The difference between immersion gold and gold plating in PCB manufacturing

Immersion gold boards and gold-plated boards are processes that are often used in PCB circuit boards. What impact will these two "gold boards" have on the circuit boards?
 
The difference between gold plating and immersion gold process
Immersion gold adopts a chemical deposition method, which generates a layer of plating through a chemical oxidation-reduction reaction method, which is generally thicker. It is a kind of chemical nickel-gold layer deposition method, which can reach a thicker gold layer.
 
Gold plating uses the principle of electrolysis, also called electroplating. Dissolve nickel and gold (commonly known as gold salt) in chemical water, immerse the circuit board in the electroplating tank and turn on the current to form a nickel-gold plating layer on the copper foil surface of the circuit board. The electro-nickel-gold coating has high hardness and resistance The advantages of abrasion and resistance to oxidation are widely used in electronic products.

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. Gold is used in the surface treatment of circuit boards. Because gold has strong conductivity, good oxidation resistance, and long life, it is generally used in key boards, gold finger boards, etc. However, the most fundamental difference between gold-plated boards and immersed gold boards is that gold-plated boards are hard Gold (wear-resistant), Immersion Gold is soft gold (not wear-resistant).

 

Why use gold-plated plates
As the integration level of IC becomes higher and higher, IC pins become more denser. The vertical spray tin process is difficult to flatten the thin pads, which brings difficulty to the placement of SMT; in addition, the shelf life of the spray tin plate is very short. The gold-plated board just solves these problems:
1. For the surface mount process, especially for ultra-small surface mount, because the flatness of the pad is directly related to the quality of the solder paste printing process, it has a decisive impact on the quality of the subsequent reflow soldering, so the whole board is gold-plated. It is common in high-density and ultra-small surface mount processes.
2. In the trial production stage, due to factors such as component procurement, it is often not that the board is soldered as soon as it comes, but often has to wait several weeks or even months before using it. The shelf life of the gold-plated board is better than that of lead. Tin alloy is many times longer, so everyone is happy to use it.
3. The cost of the gold-plated PCB in the sample stage is almost the same as that of the lead-tin alloy board. However, as the wiring becomes denser, the line width and spacing have been below 0.1mm. Therefore, the problem of gold wire short circuit is brought: as the frequency of the signal becomes higher and higher, the signal transmission in the multi-plated layer caused by the skin effect has more obvious influence on the signal quality. Skin effect refers to: high frequency alternating current, the current will tend to concentrate on the surface of the wire to flow. According to calculations, the skin depth is related to frequency.
 
Why choose Immersion Gold Board?
In order to solve the above problems of gold-plated boards, PCBs using gold-plated boards mainly have the following characteristics:
1. Immersion gold is different from the crystal structure formed by gold plating. Immersion gold will be golden yellower than gold plating.
2. Immersion gold and gold plating have different crystal structures. Immersion gold is easier to weld than gold plating and will not cause poor welding.
3. The immersion gold board only has nickel and gold on the pad, and the signal transmission in the skin effect will not affect the signal on the copper layer.
4. Immersion gold has a denser crystal structure than gold plating, and it is not easy to produce oxidation.
5. The immersion gold board only has nickel and gold on the pads, so it will not produce gold wires and cause slight shortness.
6. The immersion gold board only has nickel and gold on the pads, so the solder mask on the circuit and the copper layer are more firmly combined.
7. The project will not affect the spacing during compensation.
8. Because the crystal structure formed by immersion gold and gold plating is different, the stress of the immersion gold plate is easier to control, and for products with bonding, it is more conducive to bonding processing. At the same time, it is precisely because the immersion gold is softer than the gilding, so the immersion gold plate is not wear-resistant as the gold finger.
9. The flatness and stand-by life of the immersion gold board are as good as the gold-plated board.

Therefore, most factories currently use custom pcb assembly the immersion gold process to produce gold plates.