Gold is an excellent material for use in situations with a lot of moisture or corrosive environments. Unlike copper or nickel, gold does not corrode under normal conditions. However, it can corrode under certain conditions, mainly when it comes into contact with sulfur dioxide and chlorine gases. This can be a problem for electronic components, since these metals can easily clog up the electrical conductors. This is why hard gold plating is a popular choice in electronics for circuit boards.
Plating gold can be divided into »hard gold plating» and »soft gold plating». The PCB manufacturer has to take the characteristic of the two types of gold into consideration during PCB designing.
Hard gold plating is preferable to projects that require regular sliding wear or make/break switching events as it offers longer lifecycles based on thickness. According to laboratory testing it has better corrosion resistance than soft gold. Hard gold contains alloying elements (like nickel or cobalt) which increase its deposit hardness, but they also interfere with wire bonding.
The hardness of gold plating is important because it allows the plate to withstand high amounts of force without scratching or breaking. This is especially important for electronic parts that will be used in mobile devices or automated machines, where the plated part will be subjected to a lot of friction.
Plating gold application in PCB manufacturing. Hard gold plating is suitable for use in places where the force of friction is needed such as gold finger, keypad.
Soft gold is mainly used for wire bonding, such as BGA CSP, etc. the main component is 9999 pure gold (thickness 20uinch) and matte nickel (thickness 150uinch) the gold surface is not bright due to the addition of nickel matte. Because the hardness of pure gold is relatively soft, it is also called «soft gold». Because gold and aluminum can form a good alloy, COB will require the thickness of this layer of pure gold (soft gold) when making aluminum wire.
Electroplating procedure for soft gold and hard gold:
Soft gold: pickling → nickel plating → pure gold plating
Hard gold: pickling → nickel plating → gold pre plating → gold nickel plating or gold cobalt alloy plating