The electronics industry is focused on getting its products to be faster, smaller, more efficient, and more reliable. Thus the materials used in the electronics industry tend to be chosen with the aforementioned goals in mind. Depending on their specific role in Printed Circuit Boards (PCBs) metals need to be conductive, resistant to oxidation and corrosion, environmentally friendly, and or cost efficient.
The three most conductive metals are pure silver, pure copper, and pure gold in that order. Why then do we use ‘hard’(gold alloy) gold instead for our connectors, contact points, and in our IC’s? Though gold is less conductive than silver and copper it does not oxidize or tarnish whereas silver and especially copper are much more reactive to oxygen. This layer of tarnish diminishes the conductivity of the metals by a significant amount. Thus copper is used as a conductor in the traces of a PCB while hard gold is often used to make a corrosion resistant yet still electrically conductive layer on top of the copper. Pure gold itself is far too soft to act as a reliable barrier for copper, thus other metals such as cobalt, nickel, or iron are used to create a hard gold alloy.
For example in the case of an ENIG surface finish a layer of nickel is first electrically plated onto the copper pad/trace, this is followed by a layer of gold being immersion plated onto the nickel. The role of the nickel here is to make sure that the copper atoms cannot diffuse through the gold and thus tarnish the surface.
Not only is gold used as a surface finish for traces and pads it is also used on gold fingers, though the gold in this application is of a much higher hardness. Gold plating on edge connectors of PCBs are used to not only protect the connectors from oxidizing but to make them more resilient to the wear and tear of everyday use and protect the PCB.
The most significant disadvantage to using gold in PCBs is its cost. However since only very small amounts of gold are needed for most projects it’s cost is often insignificant when compared to the overall cost of a project. Even a very thin layer of gold is enough to provide a conductive yet corrosive resistant protection.