Even if it cannot be considered a solderable surface finishing, this process is applied, generally after the solder mask, to protect the contacting elements, especially when a high number of operations is required during the service life (sliding contacts are an example).
In this case, the deposits are obtained through an electrolytic process: at first, a layer of nickel with a thickness of at least 5 µm is deposited, followed by a layer of gold with variable thickness, depending on customer’s specification, even up to 3 µm.
The deposit of gold contains a small percentage of a metal binder (generally Cobalt) that gives the necessary characteristics of hardness (“Hard Gold“).