Key takeaways
- Tin protects the copper surface and keeps contact resistance stable, but a thin flash is not a corrosion barrier.
- Specify the plating standard, the minimum thickness and the test method, not just the word tin.
- Tin is sacrificial to copper at pores and scratches, which is why it tolerates small surface damage well.
- At high temperature, tin and copper form intermetallics that change solderability and contact behaviour.
Why tin became the default
Copper oxidises in air. The oxide is thin and reasonably conductive compared with some metals, but it grows and it changes contact resistance over time. Tin covers the copper with a layer that forms its own stable oxide, and that oxide is thin enough to break down under normal contact pressure.
Tin is also soft. Under a bolted palm it deforms into the surface roughness of the terminal, which increases the real contact area. That is useful on a busbar joint where the surfaces are not perfectly flat. It also means tin can be damaged by a rough handling process, so the finish has to survive packing and installation.
The alternative finishes each have a gap. Silver has lower resistance but tarnishes in sulphur bearing air and costs more. Nickel is harder and more corrosion resistant but has higher contact resistance. Tin sits in the middle, which is why it is the default for lugs and terminals.
What thickness actually does
A plating thickness callout sets the minimum coating on the significant surface. A thin flash, sometimes under a micrometre, covers the metal for appearance but does not provide a reliable barrier. A working coating for a lug is usually several micrometres, with the exact figure set by the product standard or the application.
Thickness is not a single number across a part. It varies with the geometry: the inside of a barrel, the edge of a palm and the thread of a screw all plate differently. That is why the specification should say which surfaces are significant and where the minimum applies.
Plating also changes dimensions. A tin layer on a threaded part affects the fit, and a thick layer inside a barrel can reduce the clearance for the conductor. For a precision part, the plating allowance belongs on the drawing.
The plating process affects coverage. Barrel plating reaches the bore and the palm, but the current density varies with the part geometry, so the coating is often thinner in a recess and thicker on an exposed edge. Rack plating gives better control on a large palm but costs more per part. If the drawing demands a minimum in a recess, say so, because the process has to be chosen around that requirement.
A minimum thickness is not an average
If the drawing says 5 micrometres minimum, the measurement points and the method must be defined. An average of 5 with thin spots at the barrel entry is not the same as a 5 micrometre minimum across the contact surface.
Choosing a thickness band
The right band depends on the environment and the duty. An indoor control panel and a coastal outdoor joint do not need the same coating, and over-specifying thickness adds cost without improving the joint.
| Duty | Typical band | Reason |
|---|---|---|
| Indoor control panel | Thin to moderate | Dry, stable environment |
| General industrial | Moderate | Humidity and occasional contamination |
| Outdoor sheltered | Moderate to heavy | Condensation and temperature cycling |
| Coastal or washdown | Heavy | Chloride exposure and wetting |
| High temperature joint | Controlled, with intermetallic check | Tin copper growth at temperature |
| Crimp barrel | Uniform, adhesion tested | Must survive deformation without cracking |
Illustrative bands only. The product standard and the end application set the real requirement.
How tin behaves in corrosion
Tin is more noble than steel but less noble than copper in the galvanic series, so at a pore or a scratch the tin becomes the anode and corrodes in preference to the copper underneath. That is useful: the protection is local and the copper stays intact until the tin nearby is consumed.
The catch is that the corrosion product has to go somewhere. In a confined joint, tin oxide can build up at the interface and raise resistance. In a wet environment with chlorides, tin can corrode more quickly, especially if the coating is thin or porous.
Tin is generally resistant to the mildly acidic and alkaline conditions found in industrial air, but it is not a marine coating in the same class as a heavy nickel or a suitable paint system. If the joint is exposed to salt spray, ask for salt spray test data and check whether the test was run on the plated part or on a flat coupon.
Contact with aluminium is the other risk. Tin plated copper against aluminium still forms a galvanic couple, and the tin layer does not change the potential difference by much. A bimetallic transition or a suitable joint compound is still needed where an aluminium conductor lands on a tin plated palm.
