Gold Plating

Gold is unique with its yellow color. Also, gold is a precious metal, which means that it will not oxidize in air, so its electrical conductivity stays uniform over long periods of time. It is ideally suited for gold electroplating applications. Gold plating offers good corrosion resistance, good solderability, and when alloyed with cobalt, it has very good wear resistance. Gold is commonly used in electrical switch contacts, connector pins and barrels, and other applications where intermittent electrical contact occurs.\

Specification: MIL-G-45204 Gold Plating, Electrodeposited

Gold Plating & Gold Electroplating - purity and coating thickness:

Co-deposited impurities can make soldering more difficult, and for this reason high purity gold plating is preferred. Soldering requirements are best achieved when gold electroplating coatings range between 0.00005 and 0.0001 inch (50 and 100 micro inches) thickness.

Gold Plating & Gold Electroplating - Hardness Grades:

A. 90 knoop, maximum
B. 91-129 knoop, inclusive
C. 130-200 knoop, inclusive
D. 201 knoop, minimum

Specification: ASTM B 488 Standard Specification for Electrodeposited
Coatings of Gold for Engineering Uses
Type-I 99.7% gold minimum; hardness grade A, B, or C *
Type-II 99.0% gold minimum; hardness grade B, C, or D *
Type-III 99.9 % gold minimum; hardness grade A only *

Specification: AMS 2422 Gold Plating for Electronic and Electrical Applications

An underplate of copper (from a copper cyanide solution) is required unless the base material is copper or a copper alloy containing less that 15% zinc.
Copper Thickness: Not less than 0.0001" A layer of nickel (from a Watt's or chloride solution) is required over the copper underplate or copper base material.
Nickel Thickness: Not less than 0.0001" The gold (from a cyanide solution) is to be deposited directly on the nickel layer.
Gold Thickness: Not less than 0.00005" on all functional surfaces. Gold Purity: Not less than 99.0% pure.

About solderability: MIL-G-45204 extract: Codeposited impurities can make soldering more difficult, and for this reason high purity gold coatings should be used for those applications. Excellent solderability is best achieved when the gold thickness is between 0.00005 and 0.0001 inch.

About Undercoating: MIL-G-45204 extract: 
a) Unless otherwise specified, gold over silver underplate combinations should be excluded from electronics hardware (3.4).
b) When gold is applied to a copper rich surface such as brass, bronze, or beryllium copper, an antidiffusion underplate such as nickel should also be applied (3.4).

Gold over silver is not recommended for electronics hardware:

Gold Plating - Underplate Recommendations Gold Electroplating - When gold is applied to a copper rich surface such as brass, bronze, or beryllium copper, metal ions from these base metals will diffuse into the gold layer and degrade its hardness and non-oxidizing properties. An antidiffusion underplate such as nickel (electroless or sulfamate) should be applied to prevent this. We recommend electroless nickel under gold where part flexure of deformation is not expected and a bight finish is desirable. Where part flexure or deformation is expected, we recommend sulfamate nickel as the underplate because of its higher ducility.