates are exposed to aqueous processing solutions or industrial solvents. This force drives the separation of resin from glass fibers or copper traces.
The pressure continues to rise until the mechanical resistance of the polymer network balances the chemical potential of the solvent. In multilayer assemblies, this localized force can exceed the mechanical tensile strength of the cured adhesive, creating microscopic fractures that allow moisture to penetrate further into the laminate stackup. This mechanism is especially active in regions surrounding plated through-holes where resin is exposed directly to wet processing chemicals.
Chemical Gradient
Concentration differences between the pure solvent outside the board and the soluble species trapped within the resin matrix drive the liquid inward. When boards are processed in warm stripping or plating baths, the chemicals penetrate into micro-voids and dissolve low molecular weight polymers. This dissolution creates a high solute concentration that draws in more water molecules to dilute the solution.
The resulting liquid accumulation builds high stress within the polymer boundaries.
Structural Failure
Mechanical failure of the laminate occurs when the internal pressure exceeds the adhesive bond strength of the materials. This phenomenon causes delamination, blistering, and copper foil peeling during subsequent thermal cycles such as solder reflow. Visual inspection or scanning acoustic microscopy reveals these internal separations as void spaces between layers.
Circuit boards with high laminate moisture or solvent retention must be baked to remove these volatile compounds before assembly.
