Terminal Plating Thickness: Microns That Matter
Short answer: plating thickness is the number that decides contact resistance, wear life and corrosion resistance on a stamped terminal, and it is specified in microns (µm). Typical values are 3–10 µm tin for general solderable terminals, 2–5 µm nickel as a barrier layer, 0.5–2 µm gold over nickel for low-resistance signal contacts, and 3–8 µm silver for high-current joints. Thicker is not automatically better: 1 µm of gold over a sound nickel barrier usually beats 3 µm of gold over none, because what fails first is the barrier and the substrate under it, not the shiny top layer.
Plating is where a lot of terminal programs quietly go wrong. The alloy is chosen carefully, the die is built, and then the finish is specified as "gold plated" with no thickness — and the parts fail in the field after a few thousand mating cycles, or corrode in a humid warehouse, or simply refuse to solder cleanly. Thickness is the variable that controls all of that, and it deserves a number on the drawing just like any dimensional tolerance.
Why Thickness, Not Just Material, Decides Performance
Contact resistance is a function of how much metal stands between the two mating surfaces and how stable that metal stays. Every plating has a different resistivity, wear rate and oxidation behaviour, so the thickness determines how long the surface keeps its properties. A thin layer wears through at the high spots where contact actually happens; a thick layer survives longer but costs more and can crack at tight bend radii.
Three failure modes drive the specification. Wear-through removes the top layer and exposes the barrier or substrate, so resistance jumps. Corrosion grows an oxide or sulfide film that raises resistance even where metal remains. Diffusion lets the substrate migrate through a thin barrier over time and temperature, which is why nickel exists as a barrier at all. Choose thickness to keep each of these under control for the service life you need, not to match a competitor's spec sheet.
The thickness also has to match how the terminal is made. A coating that survives flat strip may crack when the part is bent to a tight radius, exposing the substrate at exactly the point where stress concentrates. Ductile deposits and generous bend radii reduce that risk, and on parts that are formed after plating the spec must allow for the finish stretching on the outside of the bend.
Plating Types and Their Working Ranges
Different platings solve different problems, and each has a sensible thickness band. The values below are the ranges a terminal house will typically offer.
| Plating | Typical thickness | Role | Notes |
|---|---|---|---|
| Tin (matte or bright) | 3–10 µm | Solderable, corrosion-resistant | Cheap, soft, but whiskers possible |
| Nickel | 2–5 µm | Diffusion barrier under gold | Hard, resists wear |
| Gold over nickel | 0.5–2 µm Au | Low, stable contact resistance | Cost scales with gold thickness |
| Silver | 3–8 µm | Lowest resistance at high current | Tarnishes, needs mating care |
| Palladium-nickel | 0.5–2 µm | Wear-resistant gold alternative | Used on high-cycle connectors |
| Tin-lead (legacy) | 5–10 µm | Solderable, historic | Being phased out in many markets |
Tin is the workhorse for solderable terminals. It is inexpensive, solders easily and gives a few milliohms of contact resistance, but it is soft, so it wears in high-cycle applications and can form whiskers if stressed. Nickel is rarely the top layer; it is the barrier that stops base metal diffusing into gold. Gold is the premium contact finish: low and stable in resistance, but its cost rises almost linearly with thickness, so designers keep it thin because the nickel underneath does the heavy lifting. Silver gives the lowest resistance and is used on high-current joints, though it tarnishes and needs protected mating surfaces.
Contact Resistance, Wear and Corrosion by Thickness
The right thickness depends on the application, the mating frequency and the environment. The table below gives practical targets grouped by how the terminal is used.
| Application | Recommended finish | Typical thickness |
|---|---|---|
| Solder tab (reflow) | Tin over copper or brass | 5–10 µm |
| Crimp terminal | Tin | 3–8 µm |
| Low-cycle board connector | Tin or gold flash | 0.5–1 µm Au over 2 µm Ni |
| High-cycle signal contact | Gold over nickel | 1–2 µm Au over 2–5 µm Ni |
| High-current power joint | Silver or thick tin | 3–8 µm Ag, 8–10 µm Sn |
| Harsh or humid environment | Nickel then tin or gold | 2–5 µm Ni + top layer |
Indicative, not fixed. A connector that mates once a year can use a gold flash; one that mates thousands of times needs more gold or a palladium-nickel layer. Humid and sulfurous environments push toward nickel barriers and away from thin silver. When in doubt, specify the barrier and the mating cycle count, and let the thickness follow from those two facts.
Indicative life figures help set expectations. A 1 µm gold layer over a sound nickel barrier on a properly designed contact commonly survives the thousands of mating cycles expected of a board-to-board connector, while a bare or flash-plated surface in the same application may show rising resistance within a few hundred cycles. Treat these as planning numbers and confirm them with a life test on the real contact geometry.
Selective and Spot Plating to Control Cost
Gold is expensive, so plating the whole terminal in gold is wasteful. Selective plating puts the finish only where contact happens — the tip, the wipe zone or the crimp barrel — and leaves the rest bare or tin-plated. This is standard practice on lead frames and connectors, and it can cut precious-metal cost dramatically on a part where only a few square millimetres actually mate.
