Gold is really expensive. Why then would anyone bother to plate connectors with it? Does it have some unique electrical characteristics that make it useful?
Gold has several really unique properties that allow for it's frequent use:
- Gold is the least likely metal to oxidize. From this table, it can be shown that Gold's electro-potential value is -1.1, meaning it should not oxidize at all, even in water. Wikipedia states this is the primary reason that some electrical contacts are gold plated.
- It is an extremely conductive material, one of the best known.
- Gold is very malleable, it is very easy to get a thin sheet of it.
Bottom line, it might be a bit overkill, but there are some advantages to gold plated materials.
Gold makes good contacts because it is very nonreactive, and thus won't corrode or tarnish over time.
Copper is a better conductor than gold, but copper will form a layer of oxide or other tarnish through normal exposure to the elements that will eventually increase the contact resistance of two mating copper contacts, rendering the connection faulty. See for example, architectural copper roofs:
Where this corrosion isn't desired in architectural applications, a protective coating is applied. This isn't feasible in electronic applications since such coatings are generally non-conductive.
Gold does not corrode. Check out this shiny roof:
Although gold is expensive, its physical properties also make it easy to deposit a very thin layer. The plating can be extremely thin while still being effective. Consequently, gold plated contacts aren't as expensive as one might think. Normally the determining concern in the thickness of the plating is wear resistance.
In applications where extremely low cost is more important than maintaining a good connection over time, tin plating is frequently used. Tin is not as good as gold in the corrosion resistance department, but it is better than copper, and cheaper than gold.
Gold plated contacts provide reliable switching when the wetting current is low, because there is no oxide to breach for electrical contact to occur. Ex: a pushbutton switch used to signal a microcontroller digital input has a pullup resistor sized to flow 50 uA when the switch is closed. A switch without gold plating might not be reliable.
Currently, the best electrical conductivity is reached by materials such as nanotubes or graphene. They also have excellent mechanical properties. Unfortunately, there is still no technology of mass production, and thus we must rely on metals.
The best metal for this purpose is silver (Ag). It has the highest conductivity, even better than copper and much better than gold. It is broadly used in Russian military connectors of ShR ШР series. The weakness of silver coated connectors is that they are very sensitive to sulfur (S) compounds, which are present in human sweat (even its vapour) and breath, which produce the worst kind of patina which you can find on silver contacts.
Gold (Au) has worse conductivity than silver (still very good) but is free from the above drawbacks:
- no corrosion at all, produces no patina;
- contacts can be touched with bare hands,
- reliable in humid environments (household, automotive, marine),
- well known technology for refining and coating with it.
GOLD is an excellent conductor, plating of contacts with Gold is mostly found in low current and low voltage (less than 100 Ma & less than 50 Volt) circuits where there is little chance of arcing (which would burn/melt the plating off the contact).
There is a product on the market called "COOL-AMP", which allows you to effectively SILVER PLATE contacts and most any other metal surfaces when properly applied (expensive stuff but works very well).
Gold is fine, but gold plated pins in connectors is usually on a layer of nickel - and nickel is ferromagnetic. I have recently been studying sideband noise and as it turns out the limitation of my interferometric setup is noise due to nickel beneath the gold in BNC connectors. Have a look here: https://www.youtube.com/watch?v=GCtJ1aQ5fek The problem might not exist on microwaves where the gold layer could be thick enough to carry most of the current. At modest frequencies the nickel would produce intermodulation and overtones. I do not know why we can not have gold on brass or god on copper for connectors. In the old days we had silver on brass (I think) but silver is problebatix because it forms silver sulphide on the surface.
Gold coating used is so this that once plugged it couple of times, the useful gold film gets worn of, exposing usually just very cheap metal. You can't see it, the scratches are microscopic but contact is no longer made with gold. Gold plated connectors are nothing but a gimmick and in fact gold plated connectors are usually made of much less conductive metals than ordinary type.