Since the electric field between the power layer and the ground layer changes, electromagnetic interference is radiated outward at the edge of the board. In PCB design, the power layer can be retracted so that the electric field is only conducted within the ground layer. If the power layer is shrunk by 20H relative to the ground layer, 70% of the electric field can be confined to the grounding edge; if it is shrunk by 100H, 98% of the electric field can be confined (where H is the thickness of the medium between the power layer and the ground layer), for EMC (Electro Magnetic Compatibility).
The adoption of 20H rule means to ensure that the edge of the power supply plane is at least 20 times the distance between the two planes than the edge of the 0V plane. This rule is often required to reduce side firing from the plane structure of 0V/ power supply (suppression of edge radiation effect).
However, the 20H rule will only provide significant results under certain conditions. These specific conditions include:
1. The rise/fall time of current fluctuations in the power bus is less than 1ns.
2. The power plane is on the internal of the PCB, and the upper and lower layers adjacent to it are 0V planes. The two 0V planes extend outward at least 20 times the distance between their respective planes from the power plane.
3. The power bus structure does not resonate at any frequency of concern.
4. The total derivative of the PCB is at least 8 layers or more.
