On some PCB layouts, the power plane is inset relative to the GND plane. This arrangement is mainly related to the 20H rule, which is intended to reduce electromagnetic radiation from the board.
The 20H Rule
The 20H rule was proposed primarily to reduce electromagnetic radiation from the PCB. When high-speed current flows through a board, an associated electromagnetic field is generated. At the edges of the power and ground planes, this field can extend beyond the board structure and contribute to electromagnetic interference.
Figure 1 | Edge radiation when the power and ground planes have the same outline
When the ground plane and power plane have the same outline, the electric field between them can extend toward the board edge and radiate outward as electromagnetic interference. A common approach is therefore to pull the power plane inward by a defined distance so that the electric field remains more effectively over the ground plane. This reduces edge radiation and can improve electromagnetic compatibility (EMC).
The inset distance is defined by the 20H rule. Here, H represents the dielectric thickness between the power plane and the ground plane. The rule states that the edge of the power plane should be set back from the edge of the 0 V plane by at least 20 times the spacing between the two planes.
For example, if the dielectric spacing between the power and ground planes is H, the recommended power-plane setback is 20H. Increasing the separation between the plane edges reduces the amount of electric field that extends beyond the ground-plane boundary. The objective is not simply to make the power plane smaller, but to keep more of the electromagnetic field contained within the PCB structure.
Figure 2 | Field confinement with a 20H or 100H power-plane inset
After applying the inset, most of the electromagnetic field is kept over the ground plane rather than extending outward from the board edge. However, the effect is not complete. A 20H inset relative to the ground plane reduces field density by about 70%, rather than eliminating it entirely. If a greater degree of field confinement is required, a 100H inset can be used. In general, a 100H inset confines about 98% of the electric field.
This explains why a power plane may be intentionally smaller than the corresponding ground plane. The ground plane provides a larger reference area, while the power plane is pulled back from its edge to reduce the amount of electric field that can escape from the plane structure and contribute to external EMI.
Practical Limitations of the 20H Rule
A strict 20H setback is not always practical in a real PCB layout. Depending on the stackup, the dielectric spacing between the power and ground planes may make 20H a relatively large distance. Applying the full setback can reduce the available power-plane area and leave less room for routing.
For this reason, a common practical compromise is to inset the power plane by 1 mm relative to the GND plane. Although this does not correspond to a universal 20H distance for every stackup, it can still provide a useful improvement while preserving more usable PCB area.
The actual effectiveness of the 20H concept depends on the plane configuration and stackup. It should therefore be considered together with the dielectric thickness, plane arrangement, and available routing area rather than applied as an isolated geometric rule.
Conditions for Applying the 20H Rule
The 20H rule also has specific conditions. A clear effect is expected when the following requirements are met:
- The power plane is located on an internal PCB layer, and the layers immediately above and below it are both 0 V planes. Each of these 0 V planes must extend beyond the power-plane edge by at least 20 times the dielectric thickness between that 0 V plane and the power plane.
- The total PCB layer count is greater than or equal to 8.
These conditions are important because the 20H concept depends on the relationship between the power plane, the surrounding ground planes, and the dielectric spacing between them. Simply making a power plane smaller does not by itself guarantee the same field-confinement effect if the surrounding layer structure does not meet these conditions.
