Etch Profile
Subtractive copper removal processes produce non-vertical trace sidewalls due to lateral fluid action under protective photoresist patterns. Ratios comparing vertical etch depth to horizontal undercut quantify the degree of slope present on conductor edges across the panel. Higher ratios indicate steeper, more rectangular trace profiles, which preserve planned conductor geometries on high-density signal layers.
Measurement terminates at the interface where copper meets the underlying dielectric substrate.
Metal Reduction
Etchant spray pressure and chemical concentration alter the lateral dissolution rate of exposed copper foil. Chemical etch factor dictates the over-etch allowance required on photolithography artwork to achieve target trace widths after processing. Low ratios force designers to widen initial film features to prevent excessive conductor thinning.
Process Compensation
Thick copper foils exacerbate lateral undercut, reducing final top-trace widths relative to base dimensions. Fabricators adjust photolithographic phototool dimensions based on empirical chemistry data, broadening artwork lines to account for expected sidewall erosion. Spray nozzle angle, conveyor speed, and specific gravity in continuous alkaline or acid etchers directly shift sidewall taper across the panel.
Micro-sectioning under high-magnification optical microscopes verifies compliance with target geometries across batch runs. Uncontrolled undercutting creates trapezoidal trace profiles that degrade controlled impedance values in high-frequency circuit designs.
