Terminal Plating: Gold, Silver or Tin - What to Specify

Plating is where connector programs quietly fail. The stamping can be dimensionally perfect and still be rejected because the finish was specified as "gold plated" with no thickness, no location, and no test standard. A plating note that says "gold" without saying how thick, where, over what underplate, and against which test is not a specification - it is a handover of the decision to the supplier, and the supplier will make the cheapest assumption that still passes an incoming inspection. That assumption becomes a field failure when the contact resistance climbs after 50,000 mating cycles, or the finish tarnishes in a sulfur-laden environment, or the solder joint wets poorly on the line.

This guide is written for buyers and engineers who specify stamped terminals, connector contacts, and busbar joints. It covers the five finishes that carry nearly every connector program - gold, silver, tin, nickel, and zinc - how to write a complete plating callout, how the base alloy changes the plating decision, when selective reel-to-reel plating pays for itself, and how to verify the result instead of trusting the certificate. Our own lines plate selectively on strip-fed progressive dies at 2-8 µm for gold, silver, tin, and nickel and 5-12 µm for zinc, with corrosion verified to ASTM B117 salt spray and adhesion to ASTM D3359, so the numbers below are the ones we actually run and audit.

The Snapshot

• Selective reel-to-reel plating is applied at 2-8 µm for gold, silver, tin, and nickel.
• Zinc for corrosion protection on steel runs 5-12 µm.
• Corrosion is verified to ASTM B117 neutral salt spray; adhesion to ASTM D3359 tape test.
• Selective plating puts precious metal only on the contact zone - the single biggest cost lever available to a buyer.
• Plating runs on reel-to-reel lines integrated with progressive stamping, so contacts stay on the carrier strip and are never hand-handled between forming and finishing.
• Mated contact features are held to ±0.01-0.02 mm on dies with ±0.005 mm piloted positioning, and the plated surface is protected by the same strip handling discipline.

The Five Finishes and When Each One Wins

Every plating decision is a trade between electrical performance, corrosion resistance, mechanical durability, solderability, and cost. No finish wins all five categories, and the correct answer changes with the contact force, the current, the mating cycles, and the environment on the RFQ. The table below is the starting point; the sections after it explain the reasoning behind each row.

Finish	Typical thickness	Best for	Watch out for
Gold	2-8 µm selective	Low-signal, low-force, mating-cycle-heavy contacts	Cost - never plate the whole part
Silver	2-8 µm	High-current power contacts, busbar joints	Tarnish in sulfur environments
Tin	2-8 µm	Solderable, cost-sensitive automotive terminals	Fretting under vibration; whisker control
Nickel	2-8 µm	Underplate barrier beneath gold or tin	Never omit it under precious metal
Zinc	5-12 µm	Steel brackets, chassis hardware	Specify B117 hours explicitly

Gold: the contact-metal benchmark

Gold is specified when the contact must not corrode, must not oxidize, and must keep a stable interface resistance over many mating cycles at low contact force. A low-force contact cannot break through oxide or tarnish films, so the surface has to stay clean by itself. Gold is the only finish in the table that does that across a wide environmental range, which is why it dominates signal contacts, sensor interfaces, and any terminal that carries millivolts or microamps where a fraction of an ohm of added resistance is a measurement error, not a nuisance.

The two things that limit gold are cost and wear. At 2-8 µm selective, the metal cost is contained, but plating a whole terminal in gold when only the contact zone needs it can double or triple the finishing cost of the part. That is why the location callout on the drawing matters more than the thickness callout. On the wear side, soft gold is chosen where the contact is never wiped under load; harder gold alloys are used on interfaces that see repeated insertion and withdrawal. The mechanical assumption and the plating assumption have to agree - a high-cycle interface needs a finish that survives the wipe, and the spring alloy underneath has to keep its force for the same life.

