EMC Design Myths
If an idea gets repeated often enough, people tend to accept it as fact. Here are some of the more infamous EMC design myths that refuse to go away.
90° corners in circuit board traces cause radiated emissions.
They don't. Etching problems? Not really. Signal integrity problems? Perhaps at data rates >50 Gbps.
References
How Interconnects Work: Minimal-reflection 90-degree bends in strip lines, Y. Shlepnev, March 2, 2021.
Should You Worry About 90 Degree Bends in Circuit Board Traces?, Eric Bogatin, Signal Integrity Journal, April 13, 2021.
Slaying the PCB 90⁰ Right-Angles Bend Dragon, Kella Knack, Altium Website, 2019.
PCB Routing Angle Myths: 45-Degree Angle Vs 90-Degree Angle, Zachariah Peterson, Altium Website, 2022.
The 20-H Rule: The power plane should be 20 layer-spacings farther from the board edge than the ground plane in order to reduce radiated emissions.
As far as radiated emissions from a pair of planes, making the power plane smaller has no effect on the peak emissions. It only changes the resonant frequency. If the power plane is sandwiched between two ground planes, emissions from the edge are negligible as long as the planes are well-connected in the general vicinity of the edge. There may be times when it is a good idea to keep all traces and planes away from the board edge (e.g., to prevent near-field coupling to objects adjacent to the edge). Nevertheless, the 20-H distance and the arguments that this reduces radiated emissions cannot be supported.
References
Practical analysis on 20H rule for PCB," Shinichi Ikami and Akihisa Sakurai, 2008 Asia-Pacific Symposium on Electromagnetic Compatibility and 19th International Zurich Symposium on Electromagnetic Compatibility, Singapore, 2008, pp. 180-183.
Analysis on the effectiveness of the 20-H rule for printed-circuit-board layout to reduce edge-radiated coupling, M. I. Montrose et. al., IEEE Trans. on EMC, vol. 47. no. 2, 2005. [Note: This paper goes out of its way to support the 20-H rule by focusing on the near field, and modeling strongly driven resonant structures. Even so, the data presented does not support the rule.]
20-H rule modeling and measurements, H. W. Shim and T. H. Hubing, 2001 IEEE EMC International Symposium. Symposium Record. International Symposium on Electromagnetic Compatibility, 2001, pp. 939-942.
Effects of 20-H rule and shielding vias on electromagnetic radiation from printed circuit boards, Huabo Chen and Jiayuan Fang, IEEE 9th Topical Meeting on Electrical Performance of Electronic Packaging, 2000, pp. 193-196.