With the continuous improvement of human requirements for the 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, which seems to be a distant thing, it should be noted that long-term slow changes will lead to great changes. With the increasing calls for environmental protection, the surface treatment process of PCB will certainly change dramatically in the future.
Purpose of surface treatment
The most 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 it needs to be treated in other ways. 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.
Common surface treatment process
At present, there are many PCB surface treatment processes, the common ones are hot-air leveling, organic coating, electroless nickel plating / gold dipping, silver dipping and tin dipping, 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 PCB surface and leveling (blowing) it with heated compressed air to form a coating layer that is resistant to copper oxidation and provides good solderability. After hot air leveling, the solder and copper form copper tin intermetallic compound at the junction. 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 out the liquid solder before the solder solidifies; The wind knife 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. Generally speaking, 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 → flux coating → tin spraying → cleaning.
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; The organic coating process is simple and the cost is low, which makes it widely used in the industry. The early organic coating molecules are imidazole and benzotriazole, which play an anti rust role. 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, 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 hundreds of organic coating molecules are concentrated on the copper surface, which can ensure multiple reflow soldering. The test shows that the latest organic coating process can maintain good performance in many lead-free welding processes. The general process of organic coating process is degreasing → micro etching → pickling → pure water cleaning → organic coating → cleaning, and 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 immersion seems to put thick armor on PCB; In addition, the electroless nickel plating / gold immersion process is not like the organic coating as the anti rust barrier layer, which 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 PCB for a long time; In addition, it also has the tolerance to the environment 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; If there is no nickel layer, the gold will diffuse into the copper in a few hours. Another advantage of electroless nickel plating / gold immersion is the strength of nickel. Only nickel with a thickness of 5 microns 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 to lead-free assembly. The general process of electroless nickel plating / gold leaching process is: acid cleaning → micro etching → prepreg → activation → electroless nickel plating → chemical gold leaching. There are mainly 6 chemical tanks, involving nearly 100 chemicals, so the process control is relatively difficult.
4. Silver immersion silver immersion process
Between organic coating and electroless nickel / gold immersion, the process is relatively simple and fast; It is not as complex as electroless nickel plating / gold immersion, nor is it a thick armor for PCB, but it can still provide good electrical performance. Silver is the little brother of gold. Even when 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, silver impregnation has good storage properties, and there will be no major problems when it is put into assembly for a few years after silver impregnation. Silver immersion is a displacement reaction, which is almost submicron pure silver coating. Sometimes, some organic substances are included in the silver immersion process, 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%.
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, in the past, the PCB appeared tin whiskers after the tin dipping process, and the migration of tin whiskers and tin during the welding process would bring reliability problems, so the use of tin dipping process was limited. Later, organic additives were added to the tin immersion solution, which can make the tin layer structure take on a granular structure, overcome the previous problems, and also have good thermal stability and solderability. The tin dipping process can form a flat copper tin intermetallic compound, which makes tin dipping have the same good solderability as hot-air leveling without the headache of flatness caused by hot-air leveling; Tin immersion also has no diffusion problem between electroless nickel plating / gold immersion metals - copper tin intermetallic compounds can be firmly combined. 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. Let's look at the nickel gold plating and electroless palladium plating processes which are relatively more applied. Nickel gold plating is the originator of PCB surface treatment technology. It has appeared since the emergence of PCB, and has gradually evolved into other methods since then. It is to plating a layer of nickel on the PCB surface conductor first 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: 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 making gold wires during chip packaging; Hard gold is mainly used for electrical interconnection at non soldered places. Considering the cost, the industry often carries out selective plating by image transfer 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 the process control of electroless nickel plating / gold leaching. Under normal circumstances, welding will lead to the embrittlement of the plated gold, which will shorten the service life, so it is necessary to avoid welding on the plated gold; However, since the gold in electroless nickel plating / gold immersion is very thin and consistent, 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 generated palladium can become a catalyst to promote the reaction, so that 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 selection 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 use 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, but 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 now 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, hot air leveling process is generally not used for HDI board. 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 it is rarely applied in practice. At present, some factories use organic coating and electroless nickel / gold dipping process to replace the hot air leveling process; Technological development has also led some factories to adopt tin and silver impregnation processes. In addition, the trend of lead-free in recent years has further restricted the use of hot air leveling. 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 this 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 on low-tech PCBs and high-tech PCBs, such as single-sided TV PCBs and high-density chip packaging boards. 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
Unlike organic coating, it is mainly used on boards with connection functional requirements and long storage life on the surface, such as the key area of mobile phones, the edge connection area of the router shell and the electrical contact area of the elastic connection of chip processors. Due to the flatness of hot-air leveling and the removal of organic coating flux, electroless nickel plating / gold immersion 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 lines. Considering that the solder joint will become brittle when removing the copper tin intermetallic compound, 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 dipping process.
4. Silver immersion
It is cheaper than electroless nickel plating / gold immersion. If PCB has connection functional requirements and needs to reduce costs, silver immersion is a good choice; In addition to the good flatness and contact of silver immersion, the silver immersion process should be selected. Silver immersion is widely used in communication products, automobiles, 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 unmatched by other surface treatments. EMS recommends the silver immersion process because it is easy to assemble and has good inspectability. However, due to defects such as tarnish and solder hole in silver immersion, its growth is slow (but not decreased). It is estimated that at present, about 10% - 15% of PCBs use silver impregnation process.
5. Tin immersion
Tin has been introduced into the surface treatment process for nearly a decade, and the emergence of this process is the result of the requirements of production automation. The tin immersion does not bring any new elements into the solder joint, which 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 immersion process is restricted due to the presence of carcinogens. It is estimated that at present, about 5% - 10% of PCBs use the tin dipping process. V Conclusion with the higher and higher 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 which surface treatment process with development prospects and stronger versatility. It is impossible to predict exactly where PCB surface treatment technology will go in the future. In any case, meeting customer requirements and protecting the environment must be done first!