Selecting Dielectric Substrates for High Speed Multilayer Stackup Design
Selecting dielectric substrates requires balancing dissipation factor, glass weave uniformity, foil roughness, and panel yields to meet high-speed impedance targets.
Prepreg Pressed Thickness
Consolidated Dimension
Partially cured resin sheets reach a specific height after being subjected to heat and pressure during the board lamination process. The prepreg pressed thickness is the final height of the insulation layer once the resin has flowed into the copper features and solidified. This value is always less than the thickness of the raw sheet because some material is displaced into the voids of the adjacent layers.
Resin Displacement
Calculations for the final height of the board must account for the percentage of copper on the surrounding layers. A prepreg pressed thickness varies depending on whether the sheet is placed against a solid ground plane or a layer with very few traces. If there is a lot of empty space, the resin will fill those gaps and the layer will end up thinner.
Fabricators use tables provided by material suppliers to estimate these changes based on the copper weight and the resin content of the specific prepreg style. This variability makes impedance control a challenge for complex multi-layer designs.
Material Verification
Measuring the actual thickness of the layers after lamination requires cross sectioning a test coupon and using a microscope. The prepreg pressed thickness must stay within a narrow range to ensure the electrical impedance of the traces is correct. Consistent results depend on the precise control of the temperature and pressure cycles in the lamination press.