Why Should Power Planes Be Smaller Than Ground Planes in PCB Design?

In multilayer PCB design, there is a widely accepted guideline: power planes should be slightly smaller than ground planes, with a uniform edge inset instead of matching the board outline.

Many engineers follow this rule in practice, but not everyone fully understands the underlying physics, when it actually matters, and how to apply it correctly in real projects.

Based on our experience, the PCBWay design team has summarized key insights on power plane layout in this article—from theory to practical implementation.

Why Power Planes Should Not Match Ground Planes in Size

At its core, the issue comes down to one thing: edge-field radiation. A power plane and its adjacent ground plane essentially form a large parallel-plate capacitor. When current switches rapidly, an alternating electric field is established between the two planes.

• If the power and ground planes are the same size, the electric field can “leak” out from the PCB edges, creating strong edge radiation.
• This radiation often manifests as EMI issues, especially in high-speed, high-frequency, or switching power designs.
• In severe cases, it can interfere with nearby ICs, antennas, and sensors, or even cause compliance test failures.

In conclusion, the stronger the edge field, the harder it is to control EMI.

The 20H Rule: The Physics Behind Power Plane Inset

One of the most well-known solutions in the industry is the 20H rule:

• H = the dielectric thickness between the power plane and the ground plane
• 20H = the amount by which the power plane is pulled back from the board edge

By applying this rule:

• Around 70% of the edge electric field can be contained between the planes instead of radiating outward
• With a more aggressive design (e.g., 100H), up to 98% suppression can be achieved

In essence, the ground plane suppresses edge electric fields and reduces their radiation from the PCB boundaries.

Applying the 20H Rule in PCB Design

Although the 20H rule is theoretically sound, it is rarely applied strictly in real designs. The reasons are practical:

• The dielectric thickness is often very small, making 20H only a fraction of a millimeter—sometimes negligible
• Excessive inset can lead to limited via placement, discontinuous power distribution, and increased voltage drop
• In low-layer or asymmetric stackups, the effectiveness of the 20H rule is inherently limited

Therefore, in practice, a more practical compromise is usually adopted:

• Shrink the power plane by approximately 0.5 mm to 1 mm relative to the ground plane
• This reduces most edge-field radiation while maintaining good layout flexibility
• It is simple, consistent, and suitable for production designs

When the 20H Rule Actually Works

Although the 20H rule is effective, it only delivers meaningful results under the right conditions. Otherwise, its impact can be minimal.

• The power plane must be sandwiched between ground planes:

The layers directly above and below the power plane should both be GND to provide effective field containment.

• Typically requires 8 layers or more:

In 4-layer or lower stackups, the power plane is often located near the outer layers, limiting the effectiveness of the 20H rule. In these cases, EMI control relies more on grounding, shielding, and filtering.

• Most relevant for high-frequency, high-speed, or high di/dt scenarios:

For low-frequency, low-speed, or small-signal designs, the impact of plane inset is usually minimal. In such cases, power integrity should take priority.

Key Design Considerations Beyond Plane Inset

• Prioritize ground plane integrity:

Any splits, gaps, or slots in the ground plane can significantly degrade EMI performance—often far more than not shrinking the power plane.

• Ensure uniform power plane inset:

Avoid uneven setbacks (e.g., large inset on one side and minimal on another). A consistent margin provides better results.

• Minimize high-speed routing near board edges:

Even with proper power plane inset, high-speed signals routed near the PCB edge can still radiate. Critical signals should be placed on inner layers whenever possible.

• Proper chassis and enclosure grounding:

Achieving good EMI performance requires more than plane inset. Effective grounding of the chassis or metal enclosure, along with shielding, is essential. 

Conlusion

Shrinking the power plane slightly relative to the ground plane helps reduce edge-field radiation and improve EMI. In practice, a 0.5–1 mm margin is usually sufficient, provided the ground plane is intact and the stackup is properly designed. Combined with careful routing and grounding, this simple adjustment enhances both performance and manufacturability.

If you need assistance with PCB layout, the PCBWay design team is ready to help.