Four-wire (Kelvin) measurement eliminates lead resistance. A known current (typically 100 mA to 1 A DC) is passed through the bond, and voltage is measured across the joint with separate sense leads. The ratio V/I gives bond resistance. Milliohm meters with Kelvin probes are standard. Measure at multiple points along the joint to detect localized high-resistance spots.

What affects bonding resistance?

Surface preparation (oxide removal), contact pressure (higher = lower resistance), surface area (larger = lower resistance), surface finish (plating type), corrosion (increases over time), and environmental conditions (humidity, salt spray). Aluminum oxide is an insulator; surfaces must be abraded immediately before assembly. Anodize must be removed from contact areas.

DC vs RF bonding resistance?

DC bonding resistance measures only contact resistance. At RF, the total bond impedance includes skin-effect resistance (increases with sqrt of frequency) and inductance of the bond strap or path. A bond that passes DC testing (less than 2.5 milliohms) may have unacceptable impedance at 100 MHz due to strap inductance. Transfer impedance testing characterizes RF performance.

EMC/EMI

Bonding Resistance

Bonding Resistance is the electrical resistance of the connection between two bonded metal surfaces, measured using four-wire (Kelvin) technique. MIL-STD-188-124B specifies a maximum of 2.5 mΩ for EMC-critical bonds. At RF frequencies, the total bond impedance includes skin-effect resistance and strap inductance, requiring transfer impedance measurement for full characterization. Surface preparation, contact pressure, and corrosion protection determine long-term bond quality.

Category: EMC/EMI

Limit: ≤2.5 mΩ

Understanding Bonding Resistance

Contact resistance arises from the microscopic nature of metal-to-metal contact. Even flat surfaces touch only at asperities (high points), creating constriction resistance. Higher clamping force deforms asperities, increasing true contact area and reducing resistance. Surface oxides (especially aluminum oxide, a strong insulator) must be removed before bonding.

For EMC, bonding resistance is tested during installation and periodically thereafter. Corrosion, vibration loosening, and thermal cycling can degrade bonds over time. Critical bonds in military and aerospace systems are inspected on maintenance schedules.

Measurement Method

Four-wire Kelvin:
R_bond = V_sense / I_test
I_test: 100 mA - 1 A DC
Sense leads separate from current leads

Pass/Fail:
MIL-STD-188-124B: ≤2.5 mΩ
Best practice: ≤1.0 mΩ

Factors Affecting Bond Quality

Factor	Effect	Mitigation
Surface oxide	+10-100× R	Abrade before assembly
Contact pressure	↓ with ↑ force	Spec torque, lock washers
Corrosion	+R over time	Plating, sealant, inspect
Dissimilar metals	Galvanic corrosion	Compatible plating

Common Questions

Frequently Asked Questions

Measurement?

Four-wire Kelvin: 100 mA-1 A test current, separate sense leads. Milliohm meter. Multiple points per joint.

What affects it?

Oxide (insulator), pressure (more = lower R), corrosion (degrades over time), surface finish (plating matters).

DC vs RF?

DC: contact R only. RF: adds skin effect + inductance. DC pass doesn't guarantee RF performance. Use transfer impedance for RF.

Related Terms

← Bonding Boost Converter →

EMC Testing

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