Gold finger in printed circuits

Gold contact surfaces are often used on circuit boards with membrane switches which are a technology of choice for industrial, commercial and consumer products. When PCBs will be repeatedly installed and removed electroplated gold is used for edge-connector contacts or as they are more commonly known: Gold fingers. It is difficult to hear the term Gold finger without remembering the villain Auric Goldfinger from the James Bond film of 1964. To begin with, the plating thickness of a PCB gold finger is typically a mere 30 micro-inch. At this thickness the hard gold is expected to survive 1,000 cycles before wear through. The disadvantage of ENIG finishing is its solderability is not as good as OSP or IMS etc.

The process begins after the copper etch with the PCBs tape laminated to leave only the desired area exposed. A Nickel underlayer is electo-plated onto the PCB with a minimum thickness of 50 microinch. Nickel not only provides mechanical support it also provides a diffusion barrier as well as an inhibitor to pore and creep corrosion. The 24 carat hard gold, immersed in a salt medium, is then electro-plated directly onto the Nickel surface.

A few design rules are required for gold fingers / edge connectors.

1. No plated through holes are allowed in the plated area
2. No solder mask or silkscreen can be present in the plated area
3. For panelization, always place gold fingers facing outward from the panel center
4. Connect all gold fingers with a 0.008” conductor trace at the edge to allow for manufacturing
5. Features can be placed on one or both sides to a depth of 25mm from the outside edge
6. Don’t use combined surface treatment like OSP+ENIG, HAL+ENIG if the design contains goldfinger.