Alternating current (AC) tends to concentrate on the surface or “skin” of a conductor, which is known as the conductor skin effect.
As the AC flows through the conductor, it induces a magnetic field. If the current increases, the resulting increase in the magnetic field creates circulating eddy currents that reduce the current flow in the center and reinforce it near the skin.
When designing your PCB, consider and mitigate the conductor skin effect, especially when dealing with high-frequency signals.
Cross-section of AC flow in a cylindrical conductor with δ representing the skin depth
The conductor skin effect happens in alternating current (AC) conductors like wires or transmission lines. When AC flows through a conductor, the current distribution within the cross-section of the conductor isn’t uniform. The current tends to concentrate on the surface or “skin” of the conductor, and the core carries less. This is known as the conductor skin effect.
The effect results from the interaction between the changing magnetic field generated by the AC and the conductor. As the AC flows through the conductor, it induces a magnetic field. If the current increases, the resulting increase in the magnetic field creates circulating eddy currents that reduce the current flow in the center and reinforce it near the skin.
The conductor skin effect is minimal at low frequencies, like those found in direct current (DC) and low-frequency AC circuits, since the current is distributed evenly across the conductor’s core. However, higher frequencies amplify the skin effect.
Conductor skin effect: Alternating current I flowing through a conductor creates a magnetic field H. If the current I increases, the resulting increase in H induces circulating eddy currents Iw that reduce the current flow in the center and reinforce it near the skin.
How the Conductor Skin Effect Impacts PCB Design
It’s essential to consider the conductor skin effect when designing PCBs, especially when dealing with high-frequency signals. As the operating frequency of devices increases, the skin effect becomes greater.
Here are several ways that the conductor skin effect impacts PCB design:
- Greater resistance: At high frequencies, the skin effect reduces the cross-sectional area of the conductor carrying the current. This increases the conductor’s resistance. As a result of higher resistance, more power is dissipated, and the PCB can get hot, reducing efficiency.
- Signal integrity: Signal integrity is crucial for high-speed digital circuits and RF applications. Signals can get attenuated and distorted because of the skin effect. Signal loss, reflections, and jitter can occur, affecting PCB performance.
- Impedance variation: There’s a possibility that the skin effect will cause impedance variation on PCBs, especially in high-frequency traces. Signal reflections and degraded signal integrity can happen due to this impedance mismatch.
- Current crowding: The skin effect concentrates the current toward the conductor’s surface. This current crowding can lead to localized hotspots and electromigration problems in PCB traces. Especially when dealing with high current densities, these can degrade the PCB’s reliability and performance over time.
Ways to Mitigate Conductor Skin Effect in PCB Design
Mitigation Techniques for Conductor Skin Effect in PCB Design
Proper Trace Sizing
Wider traces reduce the skin effect
Thicker Copper Layers
Greater copper layer thickness minimizes the skin effect
Multiple Signal Layers
Multiple PCB layers reduce current crowding and minimize the skin effect
Surface finishes with better conductivity mitigate the skin effect
Controlled Impedance Design
Controlled impedance traces compensate for the skin effect’s impedance variation
Engineers use different techniques and design considerations to mitigate the conductor skin effect in PCB design:
- Proper trace sizing: When it comes to high-frequency signals, wider traces can help reduce the skin effect and lower conductor resistance.
- Thicker copper layers: Increasing the copper layer thickness on a PCB can reduce the overall resistance of traces and minimize the conductor skin effect.
- Multiple signal layers: Multiple layers in the PCB reduce current crowding and minimize the skin effect by spreading the current across several planes.
- Surface finish: Surface finishes for PCB traces can also affect the skin effect. Surface finishes with better conductivity can help mitigate the skin effect.
- Controlled impedance design: Controlled impedance traces help maintain signal integrity by compensating for the skin effect’s impedance variation.
Understanding the conductor skin effect and how it affects PCB design is vital to optimizing the performance and reliability of high-frequency electronics.
Cadence Has the Tools to Account for Conductor Skin Effect
With the right simulation features, you can account for the conductor skin effect, giving you a more realistic view of signal behavior.
Resistance and impedance are important concepts in PCB design and analysis, and you’ll need to use pre-layout and post-layout simulation software to test your designs. Cadence's front-end design features enable you to design high-speed PCBs at the circuit level and simulate signal behavior. For post-layout analysis and simulation, you can use SI/PI Analysis Point Tools.
PCB designers can use Cadence’s suite of PCB Design and Analysis Software tools to account for the conductor skin effect. When you are ready for prototyping, OrCAD PCB Designer's speed and comprehensive functionality accelerate board layout and reduce time-to-market.
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