Plating & Surface Finish for Stamped Parts

A stamped contact that looks perfect on the inspection bench can still fail in the field. The failure rarely starts with the steel, the brass, or the phosphor bronze underneath. It starts at the surface. Three problems show up again and again in returned lots: coating delamination, where the plating lifts off the base metal under thermal cycling or a tight bend; salt-spray failure, where red rust blooms through a layer that passed the eye test; and insufficient conductivity, where a tin or nickel finish drives contact resistance past the spec and the circuit drops out. Get the finish right and the part runs for twenty years. Get it wrong and no amount of tight-tolerance blanking saves it, which is why the finish spec on the drawing matters as much as the dimension block beside it.

This guide covers the finish decisions behind precision stamped parts: what each common finish - tin, nickel, gold, silver, zinc, e-coat, and passivation - actually does, the thickness grades that define them, how the finish interacts with the base metal and the tolerance budget, when selective plating beats full plating, which qualification tests prove the coating, and how to put a finish spec on the drawing that a plater can execute without guessing. The production reference throughout is a stamping plant with managed selective reel-to-reel plating: gold, silver, tin, and nickel at 2-8 µm, zinc at 5-12 µm, validated to ASTM B117 salt-spray testing, so the finish is part of the same quality system as the die that made the part.

The Four Jobs of a Surface Finish

Before any coating is chosen, name the substrate. Cold-rolled steel rusts in days if it is left bare in a humid warehouse. Brass and phosphor bronze hold up better but still oxidize, lose solderability, and tarnish. Stainless 304 and 316 resist corrosion on their own but need passivation to reach their rated life. The plating or finish you choose is meant to do one or more of four jobs, and the job list decides everything else:

• Block corrosion - keep the base metal from oxidizing in the storage, shipping, and service environment
• Make the surface solderable - a finish that wets solder reliably within the assembly process window
• Carry current - a low-resistance, stable surface for the contact interface or the current path
• Protect a tight tolerance - a coating that does not eat the dimension budget or drift with humidity

A single finish rarely does all four well. Tin solders beautifully but conducts poorly. Gold conducts and resists corrosion but costs too much to plate everywhere. Nickel protects but does not solder easily and adds contact resistance. That is why most precision stamped parts run a duplex system: a nickel barrier under a thin functional topcoat, with the nickel doing the corrosion job and the topcoat doing the electrical or solderability job. The layer stack is a design decision, not a plater's preference, and it belongs on the drawing as explicitly as the material grade.

The base metal sets the ceiling for the whole system. A finish can only be as good as the surface it is deposited on: scale, oil, and drawing lubricant residue kill adhesion before the coating is a micron thick. Stamped parts come off the press with process oils and fine metallic debris on the surface, so the cleaning line - degrease, rinse, and in some cases acid activation - is part of the finish system, not a courtesy. When a coating delaminates, the first question is not the plating chemistry; it is whether the pre-treatment removed everything the die left behind. The metal stamping materials guide covers the substrate side of this pairing, material by material.

Base metal	Bare-surface behavior	Typical finish strategy
Cold-rolled steel	Rusts quickly; poor solderability	Zinc, nickel, or e-coat for corrosion; tin or nickel for solder
Brass	Tarnishes; oxidizes at temperature	Nickel underplate plus tin or gold topcoat
Phosphor bronze	Good spring properties; oxidizes in humid service	Tin over nickel for solder; gold for low-resistance contacts
Beryllium copper	Excellent spring; tarnishes	Gold over nickel for contact zones
Stainless steel	Self-protecting; needs passivation	Passivate to ASTM A967; no plating for most duties

Tin Plating: The Solderability Workhorse

Tin is the default finish on solderable terminals, lead frames, and connector pins, and ASTM B545 governs electrodeposited tin coatings. The standard grades thickness by minimum local thickness, not average, which matters when your feature is 0.2 mm wide and the current density across the rack is uneven. The grades that appear on real prints:

• Grade A: 2.5 µm (0.0001 in) - touch-up and non-critical surfaces
• Grade B: 5.0 µm - the workhorse for most stamped terminals
• Grade C: 7.5 µm - where bend zones and handling need margin
• Grade D: 12.5 µm (0.0005 in) - heavy-duty or severe environments

Tin melts at 232 °C, so it reflows easily and wets solder quickly at the 245 °C solder temperature called up by IPC/J-STD-002. That is why it beats nickel and gold for through-hole and wave-solder joints: it forms the intermetallic with the solder in seconds, and the joint is complete before the part can oxidize. The trade-off is conductivity - pure tin runs about 15% IACS, so it is a protective and solderable skin, not a current path. For a signal contact that carries milliamps it is fine; for a power terminal that carries tens of amps, the tin layer is a thin skin over the real conductor, and the design has to account for the contact interface, not the bulk metal.

