Etching Metric
Photolithographic processing defines undercut ratio through the lateral loss of copper beneath a developed photoresist overhang during board fabrication. Chemical etchants consume metal vertically through exposed circuit channels while simultaneously attacking the sidewalls beneath the mask protection. Acid spray pressure, cupric chloride concentration and conveyor speed directly influence this lateral consumption rate across thick copper foils.
Automated optical inspection equipment measures the resulting profile divergence against design dimensions to catch dimensional drift before outer layer lamination occurs.
Dimensional Limit
Board shops calculate undercut ratio by dividing horizontal etch loss by total vertical depth removed from the conductor trace. Maximum allowable tolerances dictate that lateral consumption must remain below half the total foil thickness to preserve rectangular cross sections required for impedance control. Differential etching speeds between top and bottom surfaces occur when chemical replenishment fails to maintain uniform etchant freshness inside narrow microstrip channels.
Microsection cross sectioning verifies this ratio destructively during first article inspection because optical surface scans cannot resolve sidewall geometry beneath the overhang.
Structural Yield
Conductor profile degradation reduces functional copper cross sectional area, which increases high frequency insertion loss and shifts characteristic impedance values away from design targets. Etchant stagnation inside high aspect ratio fine pitch spaces accelerates localized lateral attack unless agitation nozzle pressure increases proportionally. Proper chemistry replenishment maintains stable oxidation reduction potential parameters, which prevents excessive chemical undercutting during prolonged conveyor exposure times.
Trace geometry stability governs final circuit reliability across demanding thermal shock cycles.