Silver: the high-current workhorse

Silver has the highest electrical and thermal conductivity of any metal, which makes it the natural finish for power contacts and busbar joints where current density, not signal integrity, is the constraint. A silver-plated contact runs cooler at the same current than an equivalent tin or nickel surface, and the finish is far cheaper than gold. For high-current automotive and EV power terminals, silver is frequently the technically correct answer that buyers overlook because gold sounds more premium.

Silver's weakness is tarnish. Sulfur compounds in industrial atmospheres, exhaust, and some rubber compounds react with silver to form a sulfide film that increases contact resistance. The film is not catastrophic at high current and high contact force, where it is mechanically broken through, but it is a real risk on low-force interfaces and in sealed connectors where the tarnish cannot be wiped. If the application is a sealed high-current joint, silver with an anti-tarnish treatment is a proven combination. If the application is an unsealed low-force signal contact, gold is the safer call.

Tin: the solderable, cost-driven default

Tin is the default finish for terminals that will be crimped or soldered, because it wets well, forms a reliable joint, and costs a fraction of precious metal. Automotive harness terminals are the classic example: millions of pieces, moderate current, crimped connections, and a finish that must survive the connector environment without breaking the piece price. At 2-8 µm, tin gives a solderable surface with real corrosion protection on copper alloys.

Tin has two well-documented failure modes that a spec should acknowledge. The first is fretting corrosion: under vibration, the tin surface micro-moves against the mating contact, the oxide is scraped and reformed, and the accumulated oxide debris drives contact resistance up until the connection runs hot. The second is whisker growth: tin can grow hair-like conductive filaments that risk short circuits in dense electronics. Both are managed by design - alloying the tin, controlling the plating process, and matching the finish to the vibration environment - which is why the application, not the brochure, should settle the choice between tin and gold.

Nickel: the barrier nobody should skip

Nickel appears in almost every precious-metal stack, and its job is usually invisible until it is missing. Copper and its alloys diffuse through thin gold layers at operating temperature; without a nickel barrier, copper reaches the surface, oxidizes, and the contact resistance drifts upward over the life of the product. The same barrier logic applies under tin on copper alloys that see elevated temperature. Nickel at 2-8 µm stops that migration and adds hardness and corrosion resistance to the stack.

The failure mode when nickel is omitted is slow and expensive: the connector passes first-article testing, ships in volume, and the field failures start appearing after thermal cycling in service. A callout that reads "gold 2-8 µm selective over nickel underplate" names the barrier explicitly. A callout that reads only "gold plated" leaves the underplate to the supplier - and the supplier's cheapest option is rarely the one that survives ten years in the field.

Zinc: for the steel, not the copper

Zinc finishes belong on steel stampings - brackets, chassis hardware, shields - where the job is corrosion protection, not conductivity. Zinc is sacrificial: it corrodes preferentially to the steel beneath it, so a scratch through the coating does not immediately start red rust at the scratch line. At 5-12 µm it carries a meaningful service life in moderate environments, and the acceptance criterion should be written in salt spray hours, not in coating color. Specify the ASTM B117 hours to red rust explicitly, because the difference between a 48-hour finish and a 200-hour finish is invisible on the surface and decisive in the field.

What a Complete Plating Callout Contains

Every plating note should answer six questions. If any of the six is missing, the supplier fills the gap with an assumption, and assumptions become deviations that surface at PPAP or in the field. Writing all six into the note costs one line on the drawing and removes most connector plating disputes.

• Metal and thickness range - for example, gold 2-8 µm over nickel underplate. A range, not a minimum: a single "minimum 3 µm" value invites the thinnest deposit that passes, while a range defines the process window the plater is expected to hold.
• Location - selective on the contact area, tin or bare metal elsewhere. Mark the plated zone directly on the drawing or with a note that names the feature. "Gold plated" without a zone means the whole part gets plated, and you pay for precious metal that contributes nothing.
• Underplate - the nickel barrier thickness stated separately from the top coat. "Gold 2-8 µm over nickel 2-8 µm" is a complete stack; "gold 2-8 µm" is a coin flip on whether the barrier exists.
• Test standards - ASTM B117 with the required hours to red rust, and ASTM D3359 with the adhesion class. The hours and the class are the acceptance criteria; without them, "passes salt spray" means whatever the supplier's lab decided it means.
• Compliance - REACH and RoHS declarations required at PPAP. Plating chemistry is a favorite place for restricted substances to hide, and the declaration belongs in the submission package, not in a follow-up email after the first shipment.
• Base material - plating behaviour differs on C11000 ETP copper, C5191 phosphor bronze, and C17200 beryllium copper. Name the alloy and temper on the drawing so the plater selects the right pre-treatment; a surface that plates beautifully on one alloy can blister on another.

