Calculating Microvia Capture Pad Tolerances across Sequential Lamination Cycles
Capture pad size equals laser diameter plus twice the root-sum-square of tool, drill, and lamination movement tolerances.
Panel Shrinkage
Material Behavior
A dimensional change occurs in printed circuit board laminates during high-temperature press cycles due to the release of internal stresses and the curing of polymer resins. In multi-layer fabrication, panel shrinkage causes the copper traces and drilled holes on the inner layers to shift closer together than their designed coordinates. This physical movement must be measured and compensated for before the tooling holes are drilled or the outer layers are laminated.
Compensation Procedure
Fabricators calculate the expected contraction rates for each laminate type based on the glass transition temperature and the copper distribution across the sheet. Designers then apply a scaling factor to the artwork files, making the patterns slightly larger than the final specified dimensions. The scaling factor is adjusted for the X and Y axes because the weave direction of the fiberglass causes the laminate to contract unevenly in different directions.
By applying this pre-shim scale, the features align with the drill machine after lamination. This step ensures that the drill bit hits the center of the inner layer pads.
Testing Range
The dimensional changes are verified by measuring test coupons positioned on the waste margins of the panel. If the measured movement diverges from the scaling predictions, the entire batch must be inspected before drilling to avoid misregistration. This evaluation protects the production run from catastrophic interlayer short circuits.