Pre-Plated Metal Stamping: Coil Before the Die

The Snapshot

Pre-plated tin on steel (ISO 11949) runs 1.1-11.2 g/m² per side (0.15-1.5 µm), enough tin for solderability and shelf life on most small stampings.
Soft tin survives a 180° flat bend; hard nickel microcracks below roughly 2-3T radius. Match coating hardness to bend severity before you quote.
Plating after forming exposes finished parts to acid and hydrogen; moving plating to the coil deletes the ASTM B850 bake (190-210 °C, minimum 3 h) that high-strength parts need afterward.
Post-plating of small parts typically adds $0.01-$0.05 per piece plus batch minimums; the coil premium is 5-20% on strip. The crossover sits between roughly 500k and 2M parts per year.

Your stamped part comes back from the plating line looking flawless: bright tin, no pits, certificate stamped. Then it takes one 90° bend at assembly and the coating cracks at the outer radius. Red rust appears at 48 h of ASTM B117 salt spray, contact resistance drifts from 3 mΩ toward 40 mΩ, and a 10-million-piece program turns into a six-figure sorting and rework event. Coating cracking, adhesion loss, and hidden plating cost are the three reasons I keep pushing engineers to the other end of the process: plate the coil before it hits the die, not the part after it.

This guide covers the material, adhesion, tooling, and cost decisions behind pre-plated strip. If you are sourcing tinplate, nickel-plated, or selectively gold-plated stampings, the tables below give the thickness classes, the bend limits, the die parameters, and the crossover arithmetic to put in the RFQ.

What "pre-plated" means at the coil

Pre-plated strip is base metal finished before stamping: electrolytic tin, nickel, copper, or selectively placed gold applied to the full coil face on a continuous line, then sheared, bent, and drawn as if it were bare metal. The coating goes in with the coil and comes out with the part - no racks, no barrels, no second vendor, no plating certificate to chase.

Coating thickness is specified by mass or by class, and the numbers matter:

Coating system	Typical thickness	Governing standard	Typical part
Electrolytic tin on steel (tinplate)	1.1-11.2 g/m² per side (≈0.15-1.5 µm)	ISO 11949, ASTM A623	Battery contacts, EMI shield cans, fuse clips
Electrolytic tin on brass/copper	1.3-2.5 µm (classes to 13 µm)	ASTM B545	Connector shells, terminals
Electrolytic nickel on steel	1-5 µm	ASTM B689	Battery terminals, spring contacts, high-temp brackets
Selective gold over nickel	0.76 µm Au over 1.27 µm Ni (typical)	ASTM B488, ASTM B689	Contact zones on connector strip
Copper on steel	1-3 µm	ASTM B177	Welding/brazing prep, EMC shielding

ASTM A623 grades tin mill product by coating weight in pounds per base box: #25 to #100 equals 2.8-11.2 g/m² across both faces, or about 0.19-0.77 µm per side. ISO 11949, the electrolytic tinplate spec, lists coating masses up to 11.2 g/m² per side (≈1.5 µm). The tinplate you meet in connectors, cans, and fuse clips usually sits at 1.1-5.6 g/m² per side.

Why coil-first beats plate-after

Same coating chemistry, completely different process economics and failure modes:

Uniformity. Strip plating deposits on a flat surface under controlled current density, so thickness holds within roughly ±10%. Rack plating concentrates current on edges and corners - 2-3× buildup is common - and the minimum-thickness rule on the print forces you to overplate everything else to match.
No acid, no hydrogen. A finished stamped part never touches an acid bath, so the hydrogen exposure window disappears. For steels at or above ~180 ksi (1241 MPa) tensile - the range where ASTM F519-type testing starts to matter - coil-first removes both the risk and the ASTM B850 bake (190-210 °C, 3 h minimum) that post-plated parts require.
Selective coating without masking. Stripe and spot plating put gold or silver only where the contact wipes. Precious-metal cost drops 70-90% versus full coverage, and you never mask a 3D stamping.
No batch logistics. Plating minimums ($50-$150 typical), racking labor, chemistry control, and waste treatment vanish. The press feeds a continuous coil; even the scrap offal still carries reclaimable coating.

