Tin-Plated Aluminium Bus Bar

Why aluminium bus bars are tin plated

Aluminium conducts about 61 % as well as copper by volume but weighs only about 30 % as much, so for the same current an aluminium bar is roughly half the weight of a copper one. Its weakness is the joint. Freshly cut aluminium forms an oxide film within seconds; the film is thin but hard and electrically insulating, and it regrows wherever it is scratched away.

In a bolted joint, current flows only through the small spots where the two surfaces actually touch. On bare aluminium those spots must break through the oxide. Over years of heating and cooling, aluminium also creeps slightly under bolt pressure and expands more than steel bolts, so contact pressure falls, the oxide regrows and resistance rises. Higher resistance means more heat, which accelerates the process. This is why poorly made bare aluminium joints develop hot spots.

Tin breaks this cycle. It is soft, so it deforms under bolt load and makes a large true contact area. Its own oxide is thin and easily broken. And because tin sits between aluminium and copper in the galvanic series, a tinned aluminium bar bolted to a tinned copper terminal has a much smaller galvanic difference at the interface than bare aluminium on bare copper, which reduces corrosion of the joint in humid air.

Which aluminium alloy is used under the tin?

The plating only affects the surface; current flows through the aluminium. Choose the alloy for conductivity and mechanical needs, then specify the tin. Conductivity values below are typical; minimums are set by the conductor standard and reported on the certificate.

For most switchgear, UPS and battery applications, 6101 is the practical choice because the higher strength keeps bolted joints tight and resists bending under short-circuit forces. 1350 is used where every percent of conductivity counts, such as long straight runs and smelter or rectifier bus, and where the bar is supported along its length.

Copper is about 100 % IACS. To carry the same current at the same temperature rise, an aluminium bar needs roughly 1.6 times the cross-section of a copper bar, yet still weighs about half as much.

How tin is plated onto aluminium

Tin cannot be plated directly onto aluminium with ordinary methods, because the oxide film reforms instantly and the deposit would peel. Aluminium is prepared with an immersion zinc (zincate) or stannate treatment, then usually given a copper or nickel strike before tin. ASTM B253 describes these preparation routes.

1. Machining firstCutting, punching, drilling, bending and deburring are done before plating, so hole walls, edges and cut ends are fully covered.
2. Cleaning and etchingNon-etching alkaline cleaning removes oils, then a light etch and desmut remove the existing oxide and alloy smut.
3. Zincate immersionThe bar is dipped in a zincate solution that replaces the oxide with a thin zinc film. A double zincate (strip and repeat) gives a finer, more adherent layer and is standard for high-reliability parts.
4. Copper or nickel strikeA thin strike layer of copper or nickel is deposited over the zinc. It protects the zinc from the tin bath and acts as a diffusion barrier between the aluminium substrate and the tin.
5. Tin electroplatingTin is electroplated from an acid sulphate or alkaline stannate bath to the specified thickness, as matte or bright tin.
6. Rinse, dry and inspectThe bars are rinsed and dried quickly to prevent staining, then checked for coverage, adhesion and thickness, usually by X-ray fluorescence (XRF).

How thick should the tin be?

Thickness is a balance between protection and cost. Very thin tin (under about 3 µm) can be porous and is consumed by diffusion over time; very thick tin adds cost and gives little extra benefit at a bolted joint. ASTM B545 groups electrodeposited tin into service classes by minimum thickness, from thin solderable coatings up to heavy deposits for severe service, and the choice is usually made by the environment.

State whether the thickness is a minimum at the contact areas or an average over the part, and where it is measured. On a long bar the contact pads matter most; edge build-up during plating means corners will always be thicker than the centre of a flat face. Ask for an XRF thickness report against agreed measurement points.

Sizes, processing and specification options

Common metric and imperial sizes are shown with the cross-section and weight per metre of the aluminium bar before plating (density 2.70 g/cm³). Tin at 10 µm adds well under 1 % to the weight.

Imperial bars are typically 1/8, 1/4, 3/8 and 1/2 inch thick in widths of 1 to 4 inches, with 1/4 × 2 in and 1/4 × 4 in among the most requested. Current ratings depend on enclosure, ventilation, ambient temperature and permitted temperature rise, so size the bar using your switchgear standard or a recognised ampacity table rather than a single number per size.

Design and installation tips for tinned aluminium joints

Tin melts at 232 °C, well above normal busbar operating temperatures, but it softens and diffuses faster at high temperature. Keep joints within the temperature-rise limits of your switchgear standard (for example IEC 61439-1 for low-voltage assemblies) and treat discoloured or hot joints as a maintenance warning.

Bare aluminium vs tin-plated aluminium vs copper busbar

Tin-plated aluminium is the middle path many panel builders now take: the bar is aluminium for weight and cost, while the joint behaves much more like a plated copper joint. For applications needing a copper surface over the whole bar, see copper-clad aluminium bus bar, which bonds a copper layer metallurgically to an aluminium core.

Where tin-plated aluminium busbars are used

• Low-voltage switchgear and panel boards: main and distribution bars, where bolted joints must stay stable for decades
• UPS systems and data centres: power distribution where temperature rise and energy loss must be kept low
• Energy storage and battery packs: many connection points and frequent thermal cycling; tin stabilises contact resistance at module and rack connections
• EV charging and power electronics: inverter, converter and charger bus, sometimes as laminated assemblies
• Bus duct (busway): plated contact areas at joints and tap-off points
• Earthing and neutral bars: tinned bars resist corrosion and accept copper earth conductors through suitable lugs
• Rectifiers and electrochemical plants: plated contact faces on heavy aluminium bus

A typical example: a panel builder replaces 10 × 100 mm copper main bars with 6101-T6 aluminium bars of about 1.6 times the cross-section, tin-plated over the joint areas. The bars are larger in section but roughly half the weight for the same rating, and the tinned contact faces bolt directly to tinned copper breaker terminals with spring washers.

Related busbar products and guides

How to order tin-plated aluminium bus bar

1. Send the drawingAlloy and temper, bar size, length, hole and slot pattern, bends, edge finish and quantity.
2. Define the platingMatte or bright tin, minimum thickness at contact areas, underlayer (copper or nickel), and whether plating is full or selective.
3. Agree inspectionDimensional report, XRF thickness points, adhesion check and any electrical tests.
4. QuotationWe confirm processing, documents and delivery terms (EXW, FOB, CIF or DAP).
5. PackingBars are separated with paper or film interleaves, protected from moisture and packed on pallets or in wooden cases, labelled with lot and report numbers for traceability.

Tin-plated aluminium (aluminum) bus bar: FAQ