What is the difference between the plating hard gold and immersion gold(ENIG)

There are several surface treatment for PCB board: bare board (no treatment on the surface), rosin board, OSP (Organic Solderability Preservatives), HASL (lead tin, lead-free tin), plating gold, immersion gold, etc., these are relatively conscious treatment. Many people can't tell the difference between plating gold and immersion gold, thinking that it is the same thing, it is a big mistake. The following is a brief introduction to the difference between plating gold and immersion gold.

There is a distinction between soft gold and hard gold (generally hard gold is for gold fingers), the principle is to dissolve nickel and gold (commonly known as gold salt) in chemical potion, immersing the board in the plating tank and turning on the current to form a nickel-gold coating on the copper foil surface of the board. Electro-nickel gold is widely used in electronic products due to its high hardness, wear resistance and oxidation resistance.

Immersion gold is formed by a chemical redox reaction method. It is generally thicker and is a kind of chemical nickel gold layer deposition method, which can reach a thick gold layer.

In practical product applications, 90% of the gold board is immersion gold board, because the poor weldability of the plating gold is his fatal flaw, and it is also the direct cause of many companies to abandon the gold plating process!
 
The immersion gold process deposits a nickel-gold coating with stable color, good brightness, flat coating and good solderability on the surface of the printed circuit. It can be basically divided into four stages: pre-treatment (deoiling, micro-etching, activation, post-dipping), sinking nickel, sinking gold, post-treatment (waste gold washing, DI washing, drying). The thickness of the immersion gold is between 0.025 and 0.1 um.
 
Gold is applied to the surface treatment of circuit boards because gold has strong conductivity, good oxidation resistance and long life. Generally, it is applied to key plates, gold finger plates, etc. The most fundamental difference between plating gold and immersion gold is that plating gold is hard gold (wear resistant), immersion gold is soft gold (not wear resistant).

Generally, the thickness of immersion gold is much thicker than that of plating gold. The color of immersion gold is much yellower than plating gold, which will make customers more satisfied. The crystal structure is different.

Since the crystal structure formed by the immersion gold and the plating gold is different, the immersion gold is easier to weld than the plating gold, and does not cause poor soldering. At the same time, because immersion gold is softer than plating gold, so the gold fingers are generally use plating gold and more wear-resistant.

The immersion gold board only has nickel and gold on the pad, so the signal transmission is in the copper layer in the skin effect and will not affect the signal.

Compared with gold plating, immersion gold has a denser crystal structure and is less prone to oxidation.

As the PCB processing precision requiremnet is getting higher and higher, the line width and spacing have reached 3~4mil. Plating gold is prone to shorts in gold wire, the immersion gold has only nickel and gold on the pad, so it is not easy to produce a gold wire short circuit.

The plating gold has only nickel and gold on the pad, so the solder mask is more firmly bonded to the copper layer, when the engineer make compensation will not affect the spacing.

For the board with higher requirements, need a good flatness, so the immersion gold is generally used. The immersion gold generally does not appear as a black mat after assembly. The flatness and service life of the immersion gold board is better than that of the plating gold board.