The trade-off in one line: sheared edges stay bare. If your corrosion spec demands coated cut edges, post-plating has the edge - I cover that in the last section.

Coating systems in detail

Each pre-plated system exists because it solves one class of problem: solderability, corrosion, contact resistance, or joining. The table below summarizes the properties that drive selection.

The selection logic runs in one direction. If the part gets soldered, start with tin. If it runs hot, above 232 °C, move to nickel. If it needs a stable contact wipe for millions of insertions, add selective gold over nickel. If it is a welding or brazing prep, copper. The coating is a functional layer, not a decoration, so the service environment fixes the choice before the die work starts.

Coating	Hardness	Melting point	Key strength	Key limitation
Pure tin	soft (5-20 HV)	232 °C	Solderability, bend ductility, low cost	Whiskers, low temperature rating, reflow above 232 °C
Tin-lead (70/30)	soft	~183 °C	Whisker-free solderability	RoHS exemption only
Bright nickel	400+ HV	1,455 °C	Hardness, wear, high-temp stability	Microcracks at tight bends
Sulfamate nickel	~200 HV	1,455 °C	Better bend ductility than bright nickel	Softer, wears faster
Gold (selective)	~90-130 HV	1,064 °C	Stable contact resistance, no oxide	Cost, only where the contact wipes
Copper	~60-80 HV	1,085 °C	Joining prep, diffusion barrier	Oxidizes, needs top coat for corrosion

Tin: the solderability workhorse

Tin is the default pre-plate for parts that get soldered or that carry low-voltage current. It is soft, so it survives deep forming; it melts at 232 °C, which is exactly why it solders so well. The FeSn2 intermetallic layer at the tin-steel interface is the reason tinplate passes the bend-adhesion requirements of ASTM A623 without flaking.

The two things to manage on tin are whiskers and reflow. Pure tin can grow whiskers over years of service, so JESD201 whisker guidance applies: matte tin, controlled storage, and a risk review for high-reliability electronics. And if the part sees temperatures above 232 °C in service, tin reflows and the protection is gone - that is the case for nickel.

Nickel: the high-temperature and wear answer

Nickel is the opposite of tin. It survives temperatures where tin reflows, it is hard enough to resist wear, and it provides a stable diffusion barrier under gold. But hardness is a two-edged sword: bright nickel at 400+ HV microcracks at the outer fiber of a bend below roughly 3T, and the cracked coating can look like a defect even when it is harmless. Sulfamate nickel (200 HV class) reaches about 2T. If the print demands a 1T bend with nickel, drop to 0.5-1 µm or switch to tin.

Gold: the contact-resistance finish

Gold never oxidizes, which is why selective gold over nickel is the standard contact finish on connector strip. The nickel underlayer blocks copper diffusion, and the gold provides a stable, low-resistance wipe surface. Typical builds run 0.76 µm gold over 1.27 µm nickel. The stamping consideration is that gold is applied in stripes or spots before forming, so the die must keep the plated zone exactly where the contact wipes - a half-millimeter shift moves the gold off the working surface.

What the die does to the coating

Adhesion is qualified on the flat strip, before any forming. ASTM B571 bend and file tests screen the deposit, and tinplate has an extra card to play: the FeSn2 intermetallic layer at the tin-steel interface, which is why it passes the bend-adhesion requirements of ASTM A623 without flaking.

Forming limits follow the coating's hardness, not the base metal's:

Pure tin is soft and melts at 232 °C - the same reason it solders so well. At 0.2-0.8 µm it survives a 180° flat bend and deep-drawn box shapes without flaking; drawn sidewalls thin the deposit but keep it continuous. That is the entire reason tinplate food cans exist.
Nickel is the opposite. Watts or bright nickel at 400+ HV microcracks at the outer fiber below roughly 3T; sulfamate nickel (200 HV class) reaches about 2T. If the print demands a 1T bend with nickel, drop to 0.5-1 µm or switch to tin. Microcracks are usually harmless - the crack network does not reach the steel, and solderability and corrosion life survive. Delamination is the killer, and that is a process defect ASTM B571 screens out before the coil ships.
Springback barely moves: a 1 µm coating changes a 0.5 mm section by 0.2%. What does move is tool wear - nickel at 400+ HV is abrasive, so budget DLC or TiN die coating for runs past roughly one million hits.

