Quality Control
Quality assurance testing utilizes sacrificial PCB coupons placed on the panel border to verify that manufactured traces meet design impedance requirements. Through impedance coupon testing, inspectors measure the electrical properties of trace geometries using time-domain reflectometry without damaging the main production board. This validation ensures that the raw board meets performance specifications before it moves to the component assembly phase.
Reflectometry Measurement
A high-speed pulse is sent down the coupon test trace, and the instrument measures the amplitude and timing of the reflected wave to calculate the local trace impedance. This test measures the trace at several points along its length, checking for uniformity and highlighting manufacturing defects like over-etching, under-etching, or layer misalignment. Fabricators compare these measured values against the impedance limits set by the board designer, which typically allow a tolerance of ten percent.
Coupons mimic the actual routing of the board, using identical widths, copper weights, and reference planes.
Measurement Limitation
Although this technique verifies the fabrication process, it does not directly test the actual signal traces on the production board itself. Differences in etching between the panel margins and the center can create small discrepancies between the coupon and the final boards.