Written out, a complete callout reads: "Au 2-8 µm selective on contact zone over Ni 2-8 µm underplate; balance tin. Verify per ASTM B117 (minimum 48 h to red rust) and ASTM D3359 class 5B." That single line removes most connector plating disputes, and it is the format our own drawings use.

Base Material Drives Finish Performance

The plating is a surface treatment on top of a mechanical part, and the part's spring properties, conductivity, and corrosion behaviour set the boundaries for what the finish has to achieve. Specifying the finish without the base alloy is like specifying the paint before the body shop knows the metal.

Spring contacts specified in C17200 beryllium copper retain contact force to elevated temperature, which is why they carry gold on high-cycle interfaces - the mechanical assumption and the plating assumption have to agree. Beryllium copper keeps the contact pressure high enough to hold a stable interface on a gold surface for the life of the product. C5191 phosphor bronze is the workhorse for formed spring terminals: it forms well, holds a reasonable spring rate, and costs less than beryllium copper, so it pairs naturally with tin or selective gold depending on the duty cycle. C11000 ETP copper is chosen for conductivity in power terminals where silver, not gold, is the right surface, because the current-carrying job rewards conductivity and the contact rarely sees low-force signal duty. C10200 OFHC copper is reserved for purity-critical conductors where oxygen content and conductivity consistency matter more than cost.

The practical consequence is that the finish table and the material table have to be read together. A power terminal in C11000 with a gold finish is usually a specification error: silver gives the same or better electrical performance at lower cost. A signal contact in C17200 with a tin finish is a risk: the tin oxide film that forms under fretting can overwhelm the contact force the spring was designed to deliver. The alloy decides the finish as often as the environment does.

Selective Reel-to-Reel Plating: Why the Process Matters

Reel-to-reel selective plating is the process that makes precious-metal finishes affordable on high-volume stamped parts. The strip passes through the plating line in a continuous band, and the precious metal is deposited only on the zones that need it - the contact area, the solder tail, or the mating surface - while the rest of the strip stays bare or takes a cheaper finish. The masked zones are defined by the same geometry as the die, so the plated area matches the functional area of the part instead of the footprint of the whole terminal.

The cost logic is simple arithmetic. Precious metal is bought by weight, and the weight of a full-surface deposit scales with the entire part area. A terminal whose contact zone is 20% of its surface area pays five times more for full plating than for selective plating of the same thickness. At millions of pieces per year, that difference is a line item large enough to decide whether the program is profitable. Selective plating is the single biggest cost lever available to a buyer who needs a precious-metal finish.

The process advantage goes beyond cost. Because the strip is plated before or between forming stations and the parts stay on the carrier, the plated surface is never touched by hand, never racked, and never re-fixtured between the press and the finish. That protects both the formed geometry - mated features held to ±0.01-0.02 mm on dies with ±0.005 mm piloted positioning - and the plated surface from handling damage. On our lines the plating step is scheduled as part of the strip flow for high-speed progressive stamping, so a terminal program moves from coil to finished, plated, inspected parts without leaving the process chain.

Selective plating also changes the tolerance conversation. A full-plate part has a thickness callout everywhere; a selectively plated part has a thickness callout in the plated zone and a different finish elsewhere, and the boundary between the two is a controlled edge. That edge should be defined on the drawing, because the transition zone is where masking registration errors show up. Ask for the registration capability of the selective plating step before you commit a tight contact geometry to it.