The tooling for selective plating is a masking arrangement in a reel-to-reel plating line: a mask or a controlled nozzle defines the plated zone, and the strip runs past it continuously. Because the process is in-line, it integrates with the stamping step and keeps handling to a minimum. Spot and stripe plating the same part differently — tin on the solder tail, gold on the contact tip — is common and worth designing for early. Our stamped contact plating guide covers finish selection for contacts, and stamped crimp and solder tabs covers the termination side. Material choice and conductivity set the base, and we compare those in our notes on spring and contact materials.
Selective plating also lets a single terminal carry two different finishes for two different jobs — a thick tin coating on the solder or crimp zone and a thin gold layer on the wipe contact — which a whole-part finish cannot do without wasting precious metal at both ends. Designing the plated zones into the strip layout early avoids re-tooling the plating mask later.
How Thickness Is Specified and Verified
On a drawing, plating thickness belongs with the finish callout: state the layer sequence and the thickness of each layer — for example, "2 µm Ni barrier + 1 µm Au, contact zone only". Ambiguity is the enemy here; "gold plated" is not a specification, and a plating shop will assume the minimum to control cost unless told otherwise.
Verification is done by X-ray fluorescence (XRF), which measures coating thickness non-destructively on the finished part, and by coulometric or cross-section methods for confirmation. A good supplier records XRF readings across the strip and ships them with the batch. Thickness on corners and edges runs thin — more than on flat surfaces — so specify where on the part you are measuring, and keep extra metal on high-wear zones to compensate. Because BQUQ runs stamping, plating coordination and inspection under one roof in Dongguan, we can stamp the terminal to ±0.05 mm and control the finish to a stated micron target, with XRF data on request. Send the drawing and the mating conditions to sc@bquq.com for a quote within 12 working hours.
Plating Thickness and Solderability
Solderability depends on the finish staying metallic until the joint is made, and thickness plays a supporting role. A tin layer thick enough to remain continuous after storage solders reliably; too thin, and the underlying alloy can show through and wet poorly. Tin-lead is the most forgiving legacy finish, but many markets restrict it, so pure tin is now standard — with the caveat that tin can whisker under stress, which is why some high-reliability parts use a nickel barrier under a controlled tin layer.
Storage matters as much as thickness. Even a well-plated terminal will oxidise or tarnish if it sits in a humid warehouse, so packaging, desiccant and shelf-life control belong in the finish specification. If a batch refuses to wet, the finish or its storage is the first thing to check, not the solder.
Common Plating Defects and How to Prevent Them
Plating problems are predictable, and most trace back to contamination, current density or geometry rather than to the alloy itself.
Poor adhesion shows up as blistering or flaking after forming and usually means the base metal was not cleaned properly before plating, or an oxide layer formed between steps. Porosity lets the substrate corrode through a coating that looks continuous; it is worse on thin deposits and on parts with rough surfaces, so a clean, uniform base is the foundation of a sound finish. Thickness variation across a part comes from current-density differences in the plating cell and is exaggerated at edges and corners, where the deposit runs thin. Burning and rough deposits come from too high a current density, and tree-like nodules form at high spots.
Prevention is mostly about process control: pre-plate cleaning and activation, controlled current density, agitation of the bath, and masking or shielding where geometry concentrates current. For stamped parts, consistent burr height and edge quality before plating matter too, because rough edges plate unevenly. A supplier that plates in-house can tie these variables to the stamping process instead of treating plating as a separate black box.
Frequently Asked Questions
Q: How thick should gold plating be on a terminal?
A: It depends on mating cycles. A gold flash of 0.5–1 µm over 2 µm of nickel suits connectors mated only a few times, while 1–2 µm of gold over a nickel barrier handles thousands of cycles. The nickel barrier matters more than the gold thickness for long-term stability.
Q: Is thicker tin plating always better?
A: No. Tin between 3 and 10 µm is the useful range for solderable terminals. Thicker tin costs more, can crack at tight bends and increases the risk of whiskers, so specify the minimum that meets the corrosion and solderability needs of the application.
Q: Why is nickel used under gold?
A: Nickel is a diffusion barrier. Without it, base-metal atoms migrate through the thin gold layer over time and temperature and raise contact resistance. A 2–5 µm nickel layer keeps the gold's low resistance stable, which is why the barrier often determines the terminal's real life.
Q: What is selective plating and why use it?
A: Selective plating applies the finish only to the contact or solder zone rather than the whole part. Because gold is expensive, plating just the working area cuts precious-metal cost sharply on connectors and lead frames, and it is done in-line on a reel-to-reel line.
Q: How is plating thickness measured?
A: The standard shop method is X-ray fluorescence (XRF), which measures each layer non-destructively on the finished part. Cross-sectioning and coulometric tests confirm the reading. Ask for XRF data with the batch so thickness is a verified number, not a claim.
Related Resources
- Stamped contact plating: finish selection for terminals, contacts and clips.
- Terminals and contacts: stamped terminals, contacts and crimp tabs with controlled plating from a Dongguan factory.
- About BQUQ: an ISO9001-certified source factory running stamping, CNC, springs and heat sinks under one roof.
- Contact us: send your drawing and get a quote within 12 working hours.
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