Two failure modes dominate tin-plated stamped parts. The first is tin whiskers: hair-like conductive growths that can bridge fine-pitch features on lead frames and connectors. Whisker formation is mitigated by matte tin rather than bright tin, by a nickel underplate that blocks copper diffusion, and by reflow or fusing the deposit after plating. The acceptance tests are defined in the whisker standards - GEIA-STD-0005-2 and IEC 60068-2-82 - and a program shipping fine-pitch parts should specify the test and the mitigation up front rather than discovering whiskers at field return. The second failure mode is fretting corrosion: in a vibrating or thermally cycling contact, the soft tin wears through and the exposed base oxidizes, driving contact resistance up. Fretting is a contact-design problem as much as a plating problem, but the fix always involves a harder or thicker finish at the interface.

Tin also has a temperature ceiling. Continuous service above roughly 125 °C pushes tin toward oxidation, and automotive connector standards cap the highest-temperature tin grades at that region. Above the ceiling, the finish choice moves to gold or a qualified high-temperature alloy. The practical guidance for buyers: tin is the cost-effective default for solderable, moderate-environment stamped parts; it is the wrong answer for high-cycle contacts, high-temperature zones, or fine-pitch parts that cannot tolerate whisker risk.

Nickel Plating: The Barrier That Makes Everything Else Work

Where corrosion is the enemy and the part sees flexing, nickel is the layer you put down first. Two specifications cover it. ASTM B689 covers electrodeposited engineering nickel coatings with thickness classes running from 12.5 µm up to heavy engineering deposits, and QQ-N-290, the military-style nickel spec still quoted on defense and automotive prints, runs the other way for thin functional work:

• Class 5: 2.5 µm (0.0001 in) - minimal, cosmetic only
• Class 4: 5.0 µm - light-duty barrier
• Class 3: 7.5 µm - typical underplate for tin or gold
• Class 2: 12.5 µm - moderate corrosion service
• Class 1: 25 µm (0.0010 in) - severe service

Bright nickel lands at 300-500 HV, far harder than tin's 12-20 HV, so it resists abrasion and gives the topcoat something to grip. That hardness is exactly why adhesion failures drop when a nickel underplate is specified: the topcoat keys into a hard, uniform surface instead of a soft, variable one. Nickel is also the diffusion barrier that stops the topcoat metal and the base metal from swapping atoms: gold over bare copper drifts copper into the gold surface over time, and tin over bare copper grows the copper-tin intermetallic that consumes the solderable layer. A nickel underplate of 1.25-2.5 µm blocks both mechanisms, which is why the duplex stack - nickel barrier plus functional topcoat - is the default for precision stamped contacts.

Nickel has limits that matter on the drawing. Conductivity is about 22-25% IACS, roughly double tin but still a fraction of copper, so a nickel surface is a poor contact interface for high-current duty unless it is the final finish and the application accepts the resistance. Nickel does not solder easily without activated fluxes, so solderable parts get tin or gold over the nickel, not bare nickel. And bright nickel deposits carry internal stress; a thick bright nickel on a thin spring can distort the part or crack at a bend, which is why spring parts often specify semi-bright or duplex nickel instead. Adhesion of the whole stack is verified by the ASTM B571 bend test - the coating must not flake after a 180-degree bend over a mandrel - and that test belongs on the control plan for any part that is formed, handled, or springs in service.

Gold Plating: Low Resistance at a Price

Gold earns its place on sliding contacts, gold-bond pads, and RF surfaces where contact resistance must stay flat for the life of the product. Hard gold, hardened with nickel or cobalt, runs 70-200 HV and is applied thin: 0.5-2.0 µm over a nickel underplate covers the vast majority of connector contact areas. Soft gold for wire bonding runs thicker and is not wear-rated. Gold's conductivity is about 70% IACS, close to copper, and it does not oxidize, so contact resistance stays in the low milliohm range for the life of the joint. In a selective reel-to-reel line, the managed plating capability puts gold exactly where the mating surface will land, typically at 2-8 µm on the functional zones, and leaves the rest of the part bare or with a cheaper finish.

