Gold has and continues to be a principle finish for electrical components especially with the continuing miniaturization of electronics. One of the primary benefits of gold plating services is a finish that is both conductive and receptive to soldering. When soldering gold plated components there are a variety of important considerations when specified the surface finish. The primary considerations are thickness, purity and the proper selection of an underplate.
Gold plating thickness is a critical, and often misunderstood, tenant of gold soldering. In gold soldering the physical bond is made between the underlying nickel layer and the solder itself, with the gold layer serving as barrier to help maintain the solderability of the nickel layer. Typical gold thickness for solderability is in the range of 10uin to 30uin as it provides adequate protection against oxidation to preserve wetting while keeping the cost of the finish as competitive as possible.
When soldering, gold dissolves into the solder through solid state diffusion. With heavier gold deposits, more gold alloys within the solder joint. In the diffusion process the gold reacts with the solder creating a gold intermetallic amalgam. If the gold in the solder exceeds 3% by mass, the solder joint can become embrittled causing joint failure, especially in dynamically or thermally stressed joints. The level of impurity and thickness of gold are directly related, thus thickness of the gold must be balanced between corrosion/oxidation protection, contact cycle life and solderability. (Soldering to Gold – A practical Guide).