D39 — Differential pairs solved for width at the class gap

Decision

Pairs are solved at the Default net class's diff-pair gap (≥ its clearance and the board minimum); the width is solved, and only if the width would fall below the board minimum (≥ 0.1 mm) is the gap solved instead

Alternatives

Solve width and gap jointly for Zdiff and a common-mode target; fixed S = W

Reason

One free variable keeps the result unique and deterministic; the gap the board already uses is the one the router will keep