Process Window Optimization for Hybrid Low Loss Multilayer Board Fabrication
Optimizing hybrid board fabrication requires balancing thermal ramp rates, dual-gas plasma desmear, and pinless registration to eliminate delamination.
Undercut Compensation
Dimensional Adjustment
Photolithography mask modifications must compensate for the lateral removal of copper that occurs beneath the dry film resist during chemical etching. The undercut compensation process involves widening the designed trace widths on the phototool to ensure that the finished etched traces achieve their specified target dimensions. This correction prevents the traces from becoming too narrow and increasing electrical resistance.
It is an essential step in preparing digital circuit files for the production floor.
Etch Correction
Chemical spray nozzles in the etcher deliver acid that eats through the unprotected copper in a vertical direction, but it also dissolves copper horizontally. When applying undercut compensation, engineers analyze the foil thickness and etchant chemistry to estimate the amount of horizontal copper loss. The trace widths on the phototool are then increased by this calculated amount.
This ensures that the trace width is correct after the horizontal etching occurs. Without this adjustment, the resulting traces would have a smaller cross-sectional area, which could lead to overheating and signal attenuation.
Width Constraint
Narrow trace spacings limit the amount of compensation that can be applied without causing adjacent traces to touch on the mask. This technique is difficult to implement on designs with spacing below seventy-five microns.