Three things happen between your data and a controlled board. First, we calculate the stack-up backwards from your target impedance using the real laminate data, adjusting dielectric thicknesses and proposing corrected track widths where needed. Second, CAM applies etch compensation — the copper is etched from the top, so a track comes out narrower at the top than at the bottom, and the artwork is widened to land on target. Third, a TDR coupon in the panel is measured per lot.
The step designers most often get wrong is the second one. Widening tracks yourself to compensate for etching means the compensation gets applied twice and the finished impedance comes out low. Send the target, not a pre-corrected geometry.