Every shear creates a bare edge, and the edge is where corrosion starts on pre-plated parts. Hold burr under 10% of strip thickness (a typical print spec), and for humid or outdoor duty step tin weight up to 5.6 g/m² per side.

Die design and tooling for pre-plated strip

Pre-plated strip stamps almost like bare metal, but three die decisions are different. First, clearance. Shearing coated strip with standard clearance produces a larger torn zone and a taller burr, and every micron of burr is a bare-edge corrosion path. Tighter clearance (5-8% of thickness per side on tinplate) improves the shear ratio and reduces the exposed edge.

Second, tool steel and coatings. Nickel at 400+ HV wears uncoated tool steel noticeably; DLC or TiN-coated die inserts extend tool life on long runs. The coating also reduces galling on the drawn sidewalls of tinplate cans and shells, where bare copper or steel would pick up on the die.

Third, lubrication and scrap handling. Forming lubricant residues on the coil face contaminate the plated surface and hurt solderability, so the lubricant must be compatible with the coating and the cleaning step. Scrap offal from plated strip still carries reclaimable coating, so separating and returning it matters for the cost picture. Coil joining, when a coil runs out mid-shift, must not put a weld through the die; a stitch joint or a re-feed at the pilot station is the standard answer.

The cost picture: what you stop paying for

Run the two columns with your own part size, because the crossover is real:

Post-plating a small stamping	Typical magnitude (2026 shop rates)
Batch minimum	$50-$150 per lot
Barrel/rack plating per piece	$0.01-$0.05
Overplating give-away	1.5-2× spec thickness, forced by minimum-thickness rules
Plating rejects and sorting	1-3% typical - six-figure annual cost at 10M parts
Rework or scrap of a plated part	Full part value lost

The coil side is simpler: tinplate runs about 5-10% over bare cold-rolled strip, nickel-plated strip 10-20%, and there are no minimums beyond a coil split. Run the arithmetic on volume: 10 million parts at $0.02 each is $200,000 a year of plating spend, and the coil premium on a $0.10 strip part is a rounding error beside it. Below roughly 500k parts a year, batch logistics often win; between 500k and 2M is where the engineering review pays for itself.

Annual volume	Typical best route	Why
Under ~50k	Post-plating	Coil premium and engineering time rarely pay back
50k-500k	Depends on part size and coating	Run the two-column arithmetic per part
500k-2M	Engineering review zone	Coil premium vs batch minimums and sorting cost
Over 2M	Pre-plated strip	Plating spend dominates; coil premium rounds to zero

The quiet killer in the left column is the minimum-thickness rule: it forces 1.5-2× average thickness, so you buy metal you do not need. Strip plating holds a tighter distribution - on selectively plated gold that is the difference between a 0.76 µm average and a 1.5 µm average.

Where coil-first earns its keep

Battery contacts and terminals - nickel-plated steel strip, 1-2 µm per ASTM B689: uniform deposit, no rack marks, weldable tab.
EMI shield cans and lids - tinplate #50-#100 per ASTM A623: the drawn box is proof the tin survives the die, and the cover seam solders clean.
Connector shells and terminals - tin-plated brass at 1.3-2.5 µm per ASTM B545, with contact zones selectively gold-plated at 0.76 µm over 1.27 µm nickel (ASTM B488 / B689).
Fuse and relay blades - tinplate for solderability and stable contact resistance in service.
High-temperature brackets - nickel, which holds up where tin reflows (tin melts at 232 °C; nickel at 1,455 °C).

RoHS note: if you specify pure tin, JESD201 whisker guidance applies - matte tin and controlled storage matter on strip just as they do on formed parts, so put the risk review in the spec.

Specifying pre-plated strip: the print

The drawing for a pre-plated part should name five things. The base metal grade and temper. The coating metal and its thickness or coating class, referencing the governing standard (ISO 11949, ASTM A623, ASTM B545, ASTM B689, ASTM B488). The bend test requirement, usually a radius in multiples of material thickness (T). The solderability or contact-resistance requirement, so the coating is tested for function, not just thickness. And the whisker note for pure tin.

