At present, the environmental problems involved in the PCB production process are particularly prominent. Lead and bromine are the hottest topics at the moment; Lead-free and halogen-free will affect the development of PCB in many ways. Although at present, the PCB surface treatment process changes are not very large, it seems to be a relatively distant thing, but it should be noted that long-term slow changes will lead to great changes. In the case of increasing calls for environmental protection, the surface treatment process of PCB will definitely change dramatically in the future.
With the continuous improvement of human requirements for the living environment, the environmental problems involved in the current PCB production process are particularly prominent. Lead and bromine are the hottest topics at the moment; Lead-free and halogen-free will affect the development of PCB in many ways. Although at present, the PCB surface treatment process changes are not very large, it seems to be a relatively distant thing, but it should be noted that long-term slow changes will lead to great changes. In the case of increasing calls for environmental protection, the surface treatment process of PCB will definitely change dramatically in the future.
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 a strong flux can be used to remove most copper oxides in subsequent assembly, the strong flux itself is not easy to remove, so the industry generally does not use strong flux.
There are many PCB surface treatment processes, the common is hot air smoothing, organic coating, chemical nickel plating/gold dipping, silver dipping and tin dipping these five processes, the following will introduce the common five surface treatment processes one by one.
1. hot air leveling
Hot air smoothing, also known as hot air solder smoothing, is the process of coating the PCB surface with molten tin lead solder and heating compressed air to smooth (blow), so that it forms a coating layer that is resistant to copper oxidation and can provide good solderability. During hot air conditioning, a copper-tin metal intermetallic compound is formed at the joint of solder and copper. The solder protecting the copper surface is about 1-2mil thick.
The PCB should be immersed in the molten solder when the hot air is finished. The air knife blows out the liquid solder before it solidifies; The air knife minimizes the meniscus of solder on the copper surface and prevents solder bridging. Hot air is divided into vertical type and horizontal type two, it is generally considered that the horizontal type is better, mainly the horizontal hot air leveling coating is more uniform, can achieve automatic production. The general flow of hot air leveling process is: micro-etching → preheating → coating flux → 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. Organic coating is simple and inexpensive, which makes it widely used in the industry. The early organic coated molecules were imidazole and benzotriazole, which act as rust inhibitors, and the latest molecules are mainly benzoimidazole, which is a chemical bond of nitrogen functional groups to copper on the PCB. In the subsequent welding process, if there is only one layer of organic coating on the copper surface is not enough, there must be many layers. This is why it is often necessary to add copper liquid to the chemical tank. After coating the first layer, the coating layer adsorbs copper; The second layer of organic coating molecules is then combined with the copper until twenty or even hundreds of organic coating molecules are assembled on the copper surface, which can ensure multiple reflow welding. The experiments show that the latest organic coating process can maintain good performance in multiple lead-free welding processes.
The general flow 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 dipping
Electroless nickel plating/gold dipping process is not as simple as organic coating, electroless nickel plating/gold dipping seems to put on a thick armor to the PCB; In addition, the electroless nickel plating/gold dipping process is not like organic coating as an anti-rust barrier layer, which can be useful in the long-term use of PCB and achieve good electrical properties. Therefore, electroless nickel plating/gold dipping is to wrap a thick, electrically good 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. 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 would diffuse into the copper within hours. Another benefit of electroless nickel plating/gold leaching is the strength of nickel, which is only 5 microns thick and can limit Z-direction expansion at high temperatures. In addition, electroless nickel plating/gold dipping can also prevent the dissolution of copper, which will benefit lead-free assembly.
The general process of electroless nickel plating/gold leaching process is: acid cleaning → micro-etching → pre-leaching → activation → electroless nickel plating → chemical gold leaching, there are mainly 6 chemical tanks, involving nearly 100 chemicals, so the process control is more difficult.
4. Dip in silver
Silver leaching process is between organic coating and electroless nickel plating/gold leaching, the process is relatively simple and fast; It is not as complex as electroless nickel plating/gold dipping, nor is it a thick layer of armor for the PCB, but it can still provide good electrical performance. Silver is the younger brother of gold, even when exposed to heat, humidity and pollution, silver is still able to maintain good weldability, but will lose its luster. Silver leaching does not have the good physical strength of electroless nickel plating/gold leaching because there is no nickel underneath the silver layer. In addition, silver immersion has good storage, and there will be no big problems after a few years of assembly after silver immersion.
Silver leaching is a displacement reaction, it is almost submicron level of pure silver coating. Sometimes the silver leaching process also contains some organic matter, mainly to prevent silver corrosion and eliminate silver migration problems; This thin layer of organic matter is generally difficult to measure, and analysis suggests that the organism is less than 1% by weight.
5. Dip tin
Since all current solders are tin-based, the tin layer can be matched to any type of solder. From this point of view, tin leaching process has great prospects for development. However, the previous PCB after the tin dip process appeared tin whiskers, tin whiskers and tin migration during the welding process will bring reliability problems, so the use of tin dip process is limited. Later, organic additives were added to the tin immersion solution, which can make the tin layer structure have a granular structure, overcome the previous problems, and also have good thermal stability and weldability.
Tin dipping process can form a flat copper-tin metal intermetallic compound, which makes tin dipping have the same good solderability as hot air smoothing without the headache flatness problem of hot air smoothing; Tin immersion also does not have the diffusion problem between electroless nickel plating/gold immersion metals - the copper-tin metal intermetallic compounds are able to bond firmly together. The tin plate cannot be stored for too long, and must be assembled according to the order of the tin immersion.
