Key takeaways
- Pre-plated coil removes the post-plating step, the rack marks and the hydrogen risk on high strength parts.
- Sheared edges are bare. If the edge sees corrosion or a low resistance contact, pre-plating alone will not do.
- You pay for coating on the skeleton as well as the part, so the scrap ratio is part of the cost case.
- The die has to protect the surface: polished faces, no sharp draw, controlled handling and clean packaging.
What pre-plated material is
Pre-plated material is strip or coil that has been coated before it reaches the press. The substrate is usually a copper alloy, brass or steel, and the coating is a thin electroplated layer of tin, nickel, silver, gold or a gold flash over nickel. The supplier controls the coating thickness across the width and along the coil, and the press shop cuts the part out of it.
The idea is simple: the coating is applied once, in a continuous line, at a thickness that is easier to control than rack plating on small parts. The part then needs no plating after stamping, which removes a process step, a transport leg and a set of handling risks.
It is not the same as cladding. Cladding bonds a thicker layer of metal to the substrate under heat and pressure, and the cladding survives the edge because the whole thickness is bonded. Pre-plating is a thin surface deposit, and the sheared edge is bare substrate.
Why buyers choose it
The strongest reason is hydrogen. High strength steel and some copper alloys can absorb hydrogen during acid pickling and electroplating, and the absorbed hydrogen can cause delayed brittle fracture. A part that is stamped from pre-plated coil never sees the plating bath, so the risk is removed rather than baked out afterwards. For a spring or a contact carrier in a high strength temper, that is a real advantage.
The second reason is handling. Rack plating small stamped parts is labour intensive, and the rack contacts leave unplated marks. Barrel plating avoids racks but can damage thin parts and trap solution. Pre-plated coil has neither problem, and the coating thickness is usually more uniform than a barrel deposit.
The third reason is time. A pre-plated part can go from press to assembly without a plating loop, which shortens the lead time and reduces the work in progress. For a connector programme with short call offs, that can matter more than a small difference in piece price.
Where pre-plating stops working
The sheared edge is the first limit. Every cut exposes bare substrate, and on a tin-plated copper part that edge is where corrosion starts and where contact resistance is highest. If the edge is inside a housing and never sees moisture, that is acceptable. If the edge is a mating contact surface, it is not.
The second limit is forming. A coating stretches with the substrate at a gentle bend, but at a tight radius or a drawn wall the coating can thin, crack or flake. Gold over nickel is more brittle than tin, and a hard gold flash on a tight bend is a common source of porosity. The part should be designed with the coating in mind, not adapted after the fact.
The third limit is thickness. Pre-plated coatings are usually thin because they are applied at line speed over a wide strip. A tin coating of a few micrometres is normal; a heavy silver or gold deposit is not economic on the whole strip. If the duty needs a thick, pore free finish, post-plating is usually the better route.
The edge is the honest test
Look at a sheared edge under magnification before approving a pre-plated design. If the bare substrate is visible where the part must conduct or resist corrosion, the process is wrong for that face.
Pre-plated against post-plating
The two routes solve different problems. The table sets out the decision rows that matter most in a quote review.
| Decision row | Pre-plated coil | Post-plating |
|---|---|---|
| When the coating is applied | Before stamping, on the full strip | After stamping, on the finished part |
| Sheared edge | Bare substrate | Coated, including the edge |
| Hydrogen embrittlement | Avoided on high strength parts | Needs baking controls after plating |
| Coating thickness | Thin, uniform, limited range | Wider range, including heavy deposits |
| Surface damage | Risk at die entry, draw and handling | Risk at rack marks and barrel contact |
| Selective coating | Possible with stripe plating on the coil | Possible with masking, at extra cost |
| Scrap and skeleton | Coating paid on material that becomes scrap | Coating paid only on good parts |
| Lead time | Press to assembly, no plating loop | Adds a plating and inspection step |
Most programmes use a mix. A pre-plated contact body with a post-plated critical face is common, but it needs a clear drawing.
