Prepreg Resin Flow Mechanics in Multilayer PCB Lamination
Prepreg resin flow during vacuum lamination requires balancing platen ramp rates against copper fill volume to eliminate internal clearance microvoids.
Transverse Permeability
Material Property
Material properties defining the ease with which resin flows perpendicular to the plane of the fiberglass weave determine the quality of layer bonding in a PCB. High transverse permeability allows the liquid epoxy to move through the glass bundles to reach the internal copper features during lamination. This characteristic is influenced by the density of the weave and the surface treatment applied to the fibers.
Saturation Efficiency
When resin can move easily through the thickness of the prepreg, it is less likely to leave behind microscopic air pockets. In a laminate with low transverse permeability, the resin may flow along the surface of the weave but fail to penetrate the center of the bundle. This results in a structurally weak board that is prone to conductive anodic filament growth and internal shorts.
Process Impact
Fabricators must choose materials with the correct flow properties to match their specific press cycles and layer counts. Increasing the transverse permeability helps in the production of high layer count boards where the resin has to travel a longer path to fill deep cavities. If the material resistance is too high, the lamination pressure must be increased which can lead to glass distortion.
Testing this property involves measuring the time it takes for a fluid to pass through a sample of the weave under a controlled pressure gradient. This value is a necessary input for the simulation of the lamination process.