Tinned Copper: When Bare Copper Needs a Coat

• Corrosion: tin resists moisture, chlorides from sea air and sulphur gases such as H₂S far better than bare copper does
• Solderability: tin is already solder-compatible, so tinned wire takes solder quickly with mild flux, even after storage
• Stable contact resistance: bolted tinned busbar joints keep a cleaner contact face over time than bare joints in polluted air
• Compatibility with aluminium: a tinned face reduces galvanic attack at copper-to-aluminium connections
• Insulation chemistry: tin acts as a barrier between copper and rubber or some PVC compounds that react with it

On tinned wire made to ASTM B33, the coating is two layers: a thin copper-tin intermetallic layer bonded to the copper, and an outer layer of essentially pure tin. The intermetallic layer is what makes the coating adhere; the free tin on top is what gives solderability. Wire is tinned by hot dipping or electroplating, either at finished size or at an intermediate size before the final draw and anneal.

Busbars and flats are normally electro-tinned after cutting and punching, so that the holes and edges are covered too. If your busbars will be drilled after tinning, the bare holes will need attention at assembly.

ASTM B33 does not rely on chemical analysis of the tin. Instead it checks two things. Continuity: cleaned samples are dipped alternately in hydrochloric acid and sodium polysulphide solution, and any spot where copper is exposed turns black. Adherence: the wire is wrapped round a mandrel no larger than four times its diameter, then dipped in polysulphide; if the coating has cracked, the copper beneath blackens.

Like soft bare wire, tinned wire is accepted on elongation and resistivity, not on tensile strength.

A little, and more on fine wire than heavy wire. Tin has only about 15 % of copper's conductivity, and in fine wire the coating makes up a larger share of the cross-section. ASTM B33 allows a maximum resistivity of about 0.15695 Ω·g/m² for the largest sizes, roughly 97.7 % IACS, rising to about 0.16454 Ω·g/m² (around 93 % IACS) for the finest. On busbar, where the tin is a few microns on a section several millimetres thick, the effect on DC resistance is negligible.

Ask for the resistivity figure on the certificate if your design depends on it, and quote the wire size, since the permitted limit changes with diameter.

The extra cost is worth it wherever air or soil is aggressive or where joints must be soldered reliably. Typical buyers include panel builders supplying coastal and chemical plants, marine and shipyard electrical contractors, sewage and effluent treatment plant projects, battery and UPS installers, and lightning protection contractors using tinned tape on roofs near the sea. Around Bangalore, where the air is mild, bare copper is usually fine indoors; the question is where the panel will finally be installed.

Tin whiskers are worth a mention for electronics buyers. Pure electroplated tin can, over months or years, grow fine conductive filaments from its surface, which matters on closely spaced circuit parts but not on busbars or earthing conductors. Hot-dipped coatings and reflowed tin are much less prone to it. If your parts go into electronics, tell us so the coating route can be discussed before the order is placed.

Tin melts at 232 °C and softens well before that, and the tin-copper intermetallic keeps growing at high temperature, eating into the free tin that makes the surface solderable. Tinned conductors are therefore generally used up to about 150 °C. For hotter service, such as furnace wiring or high-temperature PTFE wire, nickel plated copper is the usual alternative.

We can supply tinned copper as round wire, strips and busbar flats, on soft annealed or hard base metal of E.C. grade. Sizes and coating requirements are confirmed at enquiry.