Fluid Movement
Migration of epoxy resin during the lamination process occurs when heat and pressure force the liquefied material into gaps between copper traces or into drilled holes. This resin displacement is a fundamental part of creating a solid, void-free multilayer stackup.
Void Formation
Controlled flow is necessary to encapsulate internal conductors and provide electrical insulation between the different layers. If resin displacement is excessive or poorly managed, it can lead to thin spots in the dielectric layer which reduce the breakdown voltage of the board. Manufacturing engineers monitor the pre-preg properties and the lamination press cycle to ensure the resin reaches the correct viscosity.
Insufficient flow leaves air pockets that can expand and cause delamination during later soldering steps.
Structural Consequence
Physical movement of the resin can also cause small components or internal layers to shift slightly from their intended positions. When resin displacement occurs in the vicinity of high speed signal lines, the change in local dielectric constant may alter the characteristic impedance of the circuit. Inspection of micro-sections after pressing reveals whether the resin has successfully filled all the interstitial spaces.
Maintaining a consistent glass to resin ratio is necessary for the mechanical stability of the finished product.
