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.
Copper Weight Compensation
Dimensional Adjustment
Production scaling factors applied to digital artwork adjust the physical dimensions of circuit features to counteract predictable material movements during lamination. Using copper weight compensation allows designers and fabricators to modify trace widths and spacing based on the thickness of the foil being etched. This adjustment ensures that the final physical conductor width matches the intended electrical performance after chemical etching removes material from the sides of the traces.
Heavy copper foils require greater compensation than thin foils because the longer exposure to etchant increases the lateral undercut.
Etch Allowance
Chemical removal of copper from a substrate occurs both vertically and horizontally. When copper weight compensation is calculated, the artwork is expanded by a factor roughly equal to the thickness of the copper layer. A one ounce foil typically requires an expansion of about 0.001 inches to result in a finished trace that meets the nominal width.
Failure to apply these adjustments leads to high resistance or impedance mismatches in the completed circuit. The process relies on the etch factor, which is the ratio of downward etch depth to lateral undercut. Fabricators adjust these values based on the specific chemistry of the etching line and the speed of the conveyor.
Design Constraint
Minimum spacing requirements limit the extent of the expansion. If the distance between two features is too small, copper weight compensation might cause the traces to merge on the photolithography tool even if they would have been separate after etching. Designers verify that the air gap between compensated features remains above the minimum manufacturing capability of the shop.
This boundary prevents electrical shorts in dense layouts where heavy copper is required for high current applications.