Resin Flow
Resistance to deformation in a partially cured B-stage resin determines how the material behaves under heat and pressure during lamination. Measuring prepreg viscosity allows for the prediction of the movement of the epoxy as it fills the spaces between copper traces. This property changes as the temperature rises within the press, reaching a minimum before the final curing reaction takes hold.
Precise control of this flow is necessary for uniform dielectric thickness.
Lamination Control
Thermosetting resins must reach a sufficiently low fluid state to ensure complete encapsulation of the internal circuit layers. If the prepreg viscosity is too high, the material fails to flow into tight gaps and creates voids or air pockets within the board. Conversely, excessively low values might lead to resin starvation as the fluid escapes from the edges of the stack.
This balance between flow and retention determines the structural integrity of the multilayer board.
Testing Protocol
Rheometers track the changes in flow characteristics by applying controlled shear stress to a sample during a programmed temperature ramp. Data from these tests guide the selection of the heating rate and the timing of the pressure application. Reliable lamination depends on matching the press cycle to the curing profile.
