Pogo Pin Fixtures: Contact Resistance and Maintenance

A pogo pin is a spring loaded contact that presses a probe tip against a test point, and a bed of nails fixture holds hundreds of them in a plate that is lowered onto the board. Every measurement the fixture makes passes through two contacts, so the quality of those contacts sets the quality of the test result.

Contact resistance is the number that describes it. A new probe may present 20 to 50 milliohms, and a worn or contaminated one can present several ohms, which is enough to make a good board fail a resistance measurement or hide a marginal one.

What a Pogo Pin Has to Deliver

The probe has to make a stable electrical contact, do so without damaging the pad, and survive a large number of actuations. Those three requirements pull against each other, because a harder tip and a higher contact force improve the electrical connection at the cost of pad wear.

Stability matters as much as the absolute value. A contact that reads the same every time can be compensated in the program; one whose resistance varies between actuations produces a measurement that cannot be trusted at all.

The Contact Resistance Budget

In a two wire measurement, the resistance of the probe, the tip to pad interface and the return path all add to the reading. Four wire measurement removes the probe and lead resistance from the result but still leaves the interface, which is the part that degrades with use.

A practical budget assigns a value to the interface and verifies it on a known artefact. Where a test measures a 100 milliohm net, a probe contributing 500 milliohms is not acceptable; where the measurement is a functional check with wide limits, the same probe may be perfectly adequate. The specification should follow the measurement.

Spring Force and Travel

Spring force is what breaks through the surface film on the pad and creates a stable contact area. Typical probes develop 20 to 60 grams of force at working travel, and the force rises as the spring is compressed, so the fixture must be set to a defined travel rather than simply closed until it stops.

Travel is normally 1 to 2 mm of available stroke, with the working point placed in the middle of that range. Operating at the end of the stroke leaves no margin for board thickness variation, while operating at the beginning leaves too little force for a reliable contact on a contaminated pad.

Tip Geometry and Target Condition

Tip styles are chosen to suit the target. A crown tip penetrates flux and oxide, a flat tip spreads the load on a small pad, and a spear tip concentrates force where the pad is tiny. The choice should be made against the test point design, not inherited from a previous fixture.

The condition of the pad matters as much as the probe. Solder resist covering a test point, a pad that has been tinned, or residue left by a no clean process all change the interface resistance. Test points should be left free of solder and resist, and their position should be agreed with the board designer through the test coupon and test point rules for the product.

Contamination and Cleaning

Flux, solder paste and dust accumulate on the tip and reduce the contact area. The contamination is not uniform, so probes on a fixture age at different rates depending on which pads they touch and how much flux is present in that area of the board.

Cleaning intervals should come from measurement rather than from a fixed calendar. A weekly check of contact resistance on a reference artefact shows the trend, and a probe whose value has doubled is a candidate for cleaning or replacement regardless of how the fixture looks.

Wear, Plating and Replacement Limits

The tip material and its plating determine how long the probe lasts. Gold plated tips on a spring steel barrel are common, and the plating wears through where the tip meets the pad, exposing the base metal to oxidation and corrosion.

Replacement should be triggered by a measured rise in resistance or by visible wear of the plating, not by a count of actuations alone. A fixture that runs millions of cycles on clean gold pads will outlive a fixture that runs a fraction of that on a flux contaminated surface, so the actuation count is a poor predictor.

Fixture Mechanics: Alignment and Planarity

A fixture applies force through a plate, and the plate has to be parallel to the board. Planarity errors mean that some probes are over compressed while others barely touch, and the symptom is a set of failures that moves when the board is rotated in the fixture.

Alignment is the other half of the problem. Probe positions must match the test points within a fraction of the pad size, and a misalignment of a tenth of a millimetre on a small pad reduces the contact area sharply. Verification with a target board that has known pad positions is the simplest way to confirm both. Where the fixture uses a vacuum or pneumatic clamp, the closing force should also be checked, since an uneven clamp can lift one side of the board and change the compression on every probe beneath it.

Measurement and Verification Routines

A reference artefact with known resistances allows the fixture to be checked end to end. Measuring the artefact on a schedule and recording the result for each probe gives a history that shows which probes are drifting and how quickly.

Where a full artefact check is impractical, a sample of probes across the plate can be monitored, chosen to include the corners where planarity error is largest and the areas where flux is heaviest. The results feed the cleaning and replacement plan, and the plan then replaces the guesswork.

False Failures and Their Causes

Most false failures on a stable product come from the fixture rather than from the boards. A probe with a raised resistance produces a reading above limit, a probe that is not making contact produces an open circuit, and a probe that is over compressed can damage the pad it touches.

Diagnosis follows the pattern. Failures that move with the board indicate a board issue, failures that stay on the same net indicate a probe, and failures that appear only at one corner of the fixture indicate planarity. A flying probe re-test of a few failed boards, as described in the notes on flying probe testing, separates the two quickly.

FAQ

What contact resistance should a pogo pin have? A new probe typically presents 20 to 50 milliohms at the interface. The acceptable value depends on the measurement being made, and the probe budget should be a fraction of the tolerance being tested.

How often should pogo pins be cleaned? Cleaning intervals should be set from measurement. Checking contact resistance on a reference artefact weekly shows the trend, and a probe whose resistance has doubled should be cleaned or replaced.

Why do test failures move when a board is rotated in the fixture? That pattern points to a mechanical problem such as a planarity or alignment error, because the board is being presented to a different set of probes. It is rarely an electrical fault on the board itself.

}