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Characteristics, application and development trend of PCB surface treatment process

August 22, 2022

With the continuous improvement of human requirements for living environment, the environmental problems involved in the PCB production process are particularly prominent. At present, lead and bromine are the hottest topics; Lead free and halogen-free will affect the development of PCB in many aspects. Although at present, the changes in the surface treatment process of PCB are not great, and it seems that it is still a distant thing, it should be noted that long-term slow changes will lead to great changes. With the increasing demand for environmental protection, the surface treatment process of PCB will certainly undergo great changes in the future.

Purpose of surface treatment
The basic purpose of surface treatment is to ensure good solderability or electrical performance. Since copper in nature tends to exist in the form of oxide in the air, it is unlikely to remain as original copper for a long time, so other treatments are required for copper. Although in the subsequent assembly, strong flux can be used to remove most of the copper oxide, it is not easy to remove the strong flux itself, so the industry generally does not use strong flux.
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Common surface treatment process
At present, there are many PCB surface treatment processes, including hot air leveling, organic coating, electroless nickel plating / gold dipping, silver dipping and tin dipping, which will be introduced one by one.


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 PCB and leveling (blowing) with heated compressed air to form a coating layer that is resistant to copper oxidation and provides good solderability. Copper tin intermetallic compound is formed at the junction of solder and copper by hot air leveling. The thickness of the solder protecting the copper surface is about 1-2 mil. PCB shall be immersed in molten solder during hot air leveling; The air knife blows the liquid solder before the solder solidifies; The wind blade can minimize the meniscus of solder on the copper surface and prevent solder bridging. Hot air leveling is divided into vertical type and horizontal type. It is generally considered that the horizontal type is better, mainly because the horizontal hot air leveling coating is more uniform and can realize automatic production. The general process of hot air leveling process is: Micro etching → preheating → coating flux → spraying tin → cleaning.

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2. Organic coating
The organic coating process is different from other surface treatment processes in that it acts as a barrier layer between copper and air; Organic coating technology is simple and low cost, which makes it widely used in the industry. The early organic coating molecules are imidazole and benzotriazole, which play the role of rust prevention. The latest molecule is mainly benzimidazole, which is the copper that chemically bonds the nitrogen functional group to the PCB. In the subsequent welding process, if there is only one organic coating layer on the copper surface, it is not possible. There must be many layers. This is why liquid copper 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 20 or even 100 times of organic coating molecules gather on the copper surface, which can ensure multiple reflow soldering. The experiment shows that the latest organic coating technology can keep good performance in many lead-free welding processes. The general process 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 plating / gold immersion electroless nickel plating / gold immersion process
Unlike organic coating, electroless nickel plating / gold impregnation seems to put thick armor on PCB; In addition, the electroless nickel plating / gold dipping process is not like the organic coating as the antirust barrier layer. It can be useful in the long-term use of PCB and achieve good electrical performance. Therefore, electroless nickel plating / gold immersion is to wrap a thick layer of nickel gold alloy with good electrical properties 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. 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; Without the nickel layer, the gold will diffuse into the copper within hours. Another advantage of electroless nickel plating / gold impregnation is the strength of nickel. Only 5 microns of nickel can limit the expansion in the Z direction at high temperature. In addition, electroless nickel plating / gold immersion can also prevent the dissolution of copper, which will be beneficial for lead-free assembly. The general process of electroless nickel plating / gold leaching process is: acidic cleaning → micro etching → prepreg → activation → electroless nickel plating → electroless gold leaching. There are mainly 6 chemical tanks, involving nearly 100 chemicals, so the process control is difficult.

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4. Silver leaching process
Between organic coating and electroless nickel plating / gold leaching, the process is relatively simple and fast; It is not as complicated as electroless nickel plating / gold immersion, nor does it put a thick layer of armor on the 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 it will lose luster. Silver immersion does not have the good physical strength of electroless nickel plating / gold immersion because there is no nickel under the silver layer. In addition, the silver impregnation has good storage property, and there will be no big problem when it is put into assembly for several years after the silver impregnation. Silver impregnation is a displacement reaction, which is almost submicron pure silver coating. Sometimes, some organic substances are included in the process of silver leaching, mainly to prevent silver corrosion and eliminate silver migration; 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%.

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5. Tin immersion
Since all solders are based on tin, the tin layer can match any type of solder. From this point of view, the tin dipping process has great development prospects. However, tin whiskers appear after the previous PCB is dipped in tin. During the welding process, the migration of tin whiskers and tin will bring reliability problems. Therefore, the use of tin dipping process is limited. Later, organic additives were added to the tin immersion solution, which can make the tin layer structure appear granular structure, overcome the previous problems, and have good thermal stability and solderability. The tin dipping process can form a flat copper tin intermetallic compound, which makes the tin dipping have the same good solderability as hot-air leveling without the headache of flatness caused by hot-air leveling; There is no diffusion problem between electroless nickel plating / gold dipping metals in tin dipping - copper tin intermetallic compounds can be firmly bonded together. The tin immersion plate shall not be stored for too long, and the assembly must be carried out according to the sequence of tin immersion.

