Finishing Stamped Parts: Zinc, Nickel, Tin and Selective Plating
Short answer: choose the finish by what the part must do, not by habit. Zinc electroplating at 5–12 µm is the cheapest corrosion answer for steel and typically buys 24–200+ hours of neutral salt spray depending on thickness and passivate. Nickel (3–10 µm) gives wear resistance and is the standard underplate for precious metals. Tin at 2–8 µm is the solderability finish for crimp and solder tabs. Selective plating deposits silver or gold only on the working contact zone and can cut precious-metal cost by 50–80% compared with full plating. A stamped part that needs nothing should get nothing.
A blanked and formed part leaves the die with fresh, active metal on every sheared edge, and bare steel will show rust within hours in humid air. Finishing is therefore a functional decision made on the drawing: corrosion life, solderability, contact resistance, wear, or simply shelf appearance. Plating happens after stamping (or on strip before stamping), and each route changes the part, the tool, and the price. Here is how a stamping shop actually thinks through the five common choices.
Why a Stamped Part Needs a Finish at All
Stamping does three things that matter for finishing. It cuts through the surface, exposing clean base metal at the edge; it embeds die lubricant into micro-pores and tight bends; and it can work-harden thin sections. That means a plated coil stamped afterwards will show bare metal on the cut edges — the number one misunderstanding buyers have about pre-plated strip. If corrosion of the edge is critical, you either post-plate, use a corrosion-resistant base material, or accept edge creep and design for it.
Cleaning is the step that decides plating quality. Die oil must be removed completely, usually in alkaline degreasing baths, before any coating goes on. A fingerprint or oil residue under a nickel layer becomes a corrosion blister later. Most stamping shops including ours run the cleaning-plating sequence through qualified plating partners, and the plating quality you get depends more on that cleaning discipline than on the coating chemistry itself. For the full process background, the metal stamping process guide explains where finishing sits in the production flow.
Zinc Plating: The Cheap Corrosion Answer for Steel
Zinc is sacrificial: it corrodes instead of the steel beneath it, which is why it protects scratches and edges far better than a barrier coating of equal thickness. Typical deposits are 5–12 µm. A clear trivalent passivate is the default today; yellow and black passivates add chromate-style protection without hexavalent chromium. The neutral salt spray numbers below are indicative — real hours depend on part geometry, edge ratio, and the lab running ASTM B117 — but they give a ranking that matches field experience.
Zinc has limits. It is a poor choice inside electronics because zinc whiskers can grow and short circuits; it is not solderable; and it gives only moderate wear resistance. For springs and other highly stressed parts, plating zinc onto hardened steel carries a hydrogen embrittlement risk that must be managed with a post-plate bake. When parts are barrel-plated in bulk, small stamped pieces tumble together, so very thin flat parts with fine features are better racked or plated on strip to avoid mechanical damage.
Nickel: Wear, Barrier and the Universal Underplate
Nickel is the second most common stamped-part finish, and it does three jobs. Electrolytic nickel (typically 3–10 µm, often bright) gives a hard, corrosion-resistant, reasonably wear-resistant surface that also looks good — which is why fasteners and visible hardware use it. Electroless nickel deposits uniformly on every internal surface of a complex stamped part with no electrical contact points, and the nickel-phosphorus layer is hard (commonly quoted 500–700 HV), making it the choice for parts with blind pockets, springs, and intricate geometry.
Nickel is also the standard underplate beneath silver and gold. A thin nickel layer (usually 1–3 µm minimum) blocks copper diffusion into the precious metal, and without it gold over copper develops porosity failures quickly. If your part is a stamped terminal that must carry signal currents for years, the layer stack usually reads nickel + selective gold, not gold alone. For more detail on coating logic for electrical parts, the stamped contact plating guide walks through the layer stacks used on real terminals.
Tin Plating: Solderability and Press-Fit Behavior
Tin exists on stamped parts for one main reason: solderability. A 2–8 µm matte tin layer over copper or brass wets well with standard lead-free fluxes and stays solderable through normal storage — typically 6–12 months under good warehouse conditions, after which oxide growth makes joints marginal without stronger flux. Bright tin looks shinier but matte tin is preferred for solder joints because it reflows more predictably. Tin is also used on press-fit and crimp terminals where its softness and low contact resistance help.
The tin caveat is whiskers. Pure electroplated tin can grow conductive whiskers over years, which matters in high-reliability electronics. Common mitigations are a minimum thickness around 8 µm, a post-plate reflow or anneal, and specifying matte tin with a whisker-mitigation statement in the purchase spec. Tin over nickel is a frequent combination: nickel as barrier, tin as the solderable and contact layer.
