Polymer Matrix
Partially cured resin-impregnated glass fiber sheets serve as the primary bonding material in multi-layer board lamination. B-stage thermoset prepreg exists in a dry, solid but uncured state where the chemical reaction has been halted at an intermediate stage. This material remains stable at low temperatures but melts and flows when heat and pressure are applied in a lamination press.
It binds the core layers of the printed circuit board together.
Rheological Behavior
Under the action of the heated lamination press, the dry resin liquifies and flows to fill the gaps between copper traces on the adjacent board layers. This liquid phase must last long enough to ensure complete wet-out of the surfaces and expulsion of any trapped air, yet the resin must not be so fluid that too much of it squeezes out of the panel edges. The duration and viscosity of this fluid phase are highly sensitive to the temperature ramp rate of the platens.
Proper rheological control prevents laminate voids and thickness variations across the finished board panel.
Polymerization Finish
As the press cycle continues at elevated temperatures, the cross-linking reaction resumes and drives the resin to a fully cured, rigid C-stage. Once this threshold is crossed, the chemical transition is irreversible, resulting in a stable thermoset structure with high glass transition temperature. The cured laminate provides electrical insulation and mechanical support.
