Thickness Shift
The physical change in thickness that occurs when a partially cured fiberglass and resin sheet is compressed and cured during the board lamination cycle defines a primary manufacturing offset. Calculating the prepreg pressing delta allows designers to predict the final thickness of the cured dielectric layers by accounting for resin flow into the copper pattern. This thickness change directly affects the trace impedance and the overall height of the finished board.
If this delta is ignored, the resulting board will be too thin, causing assembly and electrical failures.
Lamination Calculation
During the hot press cycle, the solid resin in the prepreg melts and flows to fill the empty spaces between adjacent copper traces on the inner layers. This displacement of resin from the top of the copper traces into the channels reduces the height of the dielectric layer directly above the metal. The amount of thickness reduction depends on the copper density of the adjacent layers and the laminate’s initial resin content.
A layer next to a dense copper plane will experience less compression than a layer next to a sparse, routed signal layer. Fabricators use specialized flow models to calculate this height reduction before selecting material sheets.
Process Variable
Press temperature, ram pressure, and heat rise rate must be controlled to maintain a consistent resin flow. Any variation in these lamination parameters will change the amount of resin squeeze-out, causing the dielectric thickness to drift. This process variation can push trace impedance outside of the required limits.
