PCB Blog - Features and uses of PCB board surface treatment technology

I. Introduction
With the continuous improvement of human's requirements for the living environment, the environmental problems involved in the current PCB board production process are particularly prominent. The topic of lead and bromine is currently hot; lead-free and halogen-free will affect the development of PCB boards in many ways. Although the changes in the surface treatment process of PCB boards are not very big at present, it seems to be a relatively distant thing, but it should be noted that long-term slow changes will lead to huge changes. Under the circumstance that the voice of environmental protection is getting higher and higher, the surface treatment process of PCB boards will definitely undergo great changes in the future.

2. The purpose of surface treatment
The basic purpose of surface treatment is to ensure good solderability or electrical properties. Since copper in nature tends to exist in the form of oxides in the air and is unlikely to remain as raw copper for long periods of time, additional copper treatments are required. Although in subsequent assembly, strong flux can be used to remove most copper oxides, but 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 board. The common ones are hot air leveling, organic coating, electroless nickel plating/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 resists copper oxidation and provides good reliability. Weldable coating. Solder and copper form a copper-tin intermetallic compound at the junction during hot air leveling. The thickness of the solder to protect the copper surface is about 1-2 mils. The PCB board is immersed in molten solder when it is hot air leveled; the air knife blows the liquid solder before the solder solidifies; the air knife can meniscus the solder on the copper surface and prevent the solder from bridging. Hot air leveling is divided into two types: vertical type and horizontal type. It is generally considered that the horizontal type is better, mainly because the horizontal hot air leveling layer is more uniform and can realize automatic production. The general process of the 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, it acts as a barrier layer between copper and air; the organic coating process is simple and low cost, which makes it widely used in the industry. The molecules of the early organic coating are imidazole and benzotriazole that play a role in rust prevention, and the molecule is mainly benzimidazole, which is a chemically bonded nitrogen functional group to the copper on the PCB board. In the subsequent soldering process, if only one organic coating layer on the copper surface is not acceptable, there must be many layers. This is why copper liquids are usually added to chemical tanks. After the coating layer, the coating layer adsorbs copper; then the organic coating molecules of the second layer combine with copper until twenty or even hundreds of organic coating molecules are assembled on the copper surface, which can ensure multiple reflow soldering . Tests show that: the organic coating process can maintain good performance in multiple lead-free soldering processes. The general process of the 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/Immersion Gold
The electroless nickel plating/immersion gold process is not as simple as organic coating. Electroless nickel plating/immersion gold seems to put a thick armor on the PCB; in addition, the electroless nickel plating/immersion gold process is not like organic coating as a rust barrier. layer, it can be useful in the long-term use of the PCB board and achieve good electrical performance. Therefore, electroless nickel plating/immersion gold is to wrap a thick layer of nickel-gold alloy with good electrical properties on the copper surface, which can protect the PCB board for a long time; in addition, it also has environmental protection that other surface treatment processes do not have. Patience. The reason for nickel plating is that gold and copper interdiffuse, and the nickel layer prevents the diffusion between gold and copper; without the nickel layer, the gold would diffuse into the copper within hours. Another benefit of electroless nickel/immersion gold is the strength of nickel, as only 5 microns of nickel can limit Z-direction expansion at high temperatures. In addition, electroless nickel/immersion gold also prevents the dissolution of copper, which will benefit lead-free assembly. The general process of electroless nickel plating / immersion gold process 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 the process control comparison difficulty.

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, nor does it put a thick layer of armor on the PCB, but It still provides good electrical performance. Silver is gold's little brother and retains good solderability even when exposed to heat, humidity and pollution, but tarnishes. 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 a few years of immersion silver. Immersion silver is a displacement reaction, it is almost a sub-micron coating of pure silver. Sometimes the immersion silver process also contains some organic matter, mainly to prevent silver corrosion and eliminate the problem of silver migration; it is generally difficult to measure this thin layer of organic matter, and the analysis shows that the weight of the organic matter is less than 1%.

5) Immersion Tin
Since all current solders are based on tin, the tin layer can be matched to any type of solder. From this point of view, the immersion tin process has great prospects for development. However, tin whiskers appeared on the previous PCB boards after the tin immersion process, and the migration of tin whiskers and tin during the soldering process would bring reliability problems, so the adoption of the tin immersion process was limited. Later, organic additives were added to the tin immersion solution, which can make the tin layer structure have 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, which makes immersion tin have the same good solderability as hot air leveling without the headache of hot air leveling; immersion tin also has no electroless nickel plating / Diffusion problem between immersion gold metals - copper-tin intermetallic compounds can be firmly bonded together. The tin immersion board cannot be stored for too long, and the assembly must be carried out according to the order of tin immersion.

6) Other surface treatment processes
There are few applications of other surface treatment processes. Let's take a look at the relatively more nickel-gold electroplating and electroless palladium plating processes. Electroplating nickel-gold is the originator of the surface treatment process of PCB boards. It has appeared since the appearance of PCB boards, and has gradually evolved into other methods. It is to first coat a layer of nickel on the conductor on the surface of the PCB and then coat with a layer of gold. Nickel plating is mainly to prevent the diffusion between gold and copper. Now 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 other