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Should You Pour Copper Under an Inductor?

2025-10-11

A Practical Guide for PCB Designers

In PCB design, whether or not to pour copper under an inductor has always been a debated topic. The right answer depends on the product type and design objectives.

When alternating current flows through an inductor, pouring copper underneath can induce eddy currents on the ground plane. These eddy currents may alter the inductance value of the power inductor, increase system losses, and introduce additional noise that interferes with signal stability.

However, from an EMC/EMI perspective, having a solid copper plane beneath the inductor can improve noise performance. Thanks to modern manufacturing processes and the widespread use of shielded inductors, magnetic leakage has been greatly reduced — meaning the effect on inductance is minimal, and copper planes can even help with heat dissipation.

Should You Pour Copper Under an Inductor? 1

Should You Pour Copper Under an Inductor? 2

🔍 Understanding Inductor Types

To make the right design choice, it’s important to know the structure of different inductors:

Unshielded (drum-core) inductors: The magnetic field is fully exposed to air, with no magnetic shielding.
Semi-shielded inductors: Magnetic shielding material is added around the coil, reducing magnetic leakage.
Molded (one-piece) inductors: The winding and magnetic material are integrated into a single molded structure, minimizing air gaps and magnetic flux leakage.

⚙️ Experimental Findings

Experiments show that when copper is poured beneath unshielded inductors, the inductance value slightly decreases due to eddy currents.
For shielded inductors, however, the inductance remains nearly unchanged.

🔬 The Impact on Power Circuit Design

When alternating magnetic flux passes through a conductor, eddy currents are induced on its surface. These currents generate opposing magnetic fields that weaken the original flux.

Take a Boost DC/DC converter as an example. During operation, the current in the inductor changes dynamically, creating magnetic flux that can leak into surrounding space.

Without copper below: The magnetic flux can spread through the system, increasing EMI noise.
With copper below: Eddy currents in the copper layer act as an electromagnetic shield, reducing downward flux leakage and minimizing high-frequency interference with nearby components.

✅ Design Recommendations

For EMI performance: ✅ Pour copper under the inductor.
For shielded inductors: ✅ Copper pour has minimal impact and helps with heat dissipation.
For unshielded inductors: ⚠️ Copper pour may slightly reduce inductance — evaluate based on your design priorities.

💡 Pro tip: Place output filters and sensitive components on the opposite PCB layer from the inductor to reduce coupling and prevent high-frequency noise propagation.