Surface treatment is one of the most important process in PCB manufacturing, It forms a critical interface between component and PCB board. Basically the surface treatment has two essential functions. One is to protect the exposed copper circuitry on printed circuit board, the other is to provide a solderable surface for PCB assembly process.
Hard gold plating (Hard electrolytic gold) consist of a layer of gold plated over a barrier coat of nickel. Hard gold is extremely durable, and it's most usually used for high-wear areas such as gold fingers and keypads.
Hard gold plating is different with immersion gold, its thickness can vary by controlling the duration of the gold plating cycle, although the typical minimum values for gold fingers are 30u" over 100u" nickel for IPC Class 1 and Class2, 50u" over 100u" nickel for IPC Class3. Hard gold is not generally applied to solderable areas, because of it's high cost and it's relatively poor solderability. The maximum thickness that IPC considers to be solderable is 17.8u", so if this type of gold must be used for soldering, the recommended nominal thickness sould be about 5-10u".
The advantages to use hard gold plating is: Hard and durable surface, without Pb, Long shelf life.
The disadvantages to use hard gold plating is: very expensive cost, extra processing in PCB production, with more difficulty than other surface finishes, not solderable above 17u".
A-TECH CIRCUITS have capability to plate hard gold maximum up to 80u", no matter for gold finger areas or whole board plated, please note that hard gold plating could not be applied to the certain selective PADs on printed circuit board if without plating tie bars on these PADs.