Temperature and intermetallics
Tin and copper form intermetallic compounds at the interface, mainly Cu6Sn5, and the layer grows faster at higher temperature. The intermetallic is harder and more brittle than either metal. In a solder joint it affects wetting and long term strength. In a plated lug it changes the surface behaviour of the coating.
For a bolted joint, a thin intermetallic layer is usually not a problem because the contact is mechanical and the layer is stable. For a soldered joint, or for a part that is stored hot for a long time, the growth matters. Ask for the temperature rating and the storage conditions if the part is held for months before use.
Tin whiskers are a separate concern. Pure tin coatings can grow thin filaments over time, especially under stress. For fine pitch electronics this is a known risk. For a power lug the risk is lower because the spacing is large, but a specification that calls for pure tin should still consider whisker mitigation if the part sits near sensitive circuitry.
Bright and matte tin behave differently as well. Bright deposits are harder and smoother, which helps appearance and wear, while matte deposits are softer and often have better solderability and lower internal stress. For a crimped lug, a matte or semi bright finish is usually the safer choice because it deforms with the barrel instead of cracking.
How to verify the finish
The common measurement methods are X-ray fluorescence, coulometric testing and a cross-section. XRF is fast and non-destructive but needs a calibration for the coating and substrate. A cross-section is slower but shows the layer and the adhesion at the same time.
Adhesion is checked by a bend, a burnish or a tape test, depending on the standard. For a crimped lug, the most useful check is after crimping: the plating should not crack or flake at the barrel bend. A coating that passes a flat coupon test can still fail on a formed part.
Record the thickness, the method and the measurement points on the inspection sheet. If the part is supplied with a certificate, check that the certificate names the standard and the minimum thickness rather than a generic statement that the part is tin plated.
- Plating standard and minimum thickness on the drawing
- Significant surfaces and measurement points defined
- Adhesion test method and acceptance
- Thickness check after forming or crimping
- Salt spray or humidity test only if the environment needs it
- Certificate naming the standard and the measured values
A specification that works
Write the finish as a standard plus a thickness plus a test. For example, name the tin electroplating standard, the minimum thickness on the contact surfaces and the adhesion test. Add a note that the coating must be continuous at the barrel and must not bridge any thread.
If the part is crimped, add a post-crimp visual check for cracks and flakes. If the joint is critical, add a contact resistance measurement at commissioning so a later change can be compared with a baseline.
Keep a sample of the plated part from each lot where the finish is critical. A colour photograph and a measured thickness at a stated point give the next inspector something to compare against, which is more useful than a certificate alone.
Finally, write the finish into the purchase order as well as the drawing. A drawing note can be missed when a part is re-quoted, and plating is one of the easiest features to downgrade without changing the shape of the part.
Frequently asked questions
How thick should tin plating be on a lug?
It depends on the environment and the product standard. Indoor parts need less than outdoor or coastal parts. The important thing is to specify a minimum thickness on defined surfaces with a test method, rather than accepting a cosmetic flash.
Does tin plating prevent corrosion completely?
No. It slows corrosion and protects the copper at pores and scratches by acting as a sacrificial layer. In aggressive chloride or wet service the coating can still degrade, so the thickness and the test evidence need to match the environment.
Is tin plating safe on a crimped lug?
Yes, if the coating is thin enough and adheres well. A thick or brittle coating can crack at the crimp. Check the plating after crimping as part of the first article, and keep the thickness within the range the lug maker specifies.
Why does tin plated copper change colour over time?
The tin oxide layer thickens and the surface can dull, and at higher temperatures intermetallic growth changes the appearance. A colour change alone is not a failure, but it should be checked against the resistance and the joint temperature.
Sources and further reading
- ISO 2093, Electroplated coatings of tin
- ASTM B545, Electrodeposited coatings of tin
- IEC 61238-1, Compression and mechanical connectors for power cables
Qumek Engineering Desk
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