Four Wire Resistance Measurement of Solder Joints

A solder joint that is failing often behaves normally until the moment it opens, and the early indication is a small increase in resistance. Measuring that increase requires a technique that does not drown it in lead resistance.

Why Two Wire Is Not Enough

A two wire measurement reads the joint plus the probes, the cables and the connections to the board. Those terms are far larger than the joint resistance, so a change of a few milliohms is invisible.

The joint resistance itself is small by design. Our joint criteria notes describe the acceptance that applies.

How a Kelvin Measurement Is Made

Four wire separates the current path from the voltage path, so the voltage is sensed only across the joint. The probes must contact inside the current path, and the two voltage connections should be as close to the joint as possible.

The contact geometry is what sets the measurement resolution. Our measurement notes describe the arrangement.

Thermal EMF and Offset

A temperature difference between the two voltage connections produces a voltage that is added to the reading. The effect is small and it is larger than the change being looked for, so it has to be managed.

Reversing the current and averaging removes the offset. Our thermal measurement notes describe the same discipline in another context.

Monitoring During a Test

The useful application is continuous monitoring during thermal cycling, where the resistance is logged and a step is detected. A single measurement before and after shows only the endpoints.

The event detector should capture short interruptions that a sampled reading misses. Our thermal cycling notes describe the design.

What a Change Means

A gradual rise indicates a crack growing across the joint, and a sudden step indicates a partial separation. Both precede an open circuit, and the criterion should be set on the measurement rather than on the failure.

Our fatigue notes describe the interpretation.

Practical Setup

The connections to the board have to survive the cycling, which usually means a soldered wire on a dedicated test pad. A probe that is merely pressed on will move as the assembly expands.

The board should be designed for the measurement. Our test point notes describe the pad.

Verification

The verification is a four wire arrangement with separated current and voltage paths, an offset removed by current reversal, a logged record rather than endpoint readings, and a criterion set on the resistance change rather than on the open circuit.

Our quality notes describe how the records are kept.

Process Control and Verification

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

FAQ

Can the measurement be made without soldered wires? It can with a stable fixture, and the fixture has to hold the contact under the expansion of the test.

How large a change matters? A change of a small fraction of the initial resistance is significant when it is repeatable, which is why the initial value is recorded with the measurement conditions.

What does gopcb provide for joint measurement? We provide a four wire arrangement with separated current and voltage paths, an offset removed by current reversal, continuous logging rather than endpoint readings, and a criterion set on a resistance change that precedes an open circuit.