The enemy of gold is porosity, not corrosion. A porous 0.5 µm gold over a thin nickel layer lets the base metal corrode underneath, and the corrosion products push up through the pores and drive contact resistance up. ASTM B735 porosity testing exists precisely to catch this: it exposes the plated surface to a reactive atmosphere and counts the corrosion spots. Below roughly 0.25 µm, gold stops protecting against porosity at all; the practical floor for wear and corrosion duty is around 0.76 µm (30 microinches), and the band buyers actually specify runs 0.5-3 µm depending on cycle life and environment. The nickel underplate, not the gold, is usually the limiting layer for salt-spray life, which is why a thick gold over thin nickel does not buy the corrosion performance buyers think it does.

Gold economics are the real selection driver. At a per-gram price an order of magnitude above tin, gold is specified only where the signal, the cycle life, or the reliability justifies it. Selective plating is how high-volume programs keep the cost sane: reel-to-reel selective gold deposits precious metal only on the contact zone, saving 40-60% of the gold compared with full-strip plating on a long terminal. The drawing has to show the plated zone for that to work - a vague callout forces the plater to guess, and the guess usually costs gold on one side or performance on the other. For the full decision logic between gold, silver, and tin on connector surfaces, the terminal plating guide walks each finish against cycle life, temperature, and current.

Silver Plating: Maximum Conductivity, Manageable Tarnish

If the requirement is the lowest possible resistance, silver wins. It runs about 105% IACS - better than copper - and it is cheap relative to gold, so silver-plated busbars, switchgear contacts, and high-current terminals get the ampacity of a noble surface at a fraction of the gold cost. Stamped silver finishes typically run 2-12 µm, and the in-house selective line plates silver at 2-8 µm on high-current zones. The busbar sizing and plating guide covers where silver earns its place on current-carrying stamped parts.

The problem is tarnish. Silver sulfide forms on exposure to sulfur compounds in the air, and the tarnish film drives contact resistance up over time. That is why silver-plated power contacts often carry an anti-tarnish treatment or a thin organic topcoat, and why silver on low-current signal contacts is usually a mistake - the tarnish risk tips low-current designs to gold, which does not form a resistive film. Silver also migrates under bias and humidity: energized silver features can grow dendrites across an insulating gap, which is a design-level failure mode you avoid by keeping silver away from fine-pitch energized geometry. Where the application is high current and low voltage drop - busbars, power terminals, switchgear - nothing beats silver; where it is a low-current signal path, gold is the safer call. The connector terminal stamping guide develops this signal-versus-power split in detail.

Zinc and Cost-Driven Finishes

Zinc is the workhorse corrosion finish for structural steel stampings - brackets, housings, mounting hardware - where electrical function is not required. It protects by sacrificial action: zinc corrodes in preference to the steel underneath, so a scratch that exposes the base does not start rust the way it would under a barrier coating. The plating line applies zinc at 5-12 µm, and the thickness choice tracks the service environment and the salt-spray hours the buyer specifies. Thin zinc on a dry indoor part is a cosmetic and light-protective finish; thicker zinc, often with a chromate or other conversion treatment, carries outdoor and automotive duty.

Zinc has two properties that trip up new specifiers. First, it is not solderable and it is not a good contact surface - it belongs on structural parts, never on current-carrying or solderable features. Second, the finish is porous and absorbs handling and storage damage, so the salt-spray result depends as much on the conversion treatment and the sealing step as on the zinc thickness. When a drawing says only "zinc plated," the plater has no way to know whether the buyer needs 24 hours or 240 hours of ASTM B117 salt spray; the spec has to name the thickness grade, the conversion treatment, and the acceptance hours. For steel parts that must both resist corrosion and stay non-conductive, e-coat below is the alternative that trades zinc's sacrificial protection for a true dielectric barrier.

E-Coat and Passivation: When the Part Must Not Conduct

E-coat (electrophoretic coating) is the finish you pick when the part must not conduct and must not rust. Cathodic epoxy e-coat throws into recesses and blind features that rack plating misses, which is a real advantage on deep-drawn or folded stamped housings. Typical film builds are 10-35 µm, cured at 160-200 °C for 20-30 minutes, and the result is a continuous dielectric barrier that delivers hundreds of hours of ASTM B117 salt-spray resistance on steel - far beyond what bare tin or thin nickel can manage. The cost is that it is an insulator by design, so you cannot e-coat a contact area: any feature that must carry current or mate electrically has to be masked, plated first, or left for a secondary selective finish. E-coat belongs on brackets, shields, motor housings, and structural stampings where appearance, corrosion, and edge coverage matter more than electrical function.