Two specification mistakes are common. First, copying a post-plate callout onto a pre-plate drawing: a minimum-thickness rule written for rack plating makes no sense on strip, where the distribution is tight and the average is the real spec. Second, leaving the coating location vague on selectively plated strip: the stripe width and its position relative to the part edge must be on the print, because the die and the plating mask have to agree.

Incoming inspection and supplier qualification

Pre-plated strip is easy to verify at incoming inspection, and the tests are fast. XRF measures coating thickness on the flat strip in seconds; a bend test per ASTM B571 checks adhesion in minutes; salt spray per ASTM B117 verifies the corrosion story; and a wetting balance or dip-and-look solder test checks function.

The supplier qualification questions are the same ones you would ask of any coating line. What is the thickness distribution across the coil width, and how is it measured? What is the lot traceability back to the mill certificate and the plating bath? What bend radius does the coating survive, and how was that qualified? What is the whisker risk position for tin? And what is the rework path if a coil is rejected?

A useful audit test: take three samples from the head, middle, and tail of a coil and XRF them at edge, center, and edge. The spread you see is the real distribution. If the edge-to-center spread is more than about ±15%, the strip line is running outside its window, and the parts made from it will show it.

When post-plating still wins

Coil-first is not universal, and a supplier that says otherwise is selling, not engineering. Post-plating stays the right call when:

Cut edges must be coated - the corrosion spec demands full coverage including sheared faces.
Volume sits under ~50k parts a year, where coil premiums and engineering time rarely pay back.
The print combines a tight bend with a thick hard coating - 5 µm nickel at 1T is a crack looking for a place to happen.
Cosmetic bright finishes need a post-polish that only makes sense on the part.
Forming leaves heavy lubricant residues that would contaminate the coil face.

None of these kill the pre-plated idea; they define its envelope. Most small-precision stamping programs live inside it.

FAQ

Does pre-plated strip survive deep drawing?

Tin does, which is why tinplate food cans are deep drawn. Nickel does not at tight radii: bright nickel microcracks below roughly 3T and sulfamate nickel below about 2T. Match the coating to the bend severity before you quote.

How much does pre-plated strip cost versus bare strip plus plating?

Tinplate runs about 5-10% over bare cold-rolled strip and nickel-plated strip 10-20%. Post-plating adds $0.01-$0.05 per piece plus batch minimums. Above roughly 500k parts a year, the coil premium is usually cheaper than the batch logistics and sorting cost.

What happens to the sheared edges?

They stay bare. Every shear exposes base metal at the cut face. That is acceptable for most indoor, low-corrosion duty; for humid or outdoor service, step the tin weight up to 5.6 g/m² per side, hold burr under 10% of thickness, and if the spec demands coated cut edges, post-plating is the answer.

Is pre-plated strip RoHS compliant?

Yes, for the coatings listed here. Pure tin, nickel, copper, and gold are all RoHS-clean. The caution is whiskers on pure tin: JESD201 guidance applies, so specify matte tin and controlled storage for high-reliability electronics.

Can you solder to pre-plated tin after stamping?

Yes, that is the point of the coating. The tin surface solders as well on the formed part as on the flat strip, provided the lubricant is cleaned off and the coating has not been burnished through at the bend. The solderability test at incoming inspection catches both.

Does pre-plated strip need special storage?

Not exotic storage, but the coil should stay dry and reasonably cool. Humid storage accelerates tin oxidation and can hurt solderability, and for pure tin the JESD201 whisker guidance recommends controlled temperature and humidity for high-reliability programs. A dry room, or simply keeping coils wrapped and off the floor, is enough for most jobs.

Your Action Roadmap

The coating failure that costs money never shows at incoming inspection - it cracks at the bend, fails salt spray, or drifts contact resistance in the field. Put the coating decision before the die and you delete the whole class of post-plating defects.

Run the five-step check on your next program. Classify the part by bend severity and corrosion duty. Pick the coating system from the tables above. Compare the two-column cost arithmetic at your real volume. Put the coating class, the bend test, and the solderability requirement on the print. And qualify the strip supplier with the XRF spread test before the first production coil.

Send us your part drawing and we will tell you in one page whether coil-first cuts your landed cost. We run tinplate, nickel-strip, and selective-gold programs weekly - free DFM review, no obligation.