Resin Density
Material density changes in the dielectric layer occur during the lamination process as heat and pressure force air out and compress the polymer chains. Resin compaction determines the final thickness of the board and the resulting characteristic impedance of the circuit traces. This process is necessary to create a solid, void-free substrate that can support multiple layers of copper.
Mechanical Compression
As the resin reaches its minimum viscosity, the pressure of the lamination press forces the material to conform to the shape of the copper features. Resin compaction is affected by the copper density on each layer because areas with less copper will see more resin movement. The final density of the material influences the thermal conductivity and the dielectric constant of the finished board.
Impedance Control
Because the distance between the signal traces and the ground planes is set during this phase, any variation in the compression will change the electrical performance. Fabricators use the known rates of resin compaction to calculate the starting thickness of the prepreg needed to reach the final design goal. Precise control of the press parameters is the only way to ensure that the board meets the tight impedance tolerances required for high-speed signals.
The stability of the lamination process is a major factor in the overall yield of the manufacturing facility.
