Why Gold Is Not the Best Switch Contact Plating

Gold does not make a better switch contact. It makes a different one. The most common plating mistake in electronic hardware is treating gold as the default upgrade, then paying for it on a circuit where silver would have been more reliable and far cheaper. Contact plating is a load decision. Match it to the current and voltage the switch actually sees, and the contact will outlast the rest of the assembly.
Here is the short version. Use gold for low level or dry circuit switching, where the current is too small to break through a surface film. Use silver, the most electrically conductive metal, for power switching at higher current. Use a silver alloy such as silver tin oxide (AgSnO2) or silver nickel (AgNi) where the switch makes and breaks an inductive or high inrush load and has to survive arcing.
Why “gold is best” is the wrong instinc\

Gold is chemically inert. It does not oxidize or tarnish, so it holds a low and stable contact resistance even when the signal is tiny. That property is what makes gold essential for dry circuits, meaning switching at very low voltage and current where there is not enough energy to punch through an oxide or tarnish film. A gold contact reads the same on day one and year ten\

The trade off is mechanical and financial. Gold is soft, so under repeated actuation or the heat of an arc it wears and erodes quickly, and it can cold weld under load. It is also the most expensive of the common options. And for anyone who assumes gold is the best conductor, it is not. On the International Annealed Copper Standard (IACS), gold rates about 70 percent, below copper at the 100 percent baseline and silver at about 105 percent. Its resistivity is about 2.44 microohm centimeters (µΩ·cm), higher than both. Gold is chosen for stability, not for conductivity\

Not all gold plating behaves the same, either. A gold flash of a few microinches gives you the surface chemistry but almost no wear life, while a harder gold layer of tens of microinches survives repeated actuation. Two parts can both read gold on the datasheet and wear out years apart, so plating thickness belongs on the checklist next to the metal itself. This is why low-signal parts lean gold. Across the common types of tactile switches, the same body is often offered with either a gold or a silver contact, so the plating, not the model number, is the real choice\

Silver, the workhorse for powe\

Silver has the highest electrical and thermal conductivity of any metal, with a resistivity of about 1.59 µΩ·cm at 20°C, lower than copper at about 1.68. That conductivity is why silver dominates power switching. Parts that carry real current, such as micro switches, usually run silver or a silver alloy, where conductivity and arc resistance matter more than dry-circuit stability\

Its weakness is tarnish. In the presence of sulfur, silver forms a thin silver sulfide film. On a power circuit this does not matter, because the current and the contact force wipe through the soft film on every actuation, a behavior often called self cleaning. On a dry circuit it matters a great deal, because a signal level current cannot break the film and the contact reads as open. This is the reason silver fails in exactly the applications where gold succeeds, and the reason the two metals are not interchangeable\

Silver alloys, for arcing and inrus\

When a switch breaks an inductive load such as a motor or a solenoid, or a capacitive inrush, an arc forms across the opening contacts. Pure silver erodes and can weld under that stress, so the industry uses silver alloys\

Silver tin oxide (AgSnO2) resists welding and arc erosion and is now the common choice for higher current and arcing loads. Silver nickel (AgNi) offers low contact resistance with good arc resistance for moderate loads. Silver cadmium oxide (AgCdO) was the traditional arcing contact for decades, but cadmium is a restricted substance under RoHS, so most new designs move to AgSnO2 or AgNi\

The rule, match plating to the loa\

The decision comes down to three cases\

1.Signal or dry circuit, meaning low voltage, low current, and no arc. Use gold, often a gold layer over a nickel barrier. For extra insurance against an intermittent open, consider bifurcated, or twin, contacts for redundancy\

2.General purpose and power switching, meaning a resistive load at moderate current. Use silver\

3.Inductive, capacitive, or high inrush loads that arc. Use a silver alloy, usually AgSnO2 or AgNi\

The number that decides the first case is the minimum applicable load, sometimes called the minimum switching load or dry circuit rating. It is the lowest voltage and current at which the manufacturer will guarantee reliable contact. If your circuit sits below that line, you need a noble metal contact, and gold is the answer. If it sits above, silver or an alloy is usually both more reliable and cheaper\

This bites hardest when you cross-reference a part from another brand. An Omron cross-reference that matches the footprint but ignores the contact material can drop a silver contact onto a dry circuit, and the part will read open in the field\

A short checklist before you spec a switc\

Run these five checks before you commit to a part\

1.Write down the actual switched voltage and current, not the supply rail. A device can run on a 12 V rail and still switch a logic input at only a few microamps, and that is a dry circuit whatever the rail says\

2.Identify whether the load is resistive, inductive, or capacitive. Arcing changes the plating answer\

3.Read the minimum applicable load on the datasheet, not only the maximum rating. The maximum tells you what will not burn out. The minimum tells you what will actually make contact\

4.For gold, check the plating thickness, not just the word gold. A flash of a few microinches wears through far faster than a harder layer of tens of microinches under mechanical cycling\

5.Ask the manufacturer which platings they offer on the series you want. Many switch makers, including Swiclick, list more than one contact plating within a single mechanical series, which lets you hold the footprint and change only the contact material\

Get the plating right and the contact stops being the weak link. Get it wrong and no amount of cycle life rating on the datasheet will save a silver contact sitting on a dry circuit, or a gold contact eroding on a motor\

Frequently asked question\

Is gold plating always better for switch contacts\

No. Gold is better only for low level and dry circuit switching, where its stable low resistance matters. On power circuits silver conducts better, survives arcing longer, and costs less\

What is a dry circuit\

A dry circuit switches at very low voltage and current, low enough that the contacts cannot break through an oxide or tarnish film on their own. These circuits need noble metal contacts, usually gold\

Why does silver tarnish, and does it matter\

Silver reacts with sulfur to form a thin silver sulfide film. On power circuits the current and contact force wipe through it on each actuation, so it does not matter. On signal circuits it can cause an open reading, so gold is used instead\

What is the minimum applicable load\

It is the lowest voltage and current at which a manufacturer guarantees reliable switching for a given contact material. It is the single most useful number for choosing plating, and it is often overlooked in favor of the maximum rating\

Which silver alloy suits an inductive load\

Silver tin oxide (AgSnO2) is the common modern choice for arcing and higher current loads because it resists welding and erosion. Silver nickel (AgNi) suits moderate loads. Silver cadmium oxide is being retired under RoHS\

Can I change contact plating without changing my PCB\

Often yes. Many switch series are offered with more than one contact plating on the same body and footprint, so you can match the plating to your load while keeping the mechanical design fixed. Confirm availability with the manufacturer for the exact series.