● Shrinkage of 20H
● Edge Radiation Effects
20H principle in PCB design refers to the shrinkage of 20H spacing of the power layer against the ground layer, to suppress the edge radiation effects. The shrinkage of power layer makes it possible that electric field is conducted only in the range of the ground layer, effectively improving the EMC. If the shrinkage of 20H can make 70% of the electric field be limited to the ground edge; the shrinkage of 100H can reach 98%.
The 20H principle is used to ensure that the edges of the power layer have the shrinkage from the 0V plane, which is at least 20 times of spacing between two layers. This principle is often required to reduce the edge radiation effects.
However, the 20H principle only provides noticeable results under certain conditions. These specific conditions include:
1. The rise/fall time of the current fluctuation in the power bus is less than 1 ns.
2. Power plane should be placed on inner layer of PCB, and is next to 0V planes. The two 0V planes extend outward at least a distance of at least 20 times their interlayer spacing from the power plane.
3. The power bus structure does not resonate at any frequency.
4. The total layers of PCB is least 8 layers or more.