The application of other surface treatment processes is less, and the following is a look at the application of relatively more nickel gold plating and chemical palladium plating processes.
Electroplated nickel gold is the originator of PCB surface treatment process, since the appearance of PCB it has appeared, and then slowly evolved into other ways. It is the PCB surface conductor is first plated with a layer of nickel and then plated with a layer of gold, 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, gold surface does not look bright) and hard gold plating (smooth and hard surface, wear-resistant, containing other elements such as cobalt, gold surface looks brighter). Soft gold is mainly used for chip packaging gold wire; Hard gold is mainly used in non-welded electrical interconnections.
Considering the cost, the industry often uses the method of image transfer for selective plating to reduce the use of gold. At present, the use of selective electroplating continues to increase in the industry, mainly due to the difficulty of controlling the electroless nickel plating/gold leaching process.
Under normal circumstances, welding will cause the electroplating to become brittle, which will shorten the service life, so avoid welding on the electroplating; However, electroless nickel plating/gold leaching rarely occurs because gold is very thin and consistent.
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 catalyzed surface by a reducing agent (such as sodium dihydrogen hypophosphate), and the newly formed palladium can be used as a catalyst to promote the reaction, so that palladium coatings of any thickness can be obtained. The advantages of electroless palladium plating are good welding reliability, thermal stability and surface flatness.
The choice of surface treatment process mainly depends on the type of final assembly components; The surface treatment process will affect the production, assembly and final use of PCB, and the following will specifically introduce the use of five common surface treatment processes.
1. hot air leveling
Hot air smoothing used to be the dominant PCB surface treatment process. In the 1980s, more than three-quarters of PCBS used hot air smoothing, but the industry has been reducing the use of hot air smoothing over the past decade, and it is estimated that about 25% to 40% of PCBS now use hot air smoothing. The hot air leveling process is dirty, smelly, and dangerous, so it has never been a favorite process, but hot air leveling is an excellent process for larger components and wider-spaced wires. In PCBS with higher density, the flatness of hot air smoothing will affect subsequent assembly; Therefore, HDI board generally does not use hot air leveling process. With the advancement of technology, the industry now has a hot air smoothing process suitable for assembling QFP and BGA with smaller spacing, but the practical application is less. At present, some factories use organic coating and electroless nickel plating/gold dipping process instead of hot air leveling process; The development of technology has also made some factories use tin leaching and silver leaching processes. Coupled with the trend of lead-free in recent years, the use of hot air leveling is further restricted. Although there are currently so-called lead-free hot air smoothing, but this can involve equipment compatibility issues.
2. organic coating
It is estimated that about 25%-30% of PCBS currently use organic coating processes, and this proportion has been increasing (it is likely that organic coating has now overtaken hot air finishing in first place). The organic coating process can be used in low-tech PCBS, but also can be used in high-tech PCBS, such as single-side TV PCB, high-density chip packaging board. For BGA, the application of organic coating is also more. PCB if there is no surface connection functional requirements or storage period limits, organic coating will be the most ideal surface treatment process.
3. electroless nickel plating/gold dipping
Electroless nickel plating/gold dipping process is different from organic coating, it is mainly used in the surface of the connection functional requirements and long storage life of the board, such as the key area of the mobile phone, the edge connection area of the router housing and the electrical contact area of the chip processor elastic connection. Due to the flatness of hot air smoothing and the removal of organic coated flux, electroless nickel plating/gold dipping was widely used in the 1990s. Later, due to the emergence of black disks and brittle nickel-phosphorus alloys, the application of electroless nickel plating/gold dipping process has been reduced, but 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 in the relatively brittle nickel-tin intermetallic compound. Therefore, portable electronic products (such as mobile phones) almost all use organic coating, silver or tin-dipped copper-tin intermetallic compound solder joints, and use electroless nickel plating/gold dipping to form key areas, contact areas and EMI shielding areas. It is estimated that about 10-20% of PCBS currently use the electroless nickel plating/gold leaching process.
4. Dip in silver
Silver dipping is cheaper than electroless nickel plating/gold dipping, and if the PCB has connection functional requirements and needs to reduce costs, silver dipping is a good choice; Coupled with the good flatness and contact of silver immersion, it is more appropriate to choose the silver immersion process. There are many applications of silver immersion in communication products, automobiles, computer peripherals, and silver immersion in high-speed signal design. Because silver immersion has good electrical properties that cannot be matched by other surface treatments, it can also be used in high-frequency signals. EMS recommends the silver immersion process because it is easy to assemble and has good inspectability. However, due to the defects of silver leaching such as tarnish and solder joint holes, its growth was slow (but did not decline). It is estimated that about 10% to 15% of PCBS currently use the silver immersion process.
5. Dip tin
The introduction of tin into the surface treatment process is a matter of nearly ten years, and the emergence of the process is the result of the requirements of production automation. Tin immersion does not introduce any element into the weld, especially suitable for communication backplanes. Tin will lose solderability outside the shelf life of the board, so better storage conditions are required for tin immersion. In addition, the use of tin leaching process is restricted due to carcinogenic substances. It is estimated that about 5%-10% of PCBS currently use the tin-dipping process.
Where the PCB surface treatment process will go in the future can not be accurately predicted now. With more and more high customer requirements, more and more stringent environmental requirements, more and more surface treatment processes, in the end, the choice of the kind of development prospects, more versatile surface treatment process, it seems a little dazzling, confusing. In any case, meeting customer requirements and protecting the environment must be done first!
English
Chinese