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Five common processes for PCB surface treatment

Mar 01 2021

Five common processes for PCB surface treatment
There are many PCB surface treatment processes, the common ones are hot air leveling, organic coating, electroless nickel/immersion gold, immersion silver and immersion tin, which will be introduced one by one below.

1. Hot air leveling: also known as hot air solder leveling, it is a process of coating molten tin-lead solder on the surface of the PCB and flattening (blowing) it with heated compressed air to form a layer that is resistant to copper oxidation and provides good The solderability of the coating. During hot air leveling, the solder and copper form a copper-tin intermetallic compound at the joint. The thickness of the solder to protect the copper surface is about 1-2 mils.
The PCB should be immersed in molten solder during hot air leveling; the air knife blows the liquid solder before the solder solidifies; the air knife can minimize the meniscus of the solder on the copper surface and prevent solder bridging. There are two types of hot air leveling: vertical and horizontal. Generally, the horizontal type is considered to be better. The main reason is that the horizontal hot air leveling is more uniform and can realize automated production. The general process of the hot air leveling process is: micro-etching→preheating→coating flux→spraying tin→cleaning.
2. Organic coating: The process is different from other surface treatment processes. It acts as a barrier layer between copper and air; the organic coating process is simple and low-cost, which makes it widely used in the industry. The early organic coating molecules were imidazole and benzotriazole, which played a role in preventing rust, and the newest molecules were mainly benzimidazole, which was copper chemically bonding nitrogen functional groups to the PCB. In the subsequent soldering process, if there is only one organic coating layer on the copper surface, it will not work, there must be many layers. This is why copper liquid is usually added to the chemical tank. After the first layer is coated, the coating layer adsorbs copper; then the organic coating molecules in the second layer are combined with copper until twenty or even hundreds of organic coating molecules gather on the copper surface, which can ensure that multiple cycles are performed. Flow welding. Tests have shown that the latest organic coating process can maintain good performance during multiple lead-free soldering processes.
The general flow of the organic coating process is: degreasing → micro-etching → pickling → pure water cleaning → organic coating → cleaning. The process control is easier than other surface treatment processes.

3. Electroless nickel/immersion gold: The process is not as simple as organic coating. The electroless nickel/immersion gold seems to put a thick armor on the PCB; in addition, the electroless nickel/immersion gold process is not like organic coating as a protection Rust barrier layer, it can be useful during long-term use of PCB and achieve good electrical properties. Therefore, electroless nickel/immersion gold is a thick, good electrical nickel-gold alloy on the copper surface, which can protect the PCB for a long time; in addition, it also has environmental tolerance that other surface treatment processes do not have. Sex. The reason for nickel plating is that gold and copper will diffuse each other, and the nickel layer can prevent the diffusion between gold and copper; if there is no nickel layer, gold will diffuse into the copper within a few hours. Another benefit of electroless nickel/immersion gold is the strength of nickel. Only 5 microns of nickel can limit the expansion in the Z direction at high temperatures. In addition, electroless nickel/immersion gold can also prevent the dissolution of copper, which will benefit lead-free assembly.
The general process of electroless nickel plating/gold immersion process is: acid cleaning→micro-etching→pre-dipping→activation→electroless nickel plating→chemical immersion gold. There are mainly 6 chemical tanks, involving nearly 100 chemicals, so the process control is compared difficult.
4. Immersion silver: The process is between organic coating and electroless nickel/immersion gold. The process is relatively simple and fast; it is not as complicated as electroless nickel/immersion gold, and it is not a thick armor for PCB. , But it can still provide good electrical performance. Silver is the little brother of gold. Even if exposed to heat, humidity and pollution, silver can still maintain good solderability, but will lose its luster. Immersion silver does not have the good physical strength of electroless nickel/immersion gold because there is no nickel under the silver layer. In addition, immersion silver has good storage properties, and there will be no major problems in assembly after several years of immersion.
Immersion silver is a displacement reaction, it is almost submicron pure silver coating. Sometimes the immersion silver process also contains some organic matter, mainly to prevent silver corrosion and eliminate silver migration problems; it is generally difficult to measure this thin layer of organic matter, and analysis shows that the weight of the organism is less than 1%.
5. Immersion tin: All current solders are based on tin, so the tin layer can match any type of solder. From this point of view, the immersion tin process is extremely promising. However, tin whiskers appear in the previous PCB after the immersion tin process, and the migration of tin whiskers and tin during the soldering process will cause reliability problems, so the use of the immersion tin process is limited. Later, organic additives were added to the immersion tin solution to make the tin layer structure in a granular structure, which overcomes the previous problems, and also has good thermal stability and solderability.

The immersion tin process can form a flat copper-tin intermetallic compound. This feature makes immersion tin have the same good solderability as hot air leveling without the headache of hot air leveling; there is no electroless nickel plating for immersion tin. /Diffusion between immersion gold metals-copper-tin intermetallic compounds can be firmly bonded together. The immersion tin board cannot be stored for too long, the assembly must be carried out according to the order of immersion tin
6.6. The application of other surface treatment processes is less. Let’s look at the relatively more application of nickel-gold plating and electroless palladium plating processes. 6 {0 U+ O2 f& g) Z5 ]4 Y1 r1 l
Electroplating of nickel and gold is the originator of PCB surface treatment technology. It has appeared since the appearance of PCB, and has gradually evolved into other methods. It is to plate a layer of nickel on the PCB surface conductor first and then a layer of gold. Nickel plating mainly prevents the diffusion between gold and copper. There are two types of electroplated nickel gold: soft gold plating (pure gold, the gold surface does not look bright) and hard gold plating (the surface is smooth and hard, wear-resistant, contains cobalt and other elements, and the gold surface looks brighter). Soft gold is mainly used for gold wire during chip packaging; hard gold is mainly used for electrical interconnection in non-welded areas.

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