PCB News - PCB circuit board surface treatment

The surface treatment process of the circuit board has not changed very much, it seems to be a relatively distant matter, but it should be noted that long-term slow changes will lead to huge changes. With the increasing demand for environmental protection, the surface treatment process of PCB will definitely undergo tremendous changes in the future.

Second, the purpose of surface treatment The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since natural copper tends to exist in the form of oxides in the air, it is unlikely to remain as original copper for a long time, so other treatments are needed for copper. Although in the subsequent assembly, strong flux can be used to remove most of the copper oxides, the strong flux itself is not easy to remove, so the industry generally does not use strong flux.

3. Five common surface treatment processes There are many surface treatment processes for PCB production. 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 Hot air leveling, also known as hot air solder leveling, 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 can Provides a coating layer with good solderability. 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 hot air leveling PCB manufacturers is: micro-etching-preheating-coating flux-spraying tin-cleaning.

2. Organic coating The organic coating process is different from other surface treatment processes in that it acts as a barrier between copper and air; the organic coating process is simple and low in cost, which makes it widely used in the industry. The early organic coating molecules were imidazole and benzotriazole, which played a role in rust prevention, and the newest molecules were mainly benzimidazole, which was the copper chemically bonding nitrogen functional groups to the PCB board.

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 coating the first layer, the coating layer adsorbs copper; then the organic coating molecules of 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-microetching-pickling-pure water cleaning-organic coating-cleaning. The process control is easier than other surface treatment processes.

3. The process of electroless nickel/immersion gold electroless nickel/immersion gold is not as simple as organic coating. Electroless nickel/immersion gold seems to put a thick armor on the PCB; PCB multilayer boards generally adopt chemical immersion gold and OSP is resistant to oxidation, and the electroless nickel/immersion gold process is not like an organic coating as an anti-rust barrier layer, it can be useful in the long-term use of PCB and achieve good electrical properties. Therefore, electroless nickel/immersion gold is to wrap 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 of circuit board manufacturers is: acid cleaning-micro-etching-pre-dipping-activation-electroless nickel plating-chemical immersion gold. There are mainly 6 chemical tanks, involving nearly 100 kinds of chemicals, so Process control is more difficult.

4. Immersion silver The immersion silver 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 does not put a thick layer on the PCB. Armor, 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 silver. 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 Since all current solders are based on tin, 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 tin immersion 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 flatness problem 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 plate cannot be stored for too long, and the assembly must be carried out according to the order of immersion tin.

6. Other surface treatment processes Other surface treatment processes have fewer applications. Let's look at the relatively more application of nickel-gold plating and electroless palladium plating processes. Electroplating of nickel and gold is the originator of PCB surface treatment technology. It has appeared since PCB appeared, and it 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. The nickel plating is mainly to prevent 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. Considering the cost, the industry often adopts the method of image transfer to perform selective electroplating to reduce the use of gold.

At present, the use of selective electroplating gold in the industry continues to increase, which is mainly due to the difficulty of controlling the electroless nickel/immersion gold process. Under normal circumstances, welding will cause the electroplated gold to become brittle, which will shorten the service life, so avoid welding on electroplated gold; but the electroless nickel/immersion gold is very thin and consistent, so brittleness rarely occurs . The process of electroless palladium plating is similar to that of electroless nickel plating. The main process is to reduce the palladium ions to palladium on the catalytic surface through a reducing agent (such as sodium dihydrogen hypophosphite). The new palladium can become a catalyst to promote the reaction, so a palladium coating of any thickness can be obtained. The advantages of electroless palladium plating are good welding reliability, thermal stability, and surface smoothness.