Selective Plating: Pay Only for the Surfaces That Work
Full plating of a stamped contact is wasteful: the working zone may be 10% of the surface, yet silver or gold is deposited everywhere. Selective plating fixes that by masking, brushing, or — most efficiently at volume — plating reel-to-reel strip only where the contact face will land after stamping. The saving is real: putting 0.3–0.8 µm gold only on the contact pad instead of over the whole terminal typically cuts precious-metal consumption by 50–80%, which dominates the finish cost line on connector parts.
The practical pattern for a high-volume contact is: stamp from copper alloy strip, apply a thin nickel barrier selectively or overall (it is cheap), then spot-deposit silver at 1–3 µm or gold at 0.1–0.8 µm on the mating zone. Because selective plating happens on strip before forming in many cases, the die designer must register the plated band to the forming stations. That is a design-for-manufacturing conversation to have early, and our stamping engineers will flag the plating pattern on your drawing before tooling, not after.
How the Finish Choice Moves Cost and Lead Time
| Finish (steel substrate) | Typical thickness | Indicative neutral salt spray to red rust | Primary job |
|---|---|---|---|
| Zinc, clear passivate | 5–8 µm | 24–72 h | Indoor corrosion protection |
| Zinc, yellow/black passivate | 8–12 µm | 72–200+ h | Corrosion, mild outdoor |
| Electrolytic nickel | 3–10 µm | 48–200 h (porosity-dependent) | Wear, appearance, underplate |
| Electroless nickel | 3–25 µm | 200–1000+ h at thicker deposits | Uniform coverage, hardness |
| Matte tin | 2–8 µm | Not a corrosion finish | Solderability, press-fit |
| Selective silver/gold over nickel | 1–3 µm Ag, 0.1–0.8 µm Au | n/a | Contact zone only |
| Finish | Cost vs zinc plating (indicative) | Typical lead-time adder | Notes |
|---|---|---|---|
| Zinc (barrel) | 1.0× baseline | 2–4 working days | Cheapest; whisker risk in electronics |
| Nickel (electrolytic) | 1.5–2.5× | 2–4 working days | Bright or semi-bright |
| Electroless nickel | 2–3× | 3–5 working days | Uniform on complex geometry |
| Matte tin | 1.5–2.5× | 2–4 working days | Check whisker spec |
| Selective precious metal | 3–10× on plated area, less overall | 3–7 working days | Big saving vs full precious plate |
The cost adders above are indicative and assume the parts are clean, standard geometry, and processed in reasonable batch sizes. Precious-metal prices move daily, so any selective gold or silver quote follows the metal market on the day of quotation — that is normal industry practice, not a hidden charge. Secondary operations after plating, like staking or welding, can disturb a coating and should be reviewed with the plater before the process is frozen; our stamping secondary operations article covers which post-die steps conflict with which finishes.
Frequently Asked Questions
Q: What zinc thickness should I specify for an outdoor steel bracket?
A: For a part exposed to weather, specify 8–12 µm zinc with a yellow or black trivalent passivate, which is indicative of 72–200+ hours to red rust in neutral salt spray. If the part must survive many years outdoors, stainless steel or a zinc-flake system will outperform zinc plating regardless of thickness, so confirm the target service life first.
Q: Does plating cover the sheared edges of a stamped part?
A: Post-plating deposits on cut edges too, though coverage at the very corner can be thinner than on flat faces. Pre-plated strip that is stamped afterwards exposes bare base metal on the cut edge, so edge corrosion is possible; post-plate the part or switch to a corrosion-resistant base material when edges are critical.
Q: Is it safe to plate high-hardness stamped springs and clips?
A: Plating hardened spring steel carries a hydrogen embrittlement risk because acid cleaning and plating can introduce hydrogen into the lattice. The standard control is to bake parts soon after plating, typically at 190–220 °C for several hours depending on hardness and thickness, and to verify with a bend or load test on sample parts from every batch.
Q: How long will tin-plated solder tabs stay solderable?
A: Typically 6–12 months of normal warehouse storage before solderability degrades noticeably. Matte tin at 2–8 µm with clean handling gives the most predictable shelf life; beyond a year, plan on a stronger flux, a fresh strip of plating, or a solderability test before the production run.
Q: Is selective plating economical for a 50,000-piece-per-year order?
A: Yes, selective plating becomes clearly worthwhile at annual volumes of roughly 50,000–100,000 parts and above when the precious-metal-coated area is a small fraction of the part, because the coating is the dominant cost. Send the drawing with the functional contact zone marked and the tooling can be designed around a registered plating band from day one.
Related Resources
- Metal stamping process guide — from strip to finished part, where finishing fits in the process.
- Stamped contact plating guide — layer stacks and plating logic for electrical contacts.
- Stamping terminals and contacts — stamped and plated parts made to your drawing.
- About BQUQ — ISO9001 source factory in Dongguan running stamping, CNC, springs and heat sinks under one roof.
- Contact us — send your drawing for a 12-hour quote at sc@bquq.com.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
