Buildup HDI Stackup Design and Layer Thickness Control
Stackup thickness control requires calculating pressed prepreg heights over local copper patterns to hold impedance and microvia drilling tolerances.
Nominal Pressed Thickness
Theoretical Dimension
Mathematical models provide a target height for a circuit board based on the combined values of its individual layers. The nominal pressed thickness represents the expected result after the prepreg layers have been squeezed and the resin has filled the gaps in the copper patterns. Mechanical designs use this number to calculate the final board height and ensure it fits within the enclosure.
Stackup Calculation
Software tools add up the thickness of the cores, foils, bond plys and prepregs while accounting for the amount of copper on each layer. A nominal pressed thickness is only an estimate because the actual flow of the resin depends on the density of the circuitry. High copper density prevents the prepreg from compressing as much, while open areas allow for more movement.
The calculation must be adjusted if the board contains large planes or very sparse routing.
Flow Management
Production teams compare the actual height of the finished panel against the calculated value to monitor the stability of the lamination process. The nominal pressed thickness provides the baseline for these measurements and helps identify if the press was too hot or if the pressure was too low. Variations from the nominal value can affect the impedance of the signals and the reliability of the plated through holes.