Precious metal plating on connectors protect the surface from corrosion. Sounds pretty basic, but there is a bit more to it.
There is the perception that gold is good and more gold must be better. Not necessarily so. It depends upon the application.
Corrosion typically occurs when corrosive elements in the air reach the connector contact base metal. Chlorine, hydrogen sulfide, nitrogen dioxide, and sulfur dioxide are examples of these corrosive chemicals that are present in small amounts in nature. If these chemicals contact the copper alloys typically used for connector contacts, a corrosive reaction occurs that generates a “bloom” of non conductive material disrupting conductivity in the connector, ultimately causing failure. You have probably seen this effect when a battery fails, generating a pile of greenish white powder.
The most common way to protect the contacts is by electroplating a combination of a heavy nickel covered with a protective layer of a noble precious metal that does not react at all with the corrosive materials in the air. Unfortunately, electroplating is subject to having pores, small openings that go from the surface all the way to the base metal. The level of porosity is determined by the composition of the plating, thickness, and process controls. A typical connector plating line will have controls in place where samples are taken from the process regularly and tested with sulfuric acid to reveal any porosity.
In the telecommunication industry, it is quite common to specify that the mating surface always be lubricated with a petroleum-based lubricant that not only reduces friction during mating, but also serves to fill pores, preventing corrosive gasses from reaching base metals.
During the connector manufacturing process, electrical contacts are normally carried on a continuous strip through the stamping, plating, and forming processes that create the connector. At some point, the contact is removed from the carrier, exposing a small amount of bare metal. Depending on the requirements for the connector, this may be tolerable. Consumer connectors, for example, are often made with pre-plated material that will reveal bare metal at all the stamped edges. This is fine for a connector that only needs to last a few years.
Connectors for high performance applications like computers and 30-year life Telco applications will normally be stamped, then plated all over to assure that the minimum of bare metal is exposed and that none of these bare metal portions are anywhere near the contact area of the connector.
Up until the 1980s, Telco connectors were slathered with 50 micro inches (1.25 mµ) of precious metal plating over the entire contact. As the price of gold soared, the connector companies aggressively moved to selective plating techniques, just putting the heavy gold in the areas where the contacts mated. In the beginning, a stripe was used. As the price of gold continued to soar, more selective spot plating techniques were developed. Today almost all gold-plated connectors have a small spot of gold slightly bigger than the contact mating point. This selectivity really helped moderate the impact of precious metal cost on the total connector cost. Precious metal, however, remains an important cost element—25% of the cost for many connector styles.
The Telco operating companies learned they could reduce their cost significantly by requiring the minimum amount of precious plating possible while still meeting performance requirements. With so many connector designs, plating formulas, and producers, it was impossible to test them all, so the industry collaborated to produce a qualification test procedure and specification that each connector manufacturer could use to qualify their own connectors. This resulted in the Bellcore TR-NWT-001217 spec in 1995. This later evolved into the Telcordia 1217 core specification in 2008. These specs were performance based so that the user only needs to specify that a connector meet this specification rather than specifying connector plating or thickness. This approach was a huge change for the Telco industry and enabled massive innovation in Telco connectors during the last 25 years that has given us much higher pin counts, lower mating forces, thinner platings that still work, and contact designs that can be optimized for electrical signal integrity rather than just mechanical performance. It has also enabled alternatives to gold plating to succeed in the market.
Be aware that there are two performance levels specified, one for dry, air-conditioned central office environments, and a second for outside plant where the equipment sees wide temperature swings and more corrosive atmospheres. Think of a cellular base station located near the beach or a chemical plant on the equator or the arctic circle.
So, to answer the question of how much precious metal is enough, just specify that your connector needs to be compliant and leave the thickness and precious metal composition to the connector manufacturers. Performance base specs exist for automotive, industrial, and military applications.