Trace Delay & Length Matching
Propagation delay, the length mismatch a skew budget allows, and when a trace turns into a transmission line.
Delay is length × √εeff
A signal on a PCB travels slower than light by the square root of the effective dielectric constant. The propagation delay per unit length is:
For FR-4 that's roughly 6 ps/mm on a microstrip and 7 ps/mm on a stripline (≈ 150 and 180 ps/inch). Stripline is slower because the field sits entirely in the dielectric; microstrip is partly in air, so its — and its delay — are lower.
Length matching
When two signals must arrive together — a differential pair, a DDR byte lane, a parallel bus — the allowed length mismatch comes straight from the skew budget:
A 10 ps budget on FR-4 microstrip is only about 1.6 mm of length difference — which is why you see serpentine meanders snaking the short traces out to match the long ones.
When does a trace become a transmission line?
A trace must be treated as a transmission line — with termination and controlled impedance — once it's electrically long compared to the signal edge. A common rule uses the rise time:
Below this length, reflections settle within the edge and you can mostly ignore them; above it, impedance control and termination stop being optional. Faster edges shrink this length fast — which is why a 100 ps edge makes even a few centimetres "long".