Stainless stamped parts do not get plated; they get passivated. ASTM A967 covers nitric and citric acid passivation of stainless steel, which strips free iron from the surface and lets the chromium form its own protective oxide film. The film is sub-micron - there is no thickness to measure the way you measure plating - but the effect is large: a 304 part straight off the press can show rust at the shear edges within a few hundred hours of salt spray, while a properly passivated 316 part runs into the thousands of hours. The shear edges are the tell: blanking and stamping expose fresh, iron-rich metal at the cut line, and that is exactly where rust starts if passivation is skipped. It is the cheapest finish on this list and the most commonly omitted on incoming prints, because buyers assume stainless means rust-proof. It does not; it means rust-proof after the surface is properly prepared. The stainless steel stamping guide covers the material and passivation pairing for stamped stainless parts.

Selective Plating, Thickness, and Tolerances

The cost of a finish tracks the amount of surface it covers and the metal it uses. Full plating covers the entire strip or the whole part, which is simple to inspect and impossible to mis-zone - and wasteful when the precious metal only needs to be in one place. Selective plating deposits the functional finish only where the drawing calls for it: gold on the contact zone, tin on the solder tail, nickel as a barrier where corrosion threatens, and nothing where the base metal can stay bare. On a long terminal, selective gold over the mating surface saves 40-60% of the gold cost versus full-strip gold, and that saving compounds across millions of parts.

Reel-to-reel plating is how selective finishing scales to high-volume stamping. The strip is plated in continuous coil form, either before stamping (pre-plated strip) or after (post-stamped reel-to-reel), with masking that defines the plated zones to the same accuracy as the die. The in-house selective reel-to-reel lines plate gold, silver, tin, and nickel at 2-8 µm and zinc at 5-12 µm, which covers the functional range of most stamped connector and automotive hardware. The pre-plated versus post-plated decision is a real design fork: pre-plated strip runs the finish through the die, so the plating is carried into bends and draws and the tooling has to tolerate the coating, while post-stamped plating plates the finished geometry and keeps the die running on bare metal. The pre-plated metal stamping guide details when each route wins on cost and quality.

Selective plating raises one requirement that full plating does not: the drawing must show the plated zone. A zone callout belongs in the same view as the bend lines, with the thickness as a minimum local value, because the plater masks to that boundary and the stamping house inspects to it. Vague callouts like "plate contact area" force guesses, and the guess either wastes gold or leaves the mating surface bare. The same zone logic applies to the tolerance block: if the plated zone lands inside a tight dimension, the finish thickness is part of that dimension, and the stack has to be designed with the coating in it.

Here is the trap that catches new designers. A 12.5 µm plating on each side of a 0.3 mm stamped spring adds 25 µm to the section, and that thickness is not perfectly uniform across a rack. On a feature held to ±0.02 mm, the finish alone consumes the whole band before stamping variation is accounted for. For tight-tolerance precision stampings the finish is pushed thin and duplex - 5 µm nickel under 2 µm tin, for example - rather than a single thick layer, and the drawing specifies the minimum local thickness from the governing standard (ASTM B545 for tin, QQ-N-290 for nickel) rather than an average. Average thickness hides thin edges, and thin edges are where failure starts.

Thickness is verified by XRF (X-ray fluorescence) on production samples, which measures the coated part non-destructively and reports the local thickness at the measured spot. For the critical zones - the contact surface, the bend radius, the solder tail - the measurement points have to be defined on the drawing, because a plater can hit 5 µm average and still be thin at the bend where the current density drops. Cross-sectioning is the arbiter for adhesion and true thickness at features XRF cannot reach, and it is the standard first-article check for duplex stacks. The inspection plan should name both: XRF on the sampling plan, cross-section at first article and after any process change.

Salt-spray testing is the acceptance test that ties the whole system together. ASTM B117 exposes coated samples to a salt fog for a specified number of hours and the samples are examined for corrosion. The hours a finish can hold depend on the substrate, the layer stack, and the conversion or sealing steps, which is why the salt-spray requirement belongs on the drawing as a number, not a hope: the buyer names the hours, the plater builds the stack that achieves them, and the lab proves it on samples from the same lot. A finish that passes the eye test but fails salt spray was never going to survive the customer's environment; catching it at qualification is the cheapest place to find out.

