Resin Rheology
Thermomechanical behavior describes the movement of resin during the heat and pressure cycles of the multi-layer bonding process. Managing lamination flow dynamics prevents the formation of voids and ensures uniform dielectric thickness across the board. This resin movement is a function of temperature, pressure and the age of the material.
Filling Mechanism
As the press heats up, the resin transitions from a solid state to a liquid with a specific viscosity. This liquid must flow horizontally to fill the spaces between copper traces and vertically to bond the layers together. If the viscosity is too high, the resin will not fill small gaps, leading to trapped air.
If the viscosity is too low, too much resin may be squeezed out the sides of the panel, resulting in a board that is too thin. This balance is required for maintaining the dielectric spacing that controls the impedance of the traces. Achieving a consistent flow requires precise control of the temperature ramp rate during the press cycle.
Process Acceptability
Variation in the flow can lead to thickness non-uniformity across the panel. Fabrication houses monitor the amount of resin squeeze-out to verify that the process is within limits. Proper flow ensures that the reinforcement glass is fully encapsulated and that the copper features are protected.
This mechanical integrity is the foundation of a reliable multi-layer assembly.
