Key takeaways
- Specify a minimum local thickness on the contact face, not an average over the whole part.
- Nickel underplate is a diffusion barrier, and it only works if it is continuous at the edges and bends.
- Porosity is a corrosion path to the base metal, so the pore count belongs in the inspection plan.
- Plating thickness sets how many arc operations the face can take before the base metal appears.
What the plating has to do
A contact plating is a barrier. It keeps the base metal from oxidising, gives the contact interface a stable surface and carries current through the small real area where two faces touch. It also takes the arc damage at make and break. Those jobs pull in different directions: a soft, conductive, thick layer is good for resistance and arc life, but it wears and costs more.
Thickness links the jobs. A thin layer saves silver or gold and gives a stable contact resistance only while it lasts. Once the arc or the wipe wears through, the base metal is exposed and the contact behaves like an unplated one, usually with worse corrosion because the two metals are now in contact.
Thickness is a minimum, not an average
Specify a minimum local thickness, not an average over the part. Plating builds on edges and thins in recesses, and the contact face is often not the thickest area. An average that meets the drawing can hide a face below the minimum, and the face is the only place where the number matters.
The measurement method matters as much as the number. X ray fluorescence is fast and non destructive, but it measures a spot, is sensitive to the base alloy and reads thin on curved or rough surfaces. Coulometric testing dissolves a defined area and is accurate for the coating, but it is destructive. A cross-section shows the top layer and the underplate together and is the reference method when a result is disputed.
State the method, the area and the number of readings on the drawing or in the inspection plan. Three readings across the contact face and one on a bend tell you more than a single reading in the middle of a flat. For a small contact, a reading that includes the edge or a hole is measuring the wrong geometry.
Nickel underplate and the diffusion barrier
Nickel is the usual barrier between a copper alloy and a tin, silver or gold top layer. It slows diffusion, so the top layer keeps its composition and the base metal does not migrate to the surface. It also hardens the surface and spreads the contact load over a larger area.
The barrier is only as good as its continuity. A thin or porous nickel layer lets copper reach the surface, and the top layer discolours or the contact resistance climbs. Nickel also adds stress and can crack at a tight bend, so the thickness and the ductility belong in the same callout. A bend that is formed after plating is the usual place where the barrier fails.
Common plating systems and their weak point
The table lists the systems that appear most often on contact drawings and the test that finds each one's weak point. The thickness ranges are typical practice for illustration, not a substitute for the product standard.
| System | Typical thickness | Porosity check | Main risk |
|---|---|---|---|
| Tin over copper | 2 to 5 µm tin | Nitric acid vapour or sulphur dioxide | Whiskers and fretting corrosion |
| Tin over nickel | 2 to 5 µm tin, 1 to 2 µm nickel | Nitric acid vapour | Pores at thin spots in the barrier |
| Silver over copper | 2 to 5 µm silver | Nitric acid vapour or salt spray | Tarnish, migration and pore corrosion |
| Silver over nickel | 2 to 5 µm silver, 1 to 2 µm nickel | Nitric acid vapour | Barrier cost and edge cracking |
| Gold over nickel | 0.5 to 1.5 µm gold, 1 to 2 µm nickel | Gel test or nitric acid vapour | Wear through at a thin edge |
Ranges are illustrative. The drawing, the product standard and the application set the requirement.
Porosity and how it is measured
Porosity is a network of pores through the plating to the base metal. It comes from bath chemistry, current density, surface finish, hydrogen and handling. A pore is a corrosion path: moisture and pollutants reach the base metal, corrosion products spread under the plating, and the contact resistance rises at the point where the current has to cross the damaged layer.
The tests use a reagent that attacks the base metal through the pores and leaves a mark. Nitric acid vapour is common for tin and silver on copper. Sulphur dioxide and salt spray are used for longer exposures. Electrographic and gel tests give a visual record that can be kept with the batch. Acceptance is a count or an area per unit, agreed before the test, because the same surface can look acceptable under one method and fail under another.
- Pore count per square centimetre of contact face
- Pore size and distribution, not only the total
- Base metal exposed at a sheared edge or a bend
- Corrosion products spreading under the layer
- Change in contact resistance after the exposure
Arc performance and plating life
At make and break the arc melts a small volume of the contact face. Plating thickness sets how many operations that volume can absorb before the base metal appears. Thin silver erodes through quickly on a load that switches often. Thick silver lasts longer but costs more and can deform under a high contact force.
The material matters as much as the thickness. Silver alloys resist erosion better than pure silver but have higher resistance. Tin resists arc erosion poorly, which is why tin plated contacts are usually for low power or signal duty. Gold is used where the current is too low to break a film, not where an arc has to be survived.
Records that make the number usable
Ask for the plating thickness results with the method and the measurement positions, the underplate thickness, the porosity test and its acceptance, and an adhesion check. A certificate that says the part is plated to specification, without numbers, cannot be compared with the next batch.
Keep those records with the batch. If a contact fails in the field, the first question is whether the plating was thin, porous or damaged at the edge. A cross-section from the same lot answers that question faster than a new test on a returned part, and it turns a complaint into a process change.
Frequently asked questions
What thickness of silver plating should a contact have?
It depends on the load, the number of operations and the environment. Thin silver is adequate for a light signal duty, while a switching contact that sees arcs needs more thickness and a nickel barrier. The drawing should state a minimum on the contact face, not an average.
Is XRF accurate enough for contact plating?
It is useful for routine checks on a flat, known alloy, but it reads thin on curved or rough surfaces and can be affected by the base metal. Use it with a calibrated standard, and confirm disputed results with a cross-section or a coulometric test.
What is a porosity test on plated contacts?
It is an exposure to a reagent that attacks the base metal through pores in the plating. The resulting marks are counted or compared with a reference. The method, exposure time and acceptance limit have to be agreed before the test, because they change the result.
Why is nickel used under silver and gold?
Nickel slows diffusion between the copper alloy and the top layer, so the surface keeps its composition and the contact resistance stays stable. It also hardens the surface. The benefit depends on a continuous layer, so the underplate thickness and its condition at bends both matter.
Sources and further reading
- ASTM B700, Standard specification for electrodeposited coatings of silver for engineering use
- ASTM B545, Standard specification for electrodeposited coatings of tin
- ASTM B488, Standard specification for electrodeposited coatings of gold for engineering use
Qumek Engineering Desk
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