The Finish Selection Decision Map

The decision is a short ladder, and every rung is a data point you can put on the drawing. Work through it in order and the finish picks itself:

• Is the part stainless? Passivate to ASTM A967 and stop there; plating adds cost without adding function
• Does the surface need to solder? Tin, Grade B or C, over a nickel underplate if the environment is harsh
• Does it carry signal current with low, stable resistance? Gold 0.8-1.5 µm over 5-8 µm nickel, selective if the volume justifies it
• Does it carry high current and is cost tight? Silver 5-12 µm with anti-tarnish treatment, not gold
• Does it need to resist rust but stay non-conductive? E-coat 15-25 µm
• Is it structural steel with no electrical function? Zinc 5-12 µm with a named conversion treatment and salt-spray hours

Application	Finish stack	Typical thickness	Governing test
Solderable terminal	Matte tin over nickel	Sn 5-7.5 µm, Ni 1.25-2.5 µm	J-STD-002 solderability, B571 adhesion
Signal contact	Hard gold over nickel	Au 0.5-2 µm, Ni 1.25-2.5 µm	B735 porosity, contact resistance
High-current busbar	Silver, anti-tarnish	Ag 5-12 µm	Contact resistance, salt spray
Structural steel bracket	Zinc with conversion	Zn 5-12 µm	ASTM B117 salt spray
Non-conductive housing	Cathodic epoxy e-coat	10-35 µm	Salt spray, dielectric check
Stainless part	Passivation	Sub-micron oxide film	ASTM A967, salt spray

Every one of these choices is a spec line, not a description. The specification, the class or grade, the minimum local thickness, and the acceptance test all belong on the drawing or in the control plan. Vague calls like "plate it nickel" or "make it shiny" are how delamination and red rust get into the field - the plater executes what is written, and what is written has to be measurable. Failures on stamped parts trace back to a short list of root causes, and each one has a known fix.

Delamination - the coating lifting from the base metal - is almost always a pre-treatment or adhesion problem: residual die lubricant, scale, or an inactive surface under the deposit. The fix is cleaning and activation before plating, and the verification is the B571 bend test on samples from production, not from a test coupon. Red rust through a supposedly protective layer means the stack was too thin at that location or the substrate was not properly prepared; the fix is minimum local thickness callouts plus salt-spray sampling. High contact resistance on a plated contact means either the finish is the wrong material for the load (tin where gold belongs), the surface has oxidized in storage or service, or porosity has let the base metal corrode through. Whiskers on tin-plated fine-pitch parts mean the tin chemistry or the underplate was wrong; the fix is matte tin over nickel with a qualified whisker test. Tarnish or migration on silver means the environment or the geometry was wrong; the fix is anti-tarnish treatment and keeping silver away from energized fine-pitch gaps.

The common thread is that every one of these failures is visible at qualification if the right test is run, and invisible in a dimensional-only first article. A control plan that includes adhesion, salt spray, porosity, and solderability - matched to the finish and the application - catches the surface problems that a CMM cannot see. For spring parts and contacts specifically, the interaction between the finish and the mechanical function matters: a brittle deposit on a flexing beam cracks at the bend, and a thick deposit on a tight dimension eats the tolerance budget. The spring stamping guide covers the finish and stress side of stamped springs, and the battery contact stamping guide covers the corrosion and joint-resistance side of nickel-plated hardware.

How to Spec the Finish on Your Drawing

If you are sourcing precision stamped parts and the finish is not nailed down on the print, the cheapest fix is to write the spec before you quote. The complete finish spec has six lines: the finish material and type (for example, matte tin per ASTM B545); the grade or class; the minimum local thickness; the underplate and its thickness; the acceptance tests (salt spray hours, adhesion method, porosity method, solderability method); and the plated zones if selective plating is intended. Add the operating environment - temperature range, exposure, current load, and whether the part solders or mates - and the plater and the stamper can both execute without guessing.

Send us the drawing and the operating environment, and we will specify the finish stack - the ASTM or MIL grade, the duplex layers, and the minimum local thickness that protects the tolerance instead of eating it - then sample to salt spray and bend test so the first production lot is not the first field trial. The stamping lines, the managed reel-to-reel plating program, and the quality lab run under one quality system, which means the finish spec, the die, and the inspection plan are aligned before tooling is committed. Surface finish is where stamped parts live or die, and it is cheaper to get it right on paper than to explain a returned container. Request a quote with your drawing and environment and get a finish spec you can hold the supplier to.