How to Address Electromagnetic Compatibility and Interference in PCB Design

Technique 1: PCB Grounding
One of the most effective ways to reduce EMI is through PCB grounding. Start by maximizing the ground area across the PCB, which helps minimize emissions, crosstalk, and noise. Extra care should be taken to connect each component to the ground point or plane, as failing to do so negates the neutralizing benefits of a reliable ground plane.

Complex PCB designs often feature multiple stable voltage levels. Ideally, each reference voltage should have a dedicated ground plane. However, having too many ground planes can increase manufacturing costs. A balanced approach is to use three to five ground planes at strategic locations, with each plane covering multiple ground sections. This method helps control manufacturing costs while reducing EMI and EMC.

To minimize EMC, a low-impedance grounding system is essential. In multilayer PCBs, a robust ground plane is preferable to a copper balancing block or scattered ground areas, as it offers low impedance, a clear current path, and an optimal return signal source.

Signal return time is another critical factor. Signals must travel to and from their source within equivalent timeframes. Otherwise, they act like antennas, turning radiated energy into EMI. Similarly, the traces transmitting current to and from the signal source should be as short as possible. Unequal source and return path lengths can lead to ground bounce, further contributing to EMI.

Technique 2: Distinguishing EMI Sources
Since different EMI sources vary in characteristics, a sound EMC design principle is to separate analog circuits from digital circuits. Analog circuits, which often involve higher currents, should be kept away from high-speed traces or switching signals. When possible, ground signals should be used to shield them. On multilayer PCBs, analog traces should be routed over one ground plane, while switching or high-speed traces should be over another, ensuring that signals with different characteristics remain isolated.

A low-pass filter can sometimes be employed to eliminate high-frequency noise coupled from nearby traces. Such filters help suppress noise and stabilize current flow. Separating the ground planes for analog and digital signals is equally critical. Analog circuits and digital circuits exhibit unique characteristics, necessitating independent grounding. Digital signals should terminate in a digital ground, while analog signals should terminate in an analog ground.

Experienced PCB layout engineers pay close attention to high-speed signals and clocks in digital circuit design. For high-speed signals, the traces and clocks should be as short as possible and located close to ground planes. This minimizes crosstalk, noise, and radiation, keeping them under control.

Digital signals should also be kept away from power planes. Proximity between these planes can induce noise or crosstalk, weakening the signal integrity.

PCB Design

Technique 3: Prioritizing Crosstalk Reduction in Trace Design
Proper trace design is crucial for ensuring smooth current flow. For currents originating from oscillators or similar devices, it is vital to separate them from ground planes or avoid parallel routing with other traces, particularly high-speed traces. Parallel high-speed signals are prone to EMC and EMI issues, especially crosstalk. Trace resistance paths must be kept as short as possible, with return current paths equally minimized. Return path trace lengths should match the transmitting trace lengths.

In EMI contexts, one trace is often labeled as the “aggressor” while the other is the “victim.” Inductive and capacitive coupling due to electromagnetic fields can affect the victim trace, inducing forward and backward currents that lead to ripple in the signals.

In an ideal balanced environment, induced currents would cancel each other out, eliminating crosstalk. However, real-world conditions seldom allow for perfection, making it essential to minimize crosstalk. Maintaining a spacing between parallel traces that is at least twice the trace width can significantly reduce crosstalk. For instance, if a trace width is 5 mils, the spacing between parallel traces should be 10 mils or more.

Technique 4: Decoupling Capacitors
Decoupling capacitors help mitigate the adverse effects of crosstalk. These should be placed between the power and ground pins of a device to ensure low AC impedance, reducing noise and crosstalk. Using multiple decoupling capacitors across a wide frequency range ensures optimal performance.

The smallest-value capacitor should be placed as close as possible to the device to minimize inductive effects on the trace. This capacitor should connect directly to the device’s power pin or power trace, with its pads linked to vias or the ground plane. For longer traces, multiple vias can minimize grounding impedance.

Technique 5: Avoiding 90° Angles
To reduce EMI, avoid creating 90° angles in traces, vias, or other components, as sharp angles can lead to increased radiation. At these points, capacitance increases and characteristic impedance changes, causing reflections and EMI. Use two 45° angles to route traces around corners instead.

Technique 6: Careful Use of Vias
Vias are often indispensable in PCB layouts, providing conductive connections between layers. However, they introduce inductance and capacitance, and in some cases, reflections due to impedance changes in the traces.

Vias also extend trace lengths, requiring proper length matching. For differential pairs, avoid vias if possible. If unavoidable, ensure both traces in the pair use vias to compensate for delay in signal and return paths.

Technique 7: Cable and Physical Shielding
Cables carrying digital and analog currents often generate parasitic capacitance and inductance, leading to EMC issues. Twisted pair cables maintain low coupling levels, eliminating magnetic fields. High-frequency signals require shielded cables grounded at both ends to prevent EMI interference.

Physical shielding involves enclosing all or parts of the system in metal to block EMI from entering the PCB circuit. Such shielding acts like a grounded conductive container, reducing antenna loop size and absorbing EMI.

Technique 8: Shielding and Filtering

1. Adding Shielding: Use metal shields or shielding layers to reduce EMI when necessary. High-frequency components should be isolated using shielding boxes to prevent interference with other components.
2. Filters and Suppressors: Add low-pass filters to suppress high-frequency noise and suppressors to control electromagnetic interference. These measures help keep crosstalk, noise, and radiation levels within acceptable limits.

Technique 9: Simulation and Validation

1. Perform electromagnetic field and radiation analyses using simulation software after completing the PCB design to identify potential EMI issues.
2. Optimize the PCB design based on simulation results to ensure compliance with EMC requirements.

By applying these techniques, engineers can design more efficient and stable circuit boards, reducing electromagnetic interference and improving overall system performance. Follow LSTPCB for more insights into PCB, PCBA, and component design tips, and enjoy free prototyping services!