Measuring Low Resistance Between Defined Points on a Board

Measuring a short copper path or soldered connection often means resolving a resistance comparable to the test leads and contact interfaces. A two-wire reading can include enough fixture resistance to hide the feature of interest. A four-wire method improves the situation, provided the sense contacts define the intended measurement boundary.

Place the boundaries before the probes

Mark the two voltage-sense locations on a photograph or drawing. Decide whether the result should include connector contacts, solder joints, pads, vias, or only a particular trace segment. Move the force contacts outside those boundaries so their voltage drops are not mistaken for the target resistance.

Keithley’s low-level measurements handbook explains four-wire resistance measurement and thermoelectric offset compensation. Separating force and sense reduces lead-resistance error, while current reversal can address an approximately stable thermoelectric voltage. Neither technique removes every fixture or temperature-related uncertainty.

Select a current that measures the intended condition

For an illustrative 5-milliohm path, 1 A produces 5 mV and 5 mW of dissipation. Raising the current to 10 A produces 50 mV but 0.5 W. The larger signal is easier to resolve, yet the added heating may change the resistance being measured. Choose the test current and timing with that tradeoff in mind.

Ensure that other circuitry does not create a parallel path or experience an unintended applied condition. Use a suitable unpowered specimen or an explicitly designed test configuration. A meter cannot distinguish the intended conductor from another electrical route between the same nodes.

Check repeatability and offsets

1. Verify the instrument setup and fixture using an appropriate reference.
2. Repeat contact placement to estimate fixture repeatability.
3. Record board and ambient temperature and the measurement duration.
4. Use supported offset compensation when it is appropriate to the specimen.
5. Check whether the value changes as current or dwell time changes.

If a result drifts upward during a prolonged measurement, investigate heating before declaring an unstable joint. If it changes mainly when a clip is moved, revisit the sense boundary and contact quality.

The report should state resistance between named points, applied current, timing, temperature, instrument mode, and fixture arrangement. Preserve individual measurements when comparing assemblies rather than reporting only an average. That record makes the value useful for tracking a process change and prevents a milliohm number from implying more precision, or a narrower physical scope, than the setup can actually support.

For joint comparisons, maintain the same mechanical clamping condition. A change in applied pressure can change the contact being evaluated and confound the comparison.