Metal Dome Plating Selection Guide: Gold vs Silver vs Nickel vs Tin for Contact Resistance and Service Life

Ask a group of engineers what decides the performance of a metal dome switch and most will start talking about force, travel and cycle life. Those matter. But in any application where the dome switches a signal rather than a lamp, the plating is the part that decides whether the product still works after six months in the field. The dome geometry determines the click; the plating determines the contact. This guide is written for engineers specifying metal domes and dome sheet arrays, based on the failure modes Shanyo sees in production programs across consumer, automotive, medical and industrial applications.

Why Plating Matters More Than Dome Thickness

A bare stainless steel dome is a good spring and a poor contact. SUS301 stainless steel carries the mechanical properties that make a dome snap reliably, but its surface forms a chromium oxide film that is electrically resistive. When the dome closes, the contact resistance sits at 80-150 milliohms on a fresh part and climbs as the film thickens and hardens over cycling. For a keypad driving a logic input at 3.3V and microamps, that resistance is tolerable - the current is so small that the voltage drop across the contact barely registers. For a control loop, a measurement path, or a switch carrying tens of milliamps through a load, the same resistance is a design problem.

Plating exists to put a conductive, stable surface between the spring steel and the world. The four systems in commercial use - gold, silver, nickel and tin - each solve that problem differently, and the differences are exactly what the specification should be about.

The Four Plating Systems at a Glance

• Nickel plating (2-3µm): the standard finish for consumer keypads. Hard, low cost, consistent. Contact resistance typically 60-120 milliohms fresh, drifting upward over life and with environmental exposure. Good enough for logic-level keypads and short-life consumer devices.
• Silver plating (1-3µm): the lowest resistance of the common finishes, often below 30 milliohms fresh. Soft and an excellent conductor, but silver tarnishes in sulfur-bearing air, and the tarnish film is resistive. Used where low resistance matters and the environment is sealed or controlled.
• Gold plating (0.1-0.5µm over nickel): the stable finish. Gold does not oxidize, so contact resistance holds in the 20-50 milliohms range across life and environment. The cost is the gold itself, which is why thickness is tuned to the application.
• Tin plating (1-3µm): the solder-friendly finish, mostly used on domes that are intended to be reflow-soldered or where the mating pad is tin. Tin oxidizes in air and the oxide is hard, so tin-plated domes are not the choice for long-life switching at low voltages.

Contact Resistance: How It Behaves on Each Finish

Contact resistance on a dome is measured at a defined actuation force, because the contact area grows as the dome seats harder against the pad. The number on a datasheet is a point on a curve, not a constant. Four-wire measurement at 1.5 times the rated force is the common industry practice, and the current used for the measurement matters too - a measurement at 10mA is more representative of a logic application than one at 1A.

The behavior over life is where the finishes separate:

• Nickel: starts moderate, drifts upward steadily as surface films build and the hard nickel surface polishes unevenly at the contact point. Acceptable for devices that see modest actuation counts and short service lives.
• Silver: starts excellent, degrades environmentally. In clean, sealed air, silver holds low resistance for very long lives. In sulfur-bearing industrial air or near rubber and plastic outgassing, silver tarnishes and the resistance climbs - often suddenly.
• Gold: starts excellent and stays there. Gold's value is not a lower initial number than silver; it is the absence of drift. The measured stability over cycling and environmental exposure is why gold is specified wherever the contact behavior must not change.
• Tin: starts moderate and drifts with oxidation, and at low voltage the tin oxide film can act as a rectifying layer. Tin is chosen for solderability and cost, not for contact stability.

The Environmental Tests That Decide Plating Choice

Three environmental conditions do most of the damage to plated dome contacts, and the plating decision should be made against them:

• Sulfur and industrial gases: silver tarnishes, and thin gold over a porous deposit can let substrate films reach the surface. Nickel and gold-nickel stacks are the robust answers; if silver is used, the application should be sealed.
• Salt and coastal air: salt accelerates film growth on exposed surfaces. Gold over nickel with the edges of the contact area protected by the dome geometry survives best; nickel alone starts to pit under prolonged salt exposure.
• Temperature cycling and humidity: the expansion and contraction of the contact surfaces, plus condensation, accelerate film growth and can redistribute the plating at the contact point. The combination of 85°C / 85% RH and thermal cycling is the standard screening for this class of failure.

The standard Shanyo qualification battery - 500 temperature cycles, 1000 hours at 85/85, salt spray per ISO 9227 and a sulfur-exposure test - produces comparable numbers across the four finishes, and those numbers are the honest basis for a selection decision.

Matching Plating to Application

The practical selection matrix, based on the failure modes above:

• Consumer remotes, toys, household appliances: nickel plating. The current is logic-level, the life is short in electrical terms, and cost matters. Nickel is the right answer for the vast majority of consumer keypads.
• POS terminals, payment devices, high-actuation panels: gold flash over nickel. Hundreds of thousands of actuations and a requirement that the keypad electrical behavior never change make the gold cost worthwhile.
• Medical devices and instruments: gold. The contact sits inside a measurement or control path, and the device must behave identically across years of service. Gold over nickel with a documented thickness is the standard specification.
• Energy storage BMS, industrial panels, high-frequency switching: gold, with thickness tuned to the environment. Where the panel is sealed, 0.1-0.3µm is enough; in ventilated industrial cabinets, 0.3-0.5µm adds margin.
• Automotive interior (non-critical): nickel or gold-nickel depending on the module position and warranty. Underhood or door-side locations with wide temperature swings justify gold.
• Solderable and reflow applications: tin, where the dome is part of a soldered assembly rather than a mechanical contact.