The cost model, including the skeleton
A pre-plated quote is usually higher per kilogram than the same substrate uncoated, because the coating is applied to the whole strip. The part only uses part of that strip. The rest becomes skeleton, and the coating on the skeleton is paid for and then sold as scrap at a lower value than clean material.
For illustration, suppose the coating adds £4 per kilogram to the strip and the finished part weighs 2 grams. The coating on the part is about £0.008. If the strip layout uses 65 per cent of the material and 35 per cent becomes skeleton, the effective coating cost per part is about £0.012, because the coating was bought on 1.35 times the finished weight. Those figures are illustrative; the real numbers depend on the coating, the layout and the scrap credit.
Against that, remove the post-plating cost: the plating itself, the rack or barrel handling, the inspection, the packaging, the freight between press and plater, and the inventory in transit. For a small part with a simple layout, post-plating often wins on pure cost. For a part with a high scrap ratio or a hydrogen risk, pre-plating often wins on total cost and risk.
Tooling and handling controls
A pre-plated die is a surface protection problem as much as a cutting problem. The punches and die inserts should be polished, with generous entry radii and no sharp transitions that drag the coating. Clearance should be set so the shear is clean rather than torn, because a torn edge exposes more substrate and creates a rougher surface for corrosion.
The strip path needs attention too. Feed rolls, guides and strippers can scratch the coating before the part is even formed. Use soft or coated guide surfaces, set the stripper pressure to hold the strip without marking it, and keep the die clean. Scratches that look cosmetic on a tin surface become corrosion sites in service.
Handling after the press is part of the process. Parts should not be poured into a bin, because parts rubbing against each other damage the coating. Interleaved packaging, trays or bags add cost but protect the finish. If the part is gold plated, gloves and clean packaging are normal.
- Polished punches and inserts with generous entry radii
- Clearance set for a clean shear, not a torn edge
- Soft or coated guides and feed rolls
- Controlled stripper pressure and a clean die face
- No drop packing. Use trays, interleaves or bags
- Gloves and a clean bench for precious metal finishes
How to specify a pre-plated part
Name the substrate, the coating, the thickness and the underplate. For example, a copper alloy with a nickel underplate and a tin topcoat needs both thicknesses and the adhesion requirement. If the coating is selective, show the coated area on the drawing and state what is allowed at the boundary.
State what the edge must do. If bare substrate at the edge is acceptable, say so. If it is not, the part needs post-plating, a different edge design or a cladding. Do not leave the edge to be discovered at first article.
Add the tests that prove the finish: thickness, adhesion, solderability where relevant and a porosity or corrosion check for the duty. Ask for a coil certificate and a first article that shows the coating on a formed bend and on a sheared edge. Those two photographs prevent most of the arguments later.
Frequently asked questions
What is pre-plated material?
It is strip or coil that has been electroplated before stamping. The part is cut from the coated strip, so it needs no post-plating. The sheared edges are bare substrate, and the coating is usually thinner than a post-plated deposit.
Does pre-plating avoid hydrogen embrittlement?
It removes the plating bath from the part route, so the part does not pick up hydrogen during plating. That is a real advantage for high strength springs and contact carriers. The substrate and any upstream process still need to be controlled.
Can pre-plated parts be soldered?
Usually yes, if the coating is a solderable finish such as tin and the oxide is controlled. The bare sheared edge is harder to solder, and a thick oxide on a stored part can prevent wetting. Agree the shelf life and the packaging.
Is pre-plated always cheaper than post-plating?
No. You pay for coating on the skeleton as well as the part, so a high scrap ratio works against it. Post-plating can be cheaper for a small part with a simple layout. The comparison should include handling, inspection, freight, inventory and the risk of hydrogen damage.
Sources and further reading
- ASTM B545, Standard specification for electrodeposited coatings of tin
- ASTM B488, Standard specification for electrodeposited coatings of gold for engineering uses
- Qumek finish and stamping process notes for pre-plated parts
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Deciding between pre-plated coil and post-plating?
Send the drawing, the substrate, the finish and the volume. The engineering desk will reply with a cost and risk comparison for both routes.
Send the finish requirement