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Types of PCB surface coating
2021-09-05
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Author:Aure

Types of PCB surface coating

PCB factory: With the advancement of science and technology, the majority of customers also have more complicated requirements on the surface technology of the PCB. With the continuous improvement of customer requirements, it is also one of the responsibilities of the technicians. The development of PCB circuit board technology has continued to this day, and many PCB surface treatment processes have been produced. The common ones are hot air leveling, organic coating OSP, electroless nickel/immersion gold, immersion silver, immersion tin, and nickel-gold electroplating. Today we will introduce the types of common PCB surface coatings.

1. Hot air leveling HASL
      There are two types of hot-air leveling: vertical and horizontal. In principle, the horizontal type is better. The main reason is that the horizontal hot-air leveling is more uniform. Now automated production has been realized.
      The general flow of the hot air leveling process is: micro-etching→preheating→coating flux→spraying tin→cleaning.


Types of PCB surface coating

2. Organic coated OSP
      There is enough data to show that: the latest organic coating process can maintain good performance during multiple lead-free soldering processes.
      The general process 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 general process of ENIG electroless nickel plating/immersion gold process is: acid cleaning→micro-etching→pre-dip→activation→electroless nickel plating→chemical immersion gold. There are mainly 6 chemical baths, which involve nearly 100 kinds of chemicals, so process control is more difficult.

4. Immersion silver
      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 properties.
      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 be matched with 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.

6. Electroplated nickel gold
      Electrolytic Nickel/Gold is the originator of the PCB surface treatment process. It has appeared since the appearance of PCB, and has gradually evolved into other methods. As shown in Figure 7-17 in f). It is to plate a layer of nickel on the surface of the PCB 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 now: 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).

7. Other surface treatment processes
      The application of other surface treatment processes is less, and the electroless palladium plating process that is relatively more applied is shown below.
      The process of electroless palladium plating is similar to that of electroless nickel plating. The main process is to reduce the palladium ion to palladium on the catalyzed surface by 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.

The above are the types of PCB circuit board surface coatings that we usually see more, and I hope to provide you with some reference..