Gold fingers also called edge connectors, normally used in plugged in. Wonderful PCB Limited offer two types of gold finish: Electroless Nickel Immersion Gold (ENIG) as a surface finish for the whole PCB, and hard plated gold over plated nickel for edge-connector fingers. Electroless gold gives excellent solder ability, but the chemical deposition process means that it is too soft and too thin to withstand repeated abrasion. Electroplated gold is thicker and harder making it ideal for edge-connector contacts for PCB boards which will be repeatedly plugged in and removed.
Normally We plate the hard gold onto the PCB boards after the solder mask process and before we apply the surface finish to the rest of the board. Hard-gold plating is compatible with all the other surface finishes we offer. We first plate 3 – 6 microns of nickel onto the edge connector fingers and then on top of that 1 – 2 microns of hard gold. The plated gold is not 100% pure; it contains some cobalt to increase the wear-resistance of the surface. We normally bevel the edge connectors to ensure easy insertion. Beveling can be specified in the order details.
To make sure that the gold fingers exactly the same as the edge-connector profile, we rout the vertical edges of the connector on the first drill run. In some cases one or more gold fingers are shorter than the rest, so that the longer pads connect first when the PCB inserted into the connector. This means that the shorter pads cannot be connected vertically to the plating bar. They have to make the connection needed for electroplating in another direction
Below is the some of the PCB boards with gold fingers for your reference: