Test Pin Tip Geometry and the Contact Reading You Get

When a continuity test starts drifting, the first suspicion is usually the instrument or the fixture wiring. Quite often the culprit is far smaller: the tiny crown of metal at the end of a spring-loaded test pin, worn flat after a few hundred thousand cycles.

Contact area is not the whole story

It is tempting to assume that a bigger, flatter tip gives a lower resistance because more metal touches. On real surfaces the opposite frequently happens. A sharp or radiused tip concentrates the available spring force into a small area, breaks through oxide and contamination, and makes a stable metal-to-metal contact. A flat tip spreads the same force over a wider area, so the local pressure drops and the probe sits on the oxide layer instead of punching through it.

That is why tip style is chosen for the target, not for the datasheet. A pointed or crowned tip suits solder pads and plated through-holes. A flat or serrated head suits bare vias, terminals and battery contacts where a sharp point would damage the surface or skid off. A cupped tip locates on a pin or a solder ball. Swapping styles without re-validating the fixture is a reliable way to introduce intermittent failures.

Numbers that are actually worth watching

Parameter	Why it moves the reading
Spring force at working stroke	Sets contact pressure; too low gives unstable milliohm readings
Working stroke used	Most pins are specified at 2/3 stroke; running near full travel shortens life
Tip radius / crown wear	Changes local pressure and can double contact resistance
Plating (gold over nickel)	Protects against oxide; once worn through, resistance climbs with humidity
Barrel and plunger interface	Adds a second, often overlooked resistance in series

A typical gold-plated spring probe sits in the tens of milliohms when new. The absolute number is less useful than the trend. If a fixture's loop resistance creeps from 40 mΩ to 120 mΩ over a few weeks, clean the tips first, then measure a known short, then consider replacing pins. Doing it in that order saves a lot of unnecessary re-wiring.

Measure it in a way that means something

Two-wire continuity checks include the leads, the switch and the probe's own barrel resistance. For anything below an ohm, use a four-wire (Kelvin) arrangement or at least subtract a measured short-circuit baseline. Otherwise a 30 mΩ change in the probe is invisible against a 300 mΩ lead.

Also measure at the same stroke the fixture uses. A probe checked with a multimeter on the bench, plunger barely depressed, is not the probe that exists in the test jig. Worn springs lose force at working stroke before they lose it at rest.

Cleaning, and when cleaning stops working

Flux residue, oxide and dust are the normal causes of a slow rise in resistance. A soft brush with isopropyl alcohol, or a dedicated tip-cleaning pad with a light abrasive, usually brings the reading back. Avoid files, emery cloth and aggressive scraping: they remove the gold in seconds and expose nickel or brass, which then oxidise faster than the original surface.

• Log a baseline loop resistance for each fixture when it is commissioned.
• Clean on a cycle based on actuations, not on calendar weeks.
• Replace pins as a set on a densely packed fixture; mixed new and worn pins give uneven pressure.
• Check probe alignment and board support at the same time — a bent plate changes the stroke.

The quiet failures

Not every problem shows up as a rising resistance. A plunger that sticks partway gives an open circuit that clears on the next cycle, so the test passes on retest and the fault escapes. A cracked barrel can read fine at rest and go open under compression. If a fixture produces a cluster of first-pass failures that all pass when retested, look at pin travel and spring condition before blaming the boards. Intermittent behaviour in a fixture is almost always mechanical.