Setting and Measuring the Thickness Range

The thickness range on a callout is not a guess; it is a process window. Below the range, the finish stops performing its job - the gold becomes porous and the base metal corrodes through the pores, the tin loses solderability, the zinc stops protecting the steel. Above the range, you pay for metal that adds nothing, and in some cases thick deposits on formed features can crack at the bend line or interfere with the mating interface fit. The 2-8 µm band for gold, silver, tin, and nickel and the 5-12 µm band for zinc are practical windows that balance function and cost across the common terminal and bracket families.

Thickness is verified by measurement, not by certificate. X-ray fluorescence (XRF) is the standard non-destructive method for measuring plated thickness on finished parts, and it can measure the top coat and the nickel underplate in one pass. A plating callout should name the measurement method, the sampling plan, and the acceptance criteria just like a dimensional callout names the gauge. At PPAP, the plating data should include thickness readings across the strip width and along the strip length, because a reel-to-reel line can drift between the edges and the center of the strip, and the thickness at the part's contact zone is what matters, not the average of the reel.

There is a second measurement that belongs in the package: the adhesion test. ASTM D3359 is the tape test that pulls a cross-hatched grid of coating and rates the result by class, with class 5B meaning no coating is removed. Adhesion failures show up as blistering or flaking on formed features, and they are almost always a pre-treatment problem - the surface was not clean or not activated before plating. A supplier that includes D3359 results in the submission is telling you the surface preparation is controlled; a supplier that has never heard of the test is telling you something else.

Corrosion Testing, Compliance, and Documentation

Corrosion and Environmental Testing

Corrosion testing exists to prove the finish will survive the environment on the RFQ, and the environment should be stated on the RFQ. ASTM B117 neutral salt spray is the most common accelerated test: parts are exposed to a salt fog at controlled temperature, and the acceptance criterion is the number of hours to a defined level of corrosion - typically red rust on steel or base-metal corrosion at the pores on plated copper alloys. The hours belong in the callout because they are the only part of the test that is a decision; the chamber and the fog are the same for everyone.

Salt spray is a screening test, not a service-life model. A part that passes 48 hours of B117 has not been proven to last 48 months in a coastal installation; the test is used to compare finishes, to catch process drift, and to gate production lots. For that reason the spec should treat B117 hours as a minimum acceptance criterion and pair it with the application review: what atmosphere, what temperature, what condensation cycles, what chemical exposure. A connector under the hood of a vehicle lives in a different world than a terminal in a sealed telecom cabinet, and the finish that is over-specified for one is under-specified for the other.

For plated parts that also serve a mechanical function, the environmental test should run on formed parts, not on flat coupons. Plating behaves differently on a bend radius - the deposit stretches on the outside of the bend and compresses on the inside, and a marginal finish that survives flat will crack or flake at the formed feature. Testing the finished geometry is the only test that represents the shipped part.

Compliance and Documentation

Plating chemistry is regulated, and the regulations change faster than most supplier qualifications. REACH restricts substances in the European market; RoHS restricts hazardous substances in electrical and electronic equipment; both have a habit of showing up in plating processes through wet chemistry, brighteners, and pre-treatment baths. A buyer's plating callout should demand declarations at PPAP and updated declarations on any process change, because the plater's chemistry is part of your part.

The documentation package for a plated terminal should include: the plating specification and revision, the thickness measurement report by lot, the adhesion test results, the salt spray results with hours and outcome, the REACH and RoHS declarations, and the certificate of conformity. If the part is automotive, the plating data is part of the PPAP submission and should be tied to the same control plan that covers the dimensional features. A finish that is plated outside the control plan is a finish that can change without your knowledge.

Common Plating Failures and Their Root Causes

Most plating failures in the field trace back to a small set of root causes, and each one maps to a missing line in the callout. The table below is the checklist we use when a plated part comes back with a contact or corrosion complaint.

