Signal Integrity and EMI

A trace becomes a transmission line when its length exceeds about 1/6 of the distance the signal edge travels during its rise time. Then its characteristic impedance (often 50 Ω single-ended, 90 Ω for USB differential pairs) must be controlled and matched to avoid reflections.

$v \approx \frac{c}{\sqrt{\varepsilon_{eff}}} \quad (\approx 15\text{–}17\ \text{cm/ns on FR-4})$

Other practices: keep loop areas small, add series termination resistors (22–33 Ω) on fast clock lines, space parallel traces to reduce crosstalk, route differential pairs together with equal length, and filter cables where they leave the board. Products sold in many markets must pass EMC tests.