How to manage EMI at the harness-to-PCB interfaces

Electromagnetic interference (EMI) at harness-to-PCB connections creates one of the biggest challenges in modern electronics design. These interface points act as weak spots where unwanted signals can escape your carefully designed circuits, causing system failures.

This FAQ discusses four techniques that work in conjunction: shielded connectors, smart PCB layout, proper grounding, and common-mode chokes. When applying these methods as an integrated system, you can achieve reliable EMC compliance while maintaining design flexibility.

Shielded connectors provide the first line of defense

When high-frequency signals travel through connectors, they can radiate electromagnetic fields if the shielding has breaks. This radiation interferes with nearby circuits, causing system failures.

Shielded connectors resolve these problems using three main approaches:

• Complete 360-degree shielding covers all signal contact points and SMT connection areas.
• Multiple grounding points create several paths for electrical current to return safely.
• Controlled impedance design maintains consistent electromagnetic boundaries throughout the connection.

Now, Figure 1 shows real performance data at a 10 GHz frequency. The comparison reveals how much better shielded connectors perform. Conventional connectors show strong electromagnetic field emission. But the shielded design shows almost zero field radiation.

These design principles let engineers place sensitive RF components close to connector interfaces. This helps create smaller electronic devices while still meeting EMI compliance requirements. The electromagnetic isolation capability provides designers with more freedom in component placement, allowing them to adhere to regulatory standards.

Why PCB layout forms your second defense layer

When circuits switch fast, they create electrical noise that can interfere with other devices. Now, managing this EMI at the harness-to-PCB connections requires smart layout design. The good news is that proper PCB layout can solve EMI problems without adding expensive parts.

Your PCB layout creates invisible loops that act like antennas. When current flows through these loops during switching events, they generate EMI energy. The comparison in Figure 2 shows this clearly — the vertical loop design creates 1.0 nH of inductance, causing 70% voltage overshoot with heavy ringing. But look at the optimal loop: only 0.4 nH inductance produces just 30% overshoot.

Here is what this means for you:

• Cutting the loop inductance in half reduces EMI by four times.
• Shorter traces and better component placement make the difference.
• Your switching node forms capacitor plates with ground, creating an E-field radiation.

Where should you focus your efforts in PCB layout?

Look at your power delivery paths. These create the loops that matter most. When minimizing the physical area of these loops, you reduce both E-field and H-field emissions at the same time.

The current during switching transitions generates H-field emissions based on the current magnitude squared. Meanwhile, voltage differences create E-field radiation. Smart trace routing, careful via placement, and proper ground plane design give you control over both.

This approach works best for high-frequency switching applications where traditional filtering becomes expensive and less effective. You address EMI problems before they travel through your harness connections.

Why does grounding matter for EMI control?

Grounding provides a safe escape route for unwanted electrical energy away from a PCB circuit. Without this path, noise builds up inside your device and creates problems. Figure 3 shows exactly what happens when grounding fails – you get internal device bias that ruins your signal quality.

When looking at the comparison in Figure 3, the difference is obvious. The left side shows good grounding with foil tapes. The right side shows what happens without good grounding. Bad grounding creates voltage spikes and makes your signal much weaker.

Here is what happens at these connection points:

• Electrical currents try to find the easiest path back to ground.
• Poor connections create resistance that generates heat and noise.
• High-frequency signals suffer the most from grounding problems.

Proper grounding tape implementation, as shown in Figure 3, provides the low-resistance return path your signals need while keeping electromagnetic emissions under control.

Most of the present-day electronics operate at higher frequencies, making grounding even more important. When harness connections to your PCB create resistance, they generate harmonics that interfere with other circuits. The solution involves using specialized conductive materials like gaskets and adhesives that maintain consistent electrical contact.

How do common-mode chokes control EMI at cable connections?

Common-mode chokes block unwanted currents while keeping desired signals clean. This makes them essential for modern electronic designs. The component uses two windings on a shared ferrite core, as shown in Figure 4. When common mode currents flow the same way through both conductors, they create additive magnetic flux that produces high impedance. This technique blocks the unwanted EMI effectively.

Here is how they work:

• Equal inductance on both conductors
• Opposing flux cancellation for differential signals
• High impedance path for common-mode noise

Several sources create problems at the harness-to-PCB interfaces. Parasitic capacitive coupling between switching circuits and ground planes generates common-mode currents. Ground potential differences between connected systems also add to the problem. Plus, electromagnetic fields can couple directly to cable conductors, turning your wiring into unwanted antennas.

Placement matters a lot for performance. Put your CM choke close to the connector interface to stop currents before they radiate from the cable. You also need to match the choke’s frequency response to your specific EMI profile and make sure it has enough saturation current rating for your application.

Summary

These four EMI management techniques create a layered defense system that addresses interference at multiple points in your signal path. Start with shielded connectors to contain electromagnetic energy at the interface. Use a smart PCB layout to minimize loop areas that generate EMI. Implement proper grounding to provide clean return paths for high-frequency currents. Finally, add common-mode chokes to filter any remaining unwanted signals.

References

How to separate differential and common-mode harmonic noise currents, WTWH Media LLC
3M Solutions for Electromagnetic Interference – 5G Infrastructure, 3M
ZenShield®: High-Performance EMC Shielding Connectors, I-PEX Inc.
eGaN FETs Are Low EMI Solutions!, EPC Corp, Inc.

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