Failure	Symptom in the field	Root cause	Fix in the spec
Copper migration through gold	Contact resistance drifts up after thermal cycling	Nickel underplate missing or too thin	Name the underplate and its thickness
Corrosion at pores	Green or black corrosion on the contact surface	Gold too thin, porous deposit	Set the minimum thickness and measure by XRF
Tarnish on silver	Dark film, high resistance on low-force contacts	Sulfur exposure, no anti-tarnish treatment	Seal the joint or specify anti-tarnish
Fretting on tin	Intermittent contact under vibration	Tin oxide debris at the interface	Match finish to vibration duty; consider gold
Whisker growth	Short circuits in dense assemblies	Pure tin deposit, uncontrolled process	Require whisker-mitigated tin or alloy
Blistering at bends	Flaking finish on formed features	Poor pre-treatment or brittle deposit	Adhesion test on formed parts, D3359 class
Over-plating cost	Quoted price far above the budget	Full-surface precious metal specified	Define the selective zone on the drawing

Notice that every fix in the right-hand column is a line on the drawing or a test in the acceptance criteria. Plating failures are specification failures as often as they are process failures, and the cheapest time to fix them is before the die is cut and the process is qualified.

Plating Specification FAQ

Why a range instead of a minimum thickness?

A minimum-only callout lets the plater run at the bottom of the window, where the deposit is cheapest to produce and most likely to be porous. A range defines the process window the plater is expected to hold and makes drift visible in the measurement report. It also protects you from the other direction: a thick deposit on a formed contact can interfere with the fit or crack at the bend.

Is nickel underplate always required?

Under gold on copper alloys, essentially yes. Copper diffuses through thin gold at operating temperature, and the diffusion products oxidize at the surface and raise contact resistance. The nickel barrier is cheap relative to the failure it prevents. The same logic applies under tin when the part sees elevated temperature.

Silver or gold for a power contact?

For a high-current power contact, silver is usually the technically correct answer: it has the best conductivity of any metal and costs far less than gold. Gold earns its price on low-signal, low-force, high-cycle interfaces where the surface must stay clean by itself. If the power contact is sealed and the current is high, silver with anti-tarnish treatment is a proven combination.

How many salt spray hours should I specify?

As many as the environment requires and the finish can honestly deliver. The hours are the acceptance criterion, so they belong on the drawing. A good starting point is to define the environment on the RFQ - coastal, underhood, indoor, sealed - and let the supplier propose a finish and hours that pass it, then verify with a third-party test on the first article.

How do I verify the plating on receipt?

Ask for the XRF thickness report by lot, measured at the contact zone, across the strip width. Ask for the D3359 adhesion class and the B117 hours with outcome. Check that the certificate references your part number and your callout revision, not a generic material. The certificate that names your drawing is the only one that proves anything about your part.

When does selective plating pay for itself?

Whenever the functional zone is smaller than the part. The arithmetic scales with the ratio of plated area to part area: the smaller the contact zone relative to the whole terminal, the larger the saving. At high volume the saving is a line item; at low volume it still removes precious metal from scrap, since selective plating on the strip means the skeleton and the offal carry little or no precious metal.

The Bottom Line: Specify a System, Not a Metal

Do not specify a metal; specify a system. The complete callout names the base alloy, the underplate, the selective zone, the thickness range, and the test standard that proves the result - and it puts the environment on the RFQ so the finish can be chosen against the real duty, not against a brochure. That is the difference between a plating note that prevents disputes and a plating note that starts them.

For terminal and connector programs, the same discipline carries through the whole build: strip-fed progressive stamping with ±0.005 mm piloted positioning, mated features at ±0.01-0.02 mm, and selective reel-to-reel plating at 2-8 µm gold, silver, tin, and nickel or 5-12 µm zinc, all verified to ASTM B117 and ASTM D3359. If you want a plating scheme priced per option against your real mating interface and environment, start with the drawing. The adjacent guides on stamped terminal types and connector terminal stamping cover the part families this plating logic applies to, and our progressive die high-speed stamping service page shows how the strip flow, forming, and finishing integrate. Send us your drawing with the mating interface identified and we will return a plating scheme with cost deltas per option, plus the control plan and test package that go with it.