Written By: Syspcb Updated: 2025-4-1
The 20H rule mandates that the power plane edge be recessed inward by a distance equivalent to 20 times the dielectric thickness (H) between the power and ground planes. This counteracts edge radiation effects—where varying electromagnetic fields at PCB edges emit interference, violating FCC/CE EMI limits by up to 15 dB. Empirical studies confirm:
20H principle
Not all designs benefit equally. These four conditions amplify 20H’s effectiveness:
Rise/fall times <1 ns generate strong harmonics above 300 MHz—where radiation peaks.
Power planes must be sandwiched between two ground planes in an internal layer stackup, with ground planes extending 20H beyond the power zone.
Power plane cavities must avoid resonant frequencies within the operating band (e.g., 1–6 GHz for 5G devices).
8+ layers optimize field containment. In 4-layer boards, effectiveness drops by 40%.
– Measure dielectric thickness (H): Typical FR-4 H=0.2 mm → 20H=4 mm
– Design adjustment: For 100×80 mm power plane, reduce to 92×72 mm after
