Meaning
Manufacturing metrics that define the ratio of lateral etching to vertical etching during printed circuit board fabrication determine trace geometry precision. The etch undercut factor represents this ratio of horizontal copper removal beneath the protective resist mask to the total etch depth. It is used to calculate necessary spacing adjustments for high-density boards.
The metric stops applying where additive circuit fabrication methods replace chemical subtractive processes.
Geometric Distortion
Chemical subtractive processes alter the cross-sectional shape of a printed trace from a perfect rectangle to a trapezoid. When the etch undercut factor is high, the top of the trace becomes substantially narrower than the base. This geometry change reduces the cross-sectional area and increases the electrical resistance of the trace.
In high-frequency designs, this geometric distortion affects the characteristic impedance of transmission lines, leading to unwanted signal reflections. Hardware designers must model this trapezoidal shape in simulations to avoid impedance mismatch on the final board.
Process Compensation
Pre-compensating the digital artwork before manufacturing ensures that the final copper traces meet design specifications. The fabricator applies a correction to the trace widths on the photolithography masks to counteract the horizontal copper loss. This process step requires accurate measurement of the etch undercut factor of the chemical bath.
Adjusting the mask widths maintains the target impedance.
Quality Control
Micro-section analysis of sample boards confirms the accuracy of the etching process. Quality engineers cut a test coupon from the processed panel and examine the trace cross-section under a microscope. This inspection measures the etch undercut factor directly against the supplier’s process capability limits.
The results are recorded in the batch compliance document before shipment.
