Two terminals in the same housing, crimped to the same cable: one still passes a clean signal years later, the other has drifted out of specification. The difference is often a few micrometres of metal on the contact surface.
Tin and silver cover most of the market, and the choice is usually made once at the drawing stage, then copied forward for a decade. It deserves more thought: the metals fail differently.
What the plating on a terminal is actually doing
The substrate under a crimp terminal is a copper alloy: brass, phosphor bronze or a copper-iron grade. Copper oxide is resistive and mechanically weak, so a bare contact builds a film that raises resistance and then breaks down unevenly under load.
Plating presents a surface that does not build an insulating film, and survives wiping and vibration without wearing through. It is specified in two zones: the contact area where the mating terminal touches, and the crimp barrel that grips the conductor. Tin is normally used on both; silver goes on the contact area only, with the barrel left tin so the crimp stays solderable. Both sit on a copper or nickel underplate: a diffusion barrier, not decoration.
Tin plating: the default, and where it stops working
Tin is ductile, cheap, easy to solder and tolerant of process variation. It works despite being a metal that oxidises, because the tin oxide film is thin and brittle: contact force and a little wiping crack it, and metal-to-metal contact is re-established through the cracks.
That mechanism has limits. The oxide keeps growing with time and temperature, so a joint that measured well on the bench can drift after a few thousand thermal cycles. At low signal currents there is not enough energy at the interface to break the film down, which is why tin suits dry-circuit and sensor signals poorly.
The bigger problem is fretting corrosion. Under micrometre-scale motion tin wears and the debris oxidises almost immediately, accumulating until resistance climbs by orders of magnitude and the joint turns intermittent rather than open. Engine vibration and thermal expansion differences between copper and plastic are enough.
Two further points. Pure matte tin can grow whiskers, thin conductive filaments driven by internal stress — a process question answered with a nickel underplate, reflow or annealing, not by a promise that the part is “tin plated”. And soft tin creeps under load at temperature, so normal force decays. Terminals in the 331-part terminal range are commonly tin-plated for cases where these limits do not bite: protected interior harnesses, moderate current, few mating cycles.
Silver plating: lower resistance, higher stakes
Silver has the lowest electrical resistivity of any metal and does not form the hard insulating oxide that tin does, so constriction resistance is lower and stays stable at signal levels where tin needs a wetting current it lacks. It is soft, conforms to surface roughness, and is the usual choice for high current density, high ambient temperature and connectors mated many times.
What silver does not do is stay clean. In sulfur-bearing or chloride-bearing atmospheres it tarnishes, and the sulfide film is resistive: cosmetic in power circuits, a real drift mechanism at signal level — which is why silver contacts in harsh atmospheres are often gold-flashed. Under DC bias, humidity and contamination silver can also migrate and grow dendrites, a housing-level issue as much as a plating one.
Fretting corrosion: where the two metals really diverge
Put the same micromotion through a tin pair and a silver pair and the difference is stark. Tin produces insulating oxide debris, so resistance rises early and fast. Silver wears too, but its debris is metallic, so the contact zone keeps conducting far longer — silver survives fretting better, though not indefinitely.
Three design levers matter as much as plating: normal force, which keeps the contact area deformed and limits slip; lubricant, which excludes oxygen from the wear zone; and a nickel underplate, which hardens the surface stack. A lubricated tin terminal with good normal force beats a poorly designed silver one, so plating should not be specified apart from the housing and its contact geometry.
Current, temperature and derating
Plating does not set the current rating, but it sets the margin left as the rating is approached. Silver holds a lower interface resistance at high current, generating less heat and tolerating a higher ambient before the joint runs away thermally. Tin, being softer, loses normal force faster at temperature; less force means a smaller contact area, higher resistance, more heat — slow and self-reinforcing. Decide plating and derating together; the housing material caps the assembly regardless.
Choosing between them: a practical comparison
| Consideration | Tin plating | Silver plating |
|---|---|---|
| Contact resistance | Low while film is broken; drifts at low current | Lowest, stable to signal level |
| Fretting | Poor: oxide debris, resistance jumps | Better: debris stays metallic |
| Tarnish | Brittle oxide, self-limiting | Sulfide film, resistive in salt air |
| Current density | Adequate with derating | Preferred for high current and heat |
| Mating cycles | Limited by wear-through | Higher, especially over nickel |
| Cost and process | Low cost, solderable | Higher cost, tighter control |
| Main risks | Whiskers, creep, force loss | Migration, tarnish, cost |
Silver earns its premium when at least one of these is true:
- the circuit carries high current or runs hot;
- the signal is low-level enough that a resistive film would change the reading;
- the connector is mated and unmated repeatedly;
- the location is inaccessible, so early failure is expensive to find.
Tin suits interior, protected harnesses at moderate current with few mating cycles and a cost-driven programme — much of the automotive body-harness market, which is why tin remains the volume finish.
What to write on the drawing or in the RFQ
Most plating disputes trace to a requirement that named a metal and nothing else. A usable specification covers substrate alloy, underplate, contact-zone plating thickness separately from the crimp barrel, matte versus bright finish, whisker mitigation where needed, and whether a lubricant is applied. Add one line on the mating half: tin against silver works, but should be deliberate, because tin transfer creates the interface that frets.
If the connector uses a terminal position assurance device, note it in the same document. The needle seat and TPA range exists for that purpose, and a plating change is a good moment to confirm the retention feature still engages.
For a worked example of a 2.8 mm crimp terminal with its product code and cross-reference, see LX629-2.8X0.6A. The catalogue can be filtered by family, and open questions can go through the contact form, including a bill of materials for cross-reference.
Frequently asked questions
Is silver plating always better than tin?
No. Silver wins on resistance stability, current density and mating cycles, but costs more, tarnishes in sulfur and chloride atmospheres and carries a migration risk. For a protected interior harness at moderate current, tin is the better choice.
Can a tin-plated terminal be mated with a silver-plated one?
It can, and it happens in mixed supply chains, but make it deliberate. Tin transfers onto the harder silver surface and oxidises, making the interface more fretting-susceptible.
Why does a tin-plated terminal test fine on the bench and fail in service?
Because the bench test does not reproduce micromotion. A static measurement on a new joint only shows the oxide film was broken at assembly. Fretting needs vibration and thermal cycling to build insulating debris, and that takes time.
How do I confirm which plating a supplied terminal actually has?
Ask for the plating specification, not the trade description, and confirm the underplate and contact-zone thickness. Appearance cannot separate matte from bright tin, or a thin deposit from a thick one.