Cost and Lead-Time Trade-offs

Plating cost follows the material and the thickness control required. Nickel is the baseline and adds the least to the dome price. Silver sits close to nickel on cost but brings the environmental qualification burden. Gold is priced by thickness - the jump from 0.1µm to 0.5µm roughly multiplies the plating cost, which on a high-volume part shows up clearly in the unit price. The engineering question is not "is gold too expensive" but "what does the contact actually need," and the Shanyo quoting team asks for the annual actuation estimate, the environment and the electrical spec before recommending a finish.

Lead times are similar across the finishes - plating is a batch operation and adds days, not weeks, to a dome order. The larger lead-time factor is tooling for custom geometry, which runs 2-3 weeks regardless of finish.

Verification and Quality Control at Shanyo

Every plated dome lot at Shanyo is verified against four checks: plating thickness by X-ray fluorescence on sampled parts, adhesion by tape pull after a bake cycle, contact resistance by four-wire measurement at a fixed actuation force, and environmental exposure on lot samples. The results are recorded against the lot trace code, so a customer can pull the plating data for any shipped box. For gold and silver finishes, the thickness is measured per lot and recorded as part of the certificate of conformance - the datasheet claim and the measured value are the same number.

For programs where the dome array is supplied as a dome sheet array rather than loose parts, the same plating records follow the array lot, and the placement and adhesive parameters are verified on the finished sheet. The metal dome product family on the Shanyo website lists the standard finishes and force families; the engineering team confirms the right finish for a new application against the actual environment and electrical requirement.

The Mating Surface: Plating Is Half the Contact

A dome contact is a two-sided joint, and the pad side matters as much as the dome side. A gold-plated dome on a bare copper pad does not give the expected stability, because the copper surface oxidizes and the oxide dominates the joint resistance within weeks of service. The practical pairing rules are simple to state: gold or silver domes pair cleanly with gold, silver or nickel-over-copper pads; nickel domes pair with nickel or tin pads on logic-level loads; and tin-plated domes are intended for tin pads, usually in soldered or high-current assemblies. When the pad finish is fixed by the board design, it should be written into the dome specification - a dome selected against a gold pad behaves differently on a tin pad, and the difference shows up as drift in the field rather than at the factory.

Surface contamination is the quiet variable in the same joint. Fingerprint oils, solder flux residue and silicones from nearby gaskets migrate onto contact surfaces over time, and the effect is finish-dependent: a gold surface sheds contamination better than a nickel surface because it does not form a reactive film for the contaminant to bind into. Programs that assemble keypads near silicone potting or RTV sealants should confirm the dome finish against silicone outgassing, which is a known cause of intermittent contact failures on silver and nickel finishes.

Six Specification Mistakes That Show Up Later as Field Failures

Over years of dome programs, the same plating specification mistakes recur. They are worth listing because each one is cheap to fix at the drawing stage and expensive in the field:

• Specifying "gold plated" without thickness. The cost and the performance both live in the number. Write 0.1-0.5µm over nickel, or the supplier's interpretation decides for you.
• Choosing silver for its fresh resistance alone. Silver's initial number is the best of the common finishes, but the environmental drift is the real specification. If the product is not sealed, silver needs a qualification against the actual air quality.
• Quoting a contact resistance number without a test method. The actuation force, current and measurement point determine the number. A spec without them cannot be verified.
• Ignoring the pad finish. The dome finish and the pad finish are one joint. Specifying the dome without the pad is specifying half a switch.
• Assuming thicker gold is always better. Above 0.5µm, the incremental stability gain is small relative to the cost, and on some barrel processes thick gold deposits roughen the contact area. Thickness should follow the actuation budget, not a "more is safer" rule.
• Testing fresh contacts instead of aged ones. The qualification sample that matters is the one tested after environmental exposure, not the one measured the day the plating tank finishes. A fresh-contact pass tells you nothing about a two-year-old product.

The Shanyo engineering review checks every incoming dome specification against these six points before quoting. Most of the value of the review is in the questions it asks back, because the answers are the actual specification.

Summary: A Decision Framework, Not a Formula

The plating decision compresses to four questions. What current and voltage does the contact actually switch? If the current is large enough that contact resistance shows up in the circuit behavior, the finish matters. What environment does the product live in? Sealed consumer devices can tolerate silver and nickel; ventilated industrial and outdoor installations push toward gold. What is the service life and actuation budget? Short lives and low counts do not need gold. And what is the cost tolerance on the specific part? Gold is justified by contact stability, not by initial performance. Answer those four questions with numbers and the finish chooses itself. The Shanyo engineering review runs exactly this framework on every incoming dome specification, and it is available to customers evaluating the trade-off before tooling.