Why is bonding important for EMC?

Enclosure panels, cable shields, and connectors must be at the same RF potential to prevent voltage differences that drive common-mode currents and radiate emissions. A bond resistance above 2.5 milliohms (MIL-STD-188-124B requirement) allows enough potential difference to compromise shielding. At RF frequencies, bond impedance (not just DC resistance) determines effectiveness due to skin effect and inductance.

Direct vs indirect bonding?

Direct bonding: metal-to-metal contact between surfaces (bolted, welded, brazed). Requires clean, oxide-free surfaces and compatible metals. Lowest impedance. Indirect bonding: uses a bonding strap or jumper (flat braid preferred over round wire for lower inductance). Used when direct contact is impractical or when dissimilar metals require isolation to prevent galvanic corrosion.

Bonding dissimilar metals creates galvanic corrosion risk when moisture is present. The galvanic series determines corrosion severity: aluminum-to-copper is severe (1.0 V potential). Use conductive plating (cadmium, tin, zinc) or conductive gaskets to isolate dissimilar metals while maintaining RF bonding. MIL-STD-889 defines acceptable metal combinations.

EMC/EMI

Bonding (EMC)

Bonding in EMC is the process of creating low-impedance electrical connections between metal structures (enclosure panels, cable shields, connectors, ground planes) to maintain equipotential surfaces. A bond resistance below 2.5 mΩ (per MIL-STD-188-124B) ensures that RF currents flow through intended paths rather than creating common-mode emissions. At RF frequencies, bond impedance includes both resistance and inductance, making flat braid straps superior to round wire.

Category: EMC/EMI

Target: <2.5 mΩ

Understanding EMC Bonding

Every joint in a metal enclosure is a potential EMC weakness. Panel seams, access covers, and connector interfaces must maintain continuous conductivity to prevent RF leakage. A single unbonded seam can reduce enclosure shielding effectiveness by 20-40 dB at frequencies where the seam length approaches λ/2.

Bond surfaces must be clean, free of anodize or paint in the contact area, and protected against corrosion after assembly. Conductive chromate conversion (Alodine) on aluminum provides corrosion protection while maintaining conductivity. Lock washers, tooth washers, or serrated flanges ensure mechanical stability under vibration.

Bond Impedance

Z_bond = R + jωL
R = contact resistance (<2.5 mΩ)
L = strap inductance (nH)

Flat braid (25 mm wide, 100 mm):
L ≈ 15 nH, Z@100 MHz = 9.4 Ω
Round wire (#10, 100 mm):
L ≈ 100 nH, Z@100 MHz = 63 Ω

Bonding Method Comparison

Method	Impedance	Durability	Use Case
Direct (bolted)	Lowest	High	Permanent panels
Welded/brazed	Lowest	Permanent	Seams, joints
Flat braid strap	Low	Good	Movable covers
EMI gasket	Medium	Moderate	Access doors

Common Questions

Frequently Asked Questions

Why important?

Unbonded seams leak RF. >2.5 mΩ bond compromises shielding. At RF, impedance (not just DC R) matters due to inductance.

Direct vs indirect?

Direct: metal-to-metal (bolted/welded), lowest Z. Indirect: braid strap or jumper, used when direct impractical or dissimilar metals.

Dissimilar metals?

Galvanic corrosion risk. Al-to-Cu = severe. Use conductive plating (tin, zinc) or gaskets to isolate. MIL-STD-889 compatibility.

Related Terms

← Bonded Fin Bonding Resistance →

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