Flow Behavior
Rheological behavior during the lamination of multi-layer circuit boards determines how well the liquid epoxy flows between copper traces before curing. The parameter known as minimum resin viscosity represents the lowest resistance to flow that the resin reaches as it is heated under pressure. It is measured in pascal-seconds or poise during the prepreg gelation phase.
Process Control
Heat ramp rates and platen pressure directly affect the resin state. If the temperature rises too slowly, the minimum resin viscosity remains high, preventing the resin from filling the narrow gaps between copper traces. If it rises too quickly, the resin becomes too thin and flows out of the board, leaving resin-starved areas that are prone to delamination.
Dielectric Uniformity
Controlling this parameter ensures a uniform layer of dielectric material between signal traces. When the minimum resin viscosity is kept within the specified range, it provides adequate flow to encapsulate the copper edges while retaining enough thickness to prevent layer-to-layer shorts. This stability helps maintain the target characteristic impedance of signal lines by keeping the dielectric spacing consistent across the entire panel.
Laminators use prepreg materials with characterized viscosity curves to prevent voids and squeeze-out during the press cycle.
