What is the difference among hard gold/soft gold/gold plating/immersion gold/flash gold on PCB?

What is the difference among hard gold ,soft gold ,gold plating, immersion gold, flash gold on PCB surface treatment?

Electroplating nickel gold

In fact, gold plating itself can be divided into hard gold and soft gold. Because the electroplating of hard gold is actually alloy, so the hardness will be relatively hard, suitably used in the place need for friction, in the electronics industry, generally used as a contact point of the board edge (commonly known as "Gold Finger"); Soft gold is generally used for wire bonding on COB (Chip On Board), or as contact surface on keypad of mobile phone, recently ,it has been extensively used on both sides of the BGA carrier board.

Want to understand the origin of hard gold and soft gold, it is best to know the electroplating process first. Regardless of the previous pickling process, the purpose of electroplating is basically to "gold" plating on the copper, but the "gold" can not directly react with the copper foil, it must first plating a layer of "nickel "And then  plating gold up to the nickel, so we generally called the gold plating, the actual name should be called" plating nickel-gold. "

And the difference between hard gold and soft gold, it depended on the gold composition, you can choose pure gold or alloy plating, because the hardness of pure gold is softer, so it is called "soft gold "Because gold and aluminum can form a good alloy, so aluminum wire ponding on COB will be particularly asked the thickness of this gold layer .In addition, if the choice of gold-plated nickel alloy or cobalt alloy, as the alloy is harder than pure gold, so it is called "hard gold."

Soft gold: pickling → electroplating nickel → electroplating pure gold

Hard gold: pickling → electroplating nickel → pre-gold plating → gold-plated nickel or gold cobalt alloy

Immersion Gold

At present, immersion gold is mostly used to refer to ENIG (Electroless Nickel Immersion Gold). The advantage is that there is no need to use the plating process to attach nickel and gold to the copper, and its surface is also more flat than the gold plating, which is important for increasingly shrinking electronic parts and flat requirements of components. Since ENIG uses chemical permutation to produce the surface gold layer, the maximum thickness of the gold layer can not reach the same thickness as the gold plating.

Since ENIG's gold-plated layer is pure gold, it is also often classified as "soft gold", and some people take it as a surface treatment of aluminum ponding, but must strictly require its gold layer thickness at least higher than 3 to 5 microinches (μ "), generally more than 5μ" gold layer is difficult to achieve, too thin gold layer will affect the adhesion of aluminum; but gold plating can easily reach 15 microinches, but the price increases follow the thickness of the gold layer.

Flash Gold

The word "flash gold" means quick gold plating. In fact, it is a "pre gold-plated" procedure for electroplating hard gold. Refer to the electroplating process, It uses a larger current and high concentrations of gold liquid, first form a layer of more dense, but thin gold layer on Nickel layer, in order to facilitate the process of plating nickel-gold or gold cobalt alloy can be easily carried out. Since lacking the last gold plating program, the cost is much cheaper than the real gold. Furthermore, because its gold layer is very thin, it can not effectively cover the nickel layer, it is easier to lead to oxidize,affecting the solderability.

The cost of plating nickel gold is relatively high compared to other surface treatment (eg ENIG, OSP), so it is rarely used, unless otherwise used, for example, sliding contact elements, connectors (Such as gold finger ...), but by now, because of the good resistance to friction and excellent antioxidant capacity by plating nickel gold, no one can compare.