PCB Signal Propagation Velocity

Formula

Description

Electromagnetic signals on PCB traces travel slower than the speed of light in vacuum due to the dielectric material surrounding the trace. The propagation velocity is c (speed of light) divided by the square root of the effective relative permittivity. For microstrip traces (outer layer), the effective εr is between 1 and εr_substrate because the electric field partially travels through air. For stripline traces (inner layer, buried between ground planes), the effective εr equals the substrate εr. Standard FR-4 has εr ≈ 4.2-4.5 at low frequencies, giving propagation velocities of about 0.47-0.49 × c.

Variables

• v_p — Propagation velocity (m/s)
• ε_r_eff — Effective relative permittivity of the transmission line

Practical Notes

c = 299,792,458 m/s. For FR-4 microstrip (εr_eff ≈ 3.2-3.5), vp ≈ 160-168 mm/ns ≈ 6.0-6.3 in/ns. For FR-4 stripline (εr_eff ≈ 4.2-4.5), vp ≈ 141-146 mm/ns ≈ 5.5-5.8 in/ns. εr increases with frequency (dispersion) in FR-4, which can cause signal distortion at high data rates. Low-loss laminates (Rogers, Megtron) have more stable εr across frequency.