20H principle refers to the power layer relative to the formation of the distance 20H, of course, is to suppress the edge of radiation effects. At the edge of the board will radiate electromagnetic interference. The power layer is condensed so that the electric field is conducted only in the range of the ground plane. Effectively improve the EMC. If the internal shrink 20H can be 70% of the electric field is limited to the ground edge; shrink 100H can be limited to 98% of the electric field.
20H principle in PCB design

The adoption of the 20H rule is to ensure that the edge of the power plane is at least 20 times the distance between the two planes than the 0V plane edge. This rule is often required to be used as a side-shot firing technique (to suppress edge radiation effects) from reducing the 0V / power plane structure.

However, the 20H rule will only provide significant results under certain conditions. These specific conditions include:

1. The rise / fall time of the current fluctuation in the power bus is less than 1ns.

2. The power plane should be on the inner level of the PCB, and the upper and lower layers adjacent to it are 0V planes. The two 0V planes extend at least 20 times the distance from their respective power planes.

3. At any frequency of interest, the power bus structure does not resonate.

4. The total derivative of the PCB is at least 8 layers or more.