Tin vs Silver vs Nickel Plating on Copper: Which Finish for Busbars, Terminals and Cold Plates

01Start With Bare Copper

The Copper Development Association's busbar handbook takes a position that surprises most designers: plating the contact faces of a copper-to-copper bolted joint is not recommended unless corrosion in the environment requires it. A bolted copper joint conducts through many tiny cold welds between the two faces; a soft plating prevents those welds from forming and can flow under bolt load over time, reducing contact pressure. Bare copper in a clean, dry indoor enclosure, assembled with a joint compound, is the baseline the handbook's joint data assumes.[1]

Plating earns its place in four situations: the atmosphere is corrosive (humidity with sulfur, chlorine or ammonia compounds), the copper mates with a different metal, the joint is made and broken repeatedly, or a standard grants a higher temperature rise on condition that the terminations are plated. The handbook cites ANSI C37.20 as an example: a 65 °C rise is permitted with silver-plated or equivalent terminations, otherwise 30 °C. That single clause is why so much switchgear bar is silver-plated at the joints and bare elsewhere.[1]

02The Three Platings Side by Side

Sources: the scope statements of ASTM B545, B700 and B689, and the coatings appendix of the CDA busbar handbook.[1][2][3][4]

03Tin: The Default, With Two Cautions

Tin is the everyday finish on copper busbar, lugs and terminals: inexpensive, solderable, RoHS-compliant, and protective on copper in normal indoor exposure per ASTM B545. Its softness is an advantage at a bolted joint, where it deforms to fill the gaps between the two faces, and a disadvantage in exactly the way the handbook describes, since it can keep flowing after the bolts are torqued. Two more cautions come from the standards themselves.[1][2]

Whiskers. Pure tin can grow conductive filaments, microns wide and up to millimeters long, over months and years. The CDA handbook says flatly that pure tin coatings should be avoided on busbar for this reason, and the electronics industry's mitigation practice is written up in JEDEC JESD201 and JP002: a nickel underplate over copper, a matte rather than bright tin deposit, adequate thickness, and in some cases a post-plate bake. If the part sits near other conductors at a spacing a whisker could bridge, specify the mitigation, not just "tin plate."[1][5]

Porosity and thickness. ASTM B545 states that corrosion occurs at pores in the coating through galvanic couples with the copper beneath, and that porosity increases as thickness decreases, so a minimum thickness must be specified for each application. "Tin plate" with no thickness or class is an invitation to the thinnest deposit the plater can sell. B545 classifies tin coatings by service condition; pick the class for the environment and put it on the drawing.[2]

04Silver: For Current and Heat

Silver is the most conductive metal, and ASTM B700 lists electrical contact characteristics and high electrical and thermal conductivity as its reasons for existing. Its tarnish film is thin and breaks down under contact pressure, which is why silver-plated joints hold low resistance through years of thermal cycling and why the switchgear standards grant them the higher temperature rise. It is the choice for high-current disconnects, plug-in contacts and bolted joints that must run hot.[1][3]

Its two weaknesses are also in the standard's appendices. Silver tarnishes in sulfur-bearing atmospheres, forming sulfide films that do increase contact resistance where the pressure is low, so the handbook rules it out where sulfur compounds are present. And silver migrates: under a DC bias on a humid insulating surface it can grow dendrites between conductors. Neither matters at a torqued busbar joint in a clean enclosure; both matter on a terminal block with fine spacing in a damp one. B700 also defines types by purity and grades with anti-tarnish treatment; specify them rather than leaving it to the plater.[3]

05Nickel: For Hard Wear, Heat and Corrosion

Nickel is the hard one. ASTM B689 lists hardness, wear, load bearing, corrosion, heat-scaling and fretting resistance as the functional properties that separate engineering nickel from decorative nickel, and the CDA handbook calls it the preferred protective coating on busbar in corrosive atmospheres: cheap, durable, harder than the alternatives. It is the right finish on a copper part that is handled, mated repeatedly, runs hot, or lives in a plant atmosphere that would eat tin.[1][4]

The trade-off is at the joint. Nickel oxide is tough, so the handbook notes that nickel-plated joints need high contact pressure to be reliable; the bolt torque that is comfortable on bare or silver-plated copper may not be enough. Nickel is also less conductive than copper, silver or tin, which is irrelevant at a thin plating on a bolted face and relevant on a plated thread or a sliding contact.[1]

Two nickels exist. Electroplated nickel (B689) is the busbar and terminal finish. Electroless nickel (ASTM B733, AMS 2404) deposits uniformly in bores and threads without the edge build-up of electroplating, which makes it the usual nickel on a machined cold plate or manifold where the plating has to reach inside ports and channels and cannot be allowed to stack up on a seal cavity.[4]

06Underplates and Thickness

A thin nickel layer under tin or silver does three jobs: it blocks copper from diffusing into the top coat and forming intermetallics, it closes the pores that B545 warns about, and it is the primary whisker mitigation in JESD201. On any copper part that will see heat, humidity or a long service life, "over nickel underplate" belongs in the note. ASTM B689 names underplating for contacts as one of its purposes.[2][4][5]

On thickness, the standards use classes tied to service condition rather than one number, and the handbook's observation that protective coatings on busbar are relatively thin, in the range of 2 to 5 µm, is a useful sanity check. Thicker is not automatically better: it costs more, it builds up on edges and threads, and past the class the environment calls for it adds nothing. Match the class to the environment and let the plater's process meet it.[1]

07What Plating Does to the Machined Part

Plating is added metal, and on a machined part that has consequences the drawing has to anticipate.

08Writing the Plating Note

A complete note names the specification, the class or thickness, the underplate, the surfaces, and the sequence. The general format is in our finishes and coatings guide; for copper electrical parts it comes down to a few patterns:

The one line that saves the most trouble: "dimensions apply after plating" or "before plating," stated once in the notes. Without it, a thread, a bore or a seal cavity is a coin toss between the shop's interpretation and the plater's.