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Does gold really matter?

I want to take a look at two nearly identical connectorsWM1918-ND, a 3 pin Molex tin finished connector, andWM1923-ND, a 3 pin Molex gold finished connector.  These are board-mounted connectors for Molex Micro-Fit 3.0 family.  I tend to use them a great deal in my designs because they are very convenient.  The difference in cost between a tin finished connector and the gold finished connector is, in this case, about 5¢.  This is hardly worth thinking about if you are making a prototype, but if you are doing a production run it might become significant.
Properties of metals used in electronics
One of the important properties of a metal, used in electronics, is its ability to transfer heat.  Here is a comparison of thermal properties of some commonly used metals:
Rank    Metal    Thermal Conductivity [BTU/(hr·ft⋅°F)]
1    Silver              235
2    Copper           223
3    Gold               182
4    Aluminum    118
5    Nickel            45
6    Tin                 39
7    Lead              20
Electrical conductivity has a similar chart:
Rank    Metal    Conductivity σ at 20 °C
1    Silver              6.30 x 10^7
2    Copper          5.98 x 10^7
3    Gold               4.52 x 10^7
4    Aluminum    3.5 x 10^7
5    Nickel            1.46 x 10^7
6    Tin                 8.7 x 10^6
7    Lead              4.87 x 10^6

It is apparent from both of these charts, that gold is not the shining star when it comes to some of the most important characteristics of electronics.  So why use gold anyway?

Oxygen is the enemy

What gold has going for it is its ability to resist oxidation.  Both copper and silver will readily for oxides which decrease their conductivities.  For this reason, gold is used extensively (or is it expensively) in electronics.  The estimated annual consumption of gold by the electronics industry is about 320 tons.  It is used on the contacts of switches, relays and connectors.

Best practices

If you search the thickness of gold on connectors, you will find it ranges from 1µin to 228 µin.  Knowing what you need can save you a great deal of money.  A coating of 15 µin of gold is good for about 200 mating cycles, while a coating of 50 µin is good for about 2000 mating cycles.  For most applications, a 30 µin layer of gold over a 50 µin layer of nickel will give the best results for the money.
Gold is not generally plated directly onto copper. Often you will find a layer of nickel between the copper and the gold layers.  This is because copper atoms will diffuse through gold where they can then for copper oxides.  Nickel prevents this diffusion.  Nickel also acts as a pore-corrosion inhibitor, corrosive creep inhibitor and mechanical support for the gold layer.
Gold is porous.  When placed on a metal these pours can cause it to break up and fall off with time.  Having a proper under layer such as nickel will help support the gold.  Also, increasing the thickness of the gold reduces the effects of pores.

What are intermetallics?

Intermetallics are compounds that form when metals come in contact with one another.  In wire bonding, there are two famous intermetallics between gold and aluminum known as the white and the purple plague.  Gold is also soluble in solder and will form intermetallics of lead and tin.  In a 63/37 solder, gold will preferentially bond with tin and form cleavage planes that will reduce its mechanical strength and the reliability of the connection.  For this reason, you will find gold on the mechanical contacts of a connector, but never on the pins that are soldered.
We have noted that the presence of oxygen is the problem.  It will form metallic oxides with many metals which have lower conductivity than the pure metal or metallic alloy.  One of the things that increases the rate of reaction is heat.  Use of metals at elevated temperatures greatly increases the formation of oxides.  For this reason, if you are going to use your product at elevated temperatures, gold contacts are essential.

Final thoughts

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