Trace Matching
Quality control structures etched onto the PCB panel border allow for the verification of signal propagation speeds across differential pairs. A differential phase length coupon provides a physical sample to measure how closely the two traces in a pair match in electrical length. These structures help fabricators confirm that the etching process has not introduced timing mismatches that would degrade high-speed signals.
Signal Measurement
Technicians use time domain reflectometry to observe the arrival time of pulses at the end of each trace in the pair. A differential phase length coupon shows whether one trace is physically longer or has a different width than its partner. This check is necessary because even a small difference in length leads to common-mode noise and reduced signal voltage at the receiver.
Coupon Geometry
The layout of the coupon follows the same stack-up and trace width as the functional circuits on the main board to ensure the results are valid. High-frequency boards require the coupon to include transitions such as vias or connectors if those features are present in the actual signal path. Designers place these coupons at multiple locations on the panel to account for variations in copper thickness or resin content across the entire production area.
By comparing the results from different coupons, the manufacturer can verify that the fabrication process is stable and that all boards on the panel meet the phase matching requirements.
