Key takeaways
- Tin protects the copper and breaks its own oxide under clamp load, which keeps contact resistance stable at moderate current.
- Thickness is a process call, not a cosmetic one: 5 to 8 µm covers most indoor panel joints, with more for harsh or frequently mated joints.
- Adhesion matters more than thickness. A thick layer that lifts or flakes is worse than a thin layer that stays bonded.
- Mask threads, locating faces and any area that must remain bare, and say so on the drawing.
Why tin is the default
Copper oxidises in air, and the oxide is a poor conductor. A bare copper joint can work well if it is cleaned and clamped immediately, but the oxide returns and the contact resistance can drift. Tin covers the copper with a metal that forms a thin, stable oxide and deforms under clamp load, breaking that oxide at the contact points.
Tin is also easy to apply, easy to solder and relatively inexpensive. It covers the whole bar, including the edges and the hole bores, which protects the copper during storage and installation. That is why tin plating is the default for panel busbars and for many equipment connections.
Tin is not a conductor in the same class as copper. The plating adds a resistive layer, but the layer is thin and the contact points are metallic. The joint design, the clamp force and the contact area matter more than the plating thickness once the layer is continuous and adherent.
Thickness classes
Thickness is usually specified in micrometres and measured on a coupon, a witness piece or a defined area of the part. The table below gives starting classes for copper busbar. The values are illustrative and should be checked against the product standard, the customer call and the environment.
| Class | Thickness range | Typical use | What to check |
|---|---|---|---|
| Flash | 1 to 2 µm | Indoor, low contact force, short life | Coverage and adhesion more than thickness |
| Standard | 5 to 8 µm | Panel joints and indoor equipment | Thickness on the contact face and adhesion |
| High build | 8 to 12 µm | Harsh atmosphere or frequent mating | Thickness, porosity and edge coverage |
Illustrative only. The applicable product standard and the joint design set the real requirement.
Adhesion and the preparation behind it
Adhesion starts before the plating tank. The copper has to be clean, free of oxide, oil, burrs and deep scratches. A poor surface preparation can leave a layer that looks uniform and lifts at the first bend or the first bolted joint. Blisters, nodules and patches of dull finish are signs to investigate, not to accept.
A simple bend test on a coupon and a tape pull test are common checks. A thermal cycle can be more revealing, because the copper and tin expand at different rates and a weak bond can fail at the interface. If the part is formed after plating, the bend zone is the place to look for cracking or flaking.
Adhesion is more important than thickness. A 10 µm layer that lifts under a washer is a liability, while a 5 µm layer that stays bonded and covers the contact face will do the job. The drawing should call adhesion by a recognised test method, not just ask for a 'good bond'.
Contact resistance and the joint
A bolted tin joint relies on the tin deforming at the high points so that metal touches metal through the oxide. The clamp force, the contact area and the surface finish set the result. A rough surface with deep scratches may need more force to reach the same contact area, and a burr can hold the faces apart.
Contact resistance is not a fixed property of the plating. It changes with the clamp load, the temperature, the current and the number of mating cycles. A joint that measures well at installation can rise if the clamp load relaxes or the tin flows. That is why a joint design includes a defined torque, a washer that maintains load and, for critical joints, a re-torque step.
If the joint runs hot, the tin can soften and flow further, which changes the contact area. The thermal check should use the joint resistance at operating temperature, not the value from a room temperature measurement. A hot joint and a cool bar can have the same average temperature and very different local conditions.
Whiskers and the nickel barrier
Tin can grow thin whiskers over time, especially under compressive stress. A whisker is a conducting filament that can bridge a small gap and cause a short. In a busbar panel, the risk is highest where the plating is under stress, where the pitch between conductors is small and where the environment cycles in temperature.
A nickel barrier between the copper and the tin is a common mitigation. It limits the diffusion of copper into the tin and reduces the stress that drives whisker growth. A matte tin finish and a controlled thickness also help. The barrier adds a process step and a cost, so it belongs where the risk justifies it, not on every part by default.
Whisker risk is a system question. Two bars that are 20 mm apart are not the same as two bars that are 2 mm apart. If the design has fine pitch, long service life and a safety function, ask the plater about the whisker mitigation and record what was chosen.
Masking, threads and contact faces
Not every surface should be plated. A thread that is tinned may not accept the nut, or may seize after a few cycles. A locating diameter that is tinned may lose its fit. A face that has to make a low-resistance joint may need a controlled thickness or a bare copper area.
Masking is the usual answer, but it adds labour and can leave a sharp plating edge. The drawing should show the masked areas, the allowable plating thickness at the boundary and whether the edge has to be free of nodules. If a thread is masked, state whether it is plugged or stopped off, because the two leave different evidence.
Contact faces are the other call. A tin layer on a joint face is normal, but the thickness has to be controlled and the surface has to be free of burrs, pits and contamination. If the joint is silver plated instead, the tin call does not apply, and the two finishes should not be mixed on the same contact face without a reason.
What to inspect on a tin plated bar
Inspect thickness on a defined area, not at a random point. Use a gauge that is calibrated for the coating and the substrate, and record the reading and the location. Check the contact faces, the hole bores and the edges, because these are the areas that carry current or handle the joint.
Check adhesion with the agreed test, and check appearance for blisters, nodules, patches and exposed copper. A visual check is not enough on its own, but it catches the defects that a thickness gauge will miss. If the part is formed after plating, inspect the bend zone for cracks.
Keep the certificate with the shipment. The certificate should state the finish, the thickness range, the test method and the date. If a rework or a re-plate has occurred, record it, because a second pass through the plating line can change the thickness and the adhesion.
- Thickness on the contact face, hole bore and edge.
- Adhesion by the agreed test method.
- Appearance: no blisters, nodules or exposed copper.
- Bend zone free of cracks if the part is formed after plating.
- Masked areas clear and free of plating nodules.
- Certificate with finish, thickness, method and date.
Frequently asked questions
How thick should tin plating be on a busbar?
For many indoor panel joints, 5 to 8 µm is a practical starting range. Harsh atmospheres or frequently mated joints may need 8 to 12 µm. The product standard and the joint design set the real requirement.
Does tin plating increase contact resistance?
It adds a thin resistive layer, but a sound tin joint breaks the oxide at the contact points and keeps resistance stable. The clamp force and the contact area matter more than the thickness.
Why use a nickel barrier under tin?
A nickel barrier limits copper diffusion into the tin and reduces the stress that can drive whisker growth. It adds a process step, so use it where the whisker risk or the service life justifies the cost.
Can threads be tin plated?
They can, but the plating changes the fit and can cause seizure. Many drawings mask threads or specify a plugged thread. State the requirement rather than leaving it to the plater.
Sources and further reading
- ISO 2093, Electroplated coatings of tin, specification and test methods
- IEC 61439-1, Low-voltage switchgear and controlgear assemblies
- Qumek busbar finishing and inspection notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Specifying a tin finish for a busbar?
Send the drawing and the joint conditions. The engineering desk will comment on thickness, masking and the inspection calls.
Send the drawing