Have you ever noticed that some PCB designs incorporate a certain degree of indentation in the GND and power layers? Have you ever wondered why?

To understand this, we need to first understand the "20H" principle:

The 20H principle is primarily designed to reduce electromagnetic radiation from circuit boards. High-speed currents on a circuit board generate associated magnetic fields. The way these electromagnetic fields radiate at the edges of different layers is shown in the following diagram:

As you can see, when the ground and power planes are the same size, the electric field between the power and ground planes changes, causing electromagnetic interference to radiate outwards from the board edges. The common solution is to indent the power plane by a certain distance. This allows the electric field to conduct only within the ground plane's area, thus suppressing edge radiation effects and improving electromagnetic compatibility (EMC).

So, how much should we typically indent? The indentation distance is the "20H" distance we mentioned earlier. Here, H refers to the dielectric thickness between the power plane and the ground plane. The "20H rule" means ensuring that the edge of the power plane is indented at least 20 times the distance between the two planes compared to the edge of the 0V plane.

As shown in the image above, this is the indented power plane and ground plane. We can see that most of the electromagnetic field is no longer radiated outwards, thus reducing external EMI radiation. But why do we say that most of it is no longer radiated outwards? Because we found that indenting the power plane's edge relative to the ground plane by 20H reduces the electromagnetic field density by about 70%, not to zero. If we need to confine even more of the electric field, we can indent by "100H". Generally, an indentation of 100H can confine 98% of the electric field within the confinement area. This is one reason why our boards need to be indented.

However, due to the layer stack-up design, on some typical PCBs, strictly adhering to the 20H rule would prevent PCB routing. Therefore, a common practice is to recess the power GND plane 1mm inwards from the base GND plane, thus ensuring some performance improvement for the board.

We also need to note that the 20H rule only has a significant effect under certain conditions:

1. The power plane must be located inside the PCB, and the two adjacent layers above and below it must both be 0V planes. The distance extending outwards from these two 0V planes must be at least 20 times the layer spacing between them and the power plane.
2. The total number of PCB layers must be greater than or equal to 8 layers.