Contact resistance and connection reliability

Two metal surfaces pressed together touch only at a small number of microscopic high points. Even polished contacts are rough on the micrometre scale, and the real area of metal-to-metal contact is a small fraction of the apparent contact area. All current must squeeze through these spots, and whatever covers them — oxide, sulphide, contamination — sits directly in the current path. Contact resistance, and with it most of a connector's electrical reliability, is decided at this scale.

This article looks at what contact resistance consists of, which design parameters control it, how it changes over life, how it is measured, and how it determines the current a connection can carry. It is written for engineers who compare datasheets, specify contact systems or investigate field failures.

What contact resistance consists of

The resistance measured across a mated contact pair is the sum of three parts: the bulk resistance of the contact material between the measurement points, the constriction resistance caused by current lines converging on the small contact spots, and the film resistance of any layer on those spots. Bulk resistance is fixed by geometry and alloy and is stable. Constriction and film resistance — together usually called the interface resistance — vary with force, surface condition and time, and they are what reliability engineering is concerned with.

a-spot

One of the microscopic areas where the two contact surfaces actually touch in metallic contact. The term comes from Holm's classic analysis of electrical contacts.

Constriction resistance

The extra resistance caused by current flowing through small a-spots. For a single circular spot of radius a between two bodies of resistivity ρ, Holm's result is R = ρ/(2a): resistance falls as the spot grows, and lower-resistivity materials help.

Film resistance

Resistance contributed by oxide, sulphide, adsorbed contaminants or polymer films on the contact spots. Thin films may be tunnelled through; thicker films must be mechanically broken.

Bulk resistance

The ordinary resistance of the contact body, crimp barrel and termination between the measurement points.

Because a-spots are tiny, constriction resistance depends mainly on how many spots there are and how large they are, and both depend on the force pressing the surfaces together and the hardness of the materials. Film resistance depends on the chemistry of the finish and the environment. A stable contact keeps enough clean a-spots under all conditions of use; an unstable one gradually loses them.

Normal force, wipe and contact geometry

The normal force is the contact designer's main lever. More force plastically deforms more asperities, creating more and larger a-spots, and helps break through surface films. It also clamps the interface against micromotion. The cost is higher mating force, faster wear of thin platings and greater sensitivity to stress relaxation of the spring material at elevated temperature.

Wipe is the sliding distance between first touch and the fully mated position. It scrapes contaminants and thin films away so that the final resting point lands on clean metal. Some wipe is essential for any finish; for tin, which forms a hard oxide immediately, generous wipe combined with high force is how a reliable interface is created. The resting position must also be well clear of the region where the plating was disturbed during mating.

• Contact geometry: curved-on-flat or crossed-cylinder geometries give a well-defined, high-pressure contact zone; flat-on-flat contacts spread force and make resistance less predictable.
• Redundancy: two or more independent contact points per circuit (split tines, multi-finger sockets, lamella contacts) reduce the chance that a particle or film opens the circuit.
• Spring material: copper alloys such as phosphor bronze, copper-nickel-silicon and beryllium copper differ in conductivity, strength and resistance to stress relaxation. The choice governs whether the normal force is still there after years at temperature.

Plating systems and why they must match

Contact finishes divide broadly into noble finishes, which do not form insulating oxides, and non-noble finishes, which do and therefore rely on mechanical means to keep the interface clean. The choice should follow the circuit, the environment and the number of mating cycles.

Durability and degradation over life

Initial contact resistance says little about reliability. What matters is how the resistance changes with mating cycles, temperature, environment and vibration. The main degradation mechanisms are well understood:

• Wear: each mating cycle removes some plating. A rated number of mating cycles assumes the specified plating thickness and normal force; exceeding it exposes the underplate or base metal.
• Fretting: micromotion from vibration or thermal cycling repeatedly exposes fresh metal, which oxidises; debris accumulates and resistance rises. This is covered in more detail in Connector selection for high-vibration environments.
• Corrosion: pore corrosion through thin gold, sulphide formation on silver and copper, and creep corrosion from contaminated atmospheres. Laboratory exposure such as the flowing mixed gas test of IEC 60068-2-60 is used to accelerate it.
• Stress relaxation: at elevated temperature the spring slowly loses preload, so normal force decreases. The effect depends on alloy and temperature and is a key reason for maximum operating temperature limits.
• Intermetallic growth: in tin over copper, copper-tin intermetallic compounds grow with time and temperature, consuming the free tin layer.

Because of these mechanisms, connector qualification programmes measure contact resistance before and after sequences of durability cycling, thermal ageing, humidity, corrosive gas and vibration, and judge the change in resistance as well as the absolute value. A datasheet that quotes only an initial maximum contact resistance tells you about the as-new condition, not about life.

Measuring contact resistance

Milliohm-level resistances must be measured with a four-terminal (Kelvin) arrangement: current is fed through one pair of leads and voltage is sensed with a separate pair, placed at defined points either side of the interface. Two-wire measurements include lead and probe resistance that can exceed the value being measured.

The test conditions themselves can change the result. A sufficiently high voltage or current can break down thin films electrically or heat a-spots until they soften — so a measurement made at power-circuit levels can make a degraded contact look healthy. For this reason, low-level or "dry circuit" methods limit the open-circuit voltage and current so that films are not disturbed. IEC 60512-2-1 (Test 2a) specifies a millivolt-level method; EIA-364-23 specifies low-level contact resistance (LLCR), typically with an open-circuit voltage limit of 20 mV and a current limit of 100 mA. Where a defined test current is wanted instead, IEC 60512-2-2 (Test 2b) provides a specified test current method.

Current, temperature rise and derating

Every milliohm of contact and termination resistance dissipates power as I²R. Heat raises the temperature of the contact, which accelerates oxidation, intermetallic growth and stress relaxation — which in turn raise resistance. In a well-designed contact this loop stays stable; in a marginal one it runs away, which is how overheated and melted power connectors usually originate.

Connector manufacturers characterise this with temperature-rise and derating tests. IEC 60512-5-1 (Test 5a) measures temperature rise at a given current, and IEC 60512-5-2 (Test 5b) establishes a current–temperature derating curve. The derating curve shows the permissible current as a function of ambient temperature, such that ambient temperature plus self-heating stays within the maximum operating temperature of the materials.

1. 01Read the derating curve at your actual ambient temperature, not at room temperature.
2. 02Check how many contacts were loaded simultaneously in the test; loading all contacts at once reduces the per-contact current considerably compared with a single loaded contact.
3. 03Check the conductor cross-section used in the test, because the cable carries heat away from the contact.
4. 04Allow margin for degradation over life, enclosure heating and neighbouring heat sources.

Termination quality belongs to the same budget. A poorly crimped or under-sized conductor adds resistance and heat right next to the contact interface; the termination methods and their checks are discussed in Cable assembly design considerations, and the way ratings and derating curves are presented in Understanding connector datasheets.