6. Other surface treatment processes
Other surface treatment processes are less applied. The nickel gold plating and electroless palladium plating processes that are relatively more applied are as follows. Nickel gold plating is the originator of PCB surface treatment process. It has appeared since the appearance of PCB, and has gradually evolved into other methods. It is to first coat the conductor on the PCB surface with a layer of nickel and then a layer of gold. Nickel plating is mainly to prevent the diffusion between gold and copper. There are two types of nickel gold plating at present: 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 bright). Soft gold is mainly used for gold wire during chip packaging; Hard gold is mainly used for electrical interconnection at non welded places. In consideration of the cost, the industry often uses image transfer method for selective plating to reduce the use of gold.

At present, the use of selective gold plating in the industry continues to increase, which is mainly due to the difficulty in controlling the process of electroless nickel plating / gold leaching. Under normal circumstances, welding will lead to embrittlement of plated gold, which will shorten the service life. Therefore, welding on plated gold should be avoided; However, due to the thin and consistent gold of electroless nickel plating / gold immersion, embrittlement rarely occurs. The process of electroless palladium plating is similar to that of electroless nickel plating. The main process is to reduce palladium ions to palladium on the catalytic surface through a reducing agent (such as sodium dihydrogen hypophosphite). The newly formed palladium can become a catalyst to promote the reaction, so any thickness of palladium coating can be obtained. The advantages of electroless palladium plating are good welding reliability, thermal stability and surface flatness.
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Selection of surface treatment process
The choice of surface treatment process mainly depends on the type of final assembled components; The surface treatment process will affect the production, assembly and final use of PCB. The following will specifically introduce the application occasions of the five common surface treatment processes.

1. Hot air leveling
Hot air leveling once played a leading role in PCB surface treatment process. In the 1980s, more than three quarters of PCBs used hot-air leveling technology. However, the industry has been reducing the use of hot-air leveling technology in the past decade. It is estimated that about 25% - 40% of PCBs currently use hot-air leveling technology. Hot air leveling process is dirty, smelly and dangerous, so it has never been a favorite process. However, hot air leveling is an excellent process for larger components and wires with larger spacing. In the PCB with high density, the flatness of hot air leveling will affect the subsequent assembly; Therefore, the hot air leveling process is generally not used for HDI boards. With the progress of technology, the hot air leveling process suitable for assembling QFP and BGA with smaller spacing has appeared in the industry, but the actual application is less. At present, some factories use organic coating and electroless nickel plating / gold dipping process to replace the hot air leveling process; Technological development has also led some factories to adopt tin and silver impregnation processes. With the lead-free trend in recent years, the use of hot air leveling is further restricted. Although the so-called lead-free hot air leveling has appeared, it may involve the compatibility of equipment.

2. Organic coating
It is estimated that at present, about 25% - 30% of PCBs use organic coating technology, and the proportion has been rising (it is likely that organic coating has now surpassed hot-air leveling in the first place). The organic coating process can be used for low-tech PCB or high-tech PCB, such as single-sided TV PCB and high-density chip packaging board. For BGA, organic coating is also widely used. If PCB has no functional requirements for surface connection or storage period, organic coating will be the most ideal surface treatment process.
3. Electroless nickel plating / gold immersion electroless nickel plating / gold immersion process
Different from organic coating, it is mainly used on boards with functional requirements for connection and long storage life on the surface, such as mobile phone key area, edge connection area of router shell and electrical contact area of elastic connection of chip processor. Due to the flatness of hot-air leveling and the removal of organic coating flux, electroless nickel plating / gold impregnation was widely used in the 1990s; Later, due to the appearance of black disk and brittle nickel phosphorus alloy, the application of electroless nickel plating / gold dipping process was reduced. However, at present, almost every high-tech PCB Factory has electroless nickel plating / gold dipping line. Considering that the solder joint will become brittle when the copper tin intermetallic compound is removed, many problems will occur at the relatively brittle nickel tin intermetallic compound. Therefore, almost all portable electronic products (such as mobile phones) use copper tin intermetallic compound solder joints formed by organic coating, silver immersion or tin immersion, while electroless nickel plating / gold immersion is used to form key areas, contact areas and EMI shielding areas. It is estimated that at present, about 10% - 20% of PCBs use electroless nickel plating / gold impregnation process.

4. Silver immersion
It is cheaper than electroless nickel plating / gold immersion. If PCB has functional requirements and needs to reduce costs, silver immersion is a good choice; In addition to the good flatness and contact of silver impregnation, the silver impregnation process should be selected. Silver immersion is widely used in communication products, automobiles and computer peripherals, and also in high-speed signal design. Silver impregnation can also be used in high-frequency signals because of its excellent electrical properties that can not be matched by other surface treatments. EMS recommends the silver impregnation process because it is easy to assemble and has good inspectability. However, due to the defects such as tarnish and solder hole in the silver impregnation, its growth is slow (but not decreased). It is estimated that about 10% - 15% of PCBs currently use silver impregnation process.

5. Tin immersion
It has been nearly ten years since tin was introduced into the surface treatment process. The appearance of this process is the result of the requirements of production automation. The tin impregnation does not bring any new elements into the welding place, and is especially suitable for the communication backplane. Tin will lose solderability beyond the storage period of the board, so better storage conditions are required for tin immersion. In addition, the use of tin impregnation process is restricted due to carcinogenic substances. It is estimated that about 5% - 10% of PCBs currently use the tin dipping process. V Conclusion: with the increasingly high requirements of customers, stricter environmental requirements and more and more surface treatment processes, it seems that it is a bit confusing and confusing to choose the surface treatment process with better development prospects and stronger universality. Where the PCB surface treatment process will go in the future can not be accurately predicted now. In any case, meeting customer requirements and protecting the environment must be done first!