Electromagnetic Radiation on PCB edges: 20H Rule and Via Stitching

Since past few days I was going through the study on Electromagnetic radiation occurring on PCB edge; here is the brief on what the paper suggests to follow.

One of the primary modes of radiation from printed circuit boards (PCBs) are because of emissions along the edges of PCBs. One of the primary printed circuit board mechanisms [1] that produce radiated emissions from PCB edges are via currents that excite radially propagating electromagnetic waves between power and/or ground plane structures.

When these waves propagates to the edge of the PCB, a portion of the energy radiates into space (usually the cavity of the electronic enclosure housing the PCBs), and a portion is reflected back into the PCB, where it induces currents into the same vias that were the original source of the initial radially propagating wave.

One of the primary modes of radiation from printed circuit boards are because of emissions along the edges.

These induced currents then conductively flow into the components mounted on the surface of the PCB (conductive emission), producing secondary radiation. The waves inside the PCB excite resonant modes whose frequencies are dependent on the width and length of the PCB.

20-H Rule

The 20-H rule [3] states that the power planes are pulled back by edges from the ground planes by about 20 times the distance between the planes. Consider the simple structure consisting of one ground plane and one power plane shown in Figure (a). The 20-H rule structure is represented in Figure (b).

Pulling back one of the power plane forms a smoother impedance transition region. While this increases edge emissions, it reduces PCB resonance effects. Power plane pulling back should be on all the four sides of the PCB.

Via Edge Stitching on the PCB edge

The most common technique of reducing edge radiation is fencing, where a series of shorting vias are used to connect top/bottom ground planes into a Faraday shield. But, this increases internal reflections. 

This is as similar as the ground Via stitching around the critical net track.

For the two-plane structure[4], 20-H rule yields much more radiation than the normal structure. For the multiple plane case, no significant change in radiation is found if the 20-H rule is applied to the internal planes. Also the numerical result shows that the 20H Rule and ground Stitching Vias would cut down the radiation effectively.

References:

[1]: Franz Gisin, Zorica Pantic-Tanner "Minimizing EMI Caused by Radially Propagating Waves Inside High Speed Digital Logic PCBs"

[2]: Gisin, Franz, Pantic-Tanner, Z., “Radiation From Printed Circuit Board Edge Structures”, 2001 IEEE EMC

[3]  Mark I. Montrose, "Printed Circuit Board Design Techniques for EMC Compliance", 1996 IEEE EMC

[4] Huabo Chen et. al. "Effects of 20-H Rule and Shielding Vias on Electromagnetic Radiation From Printed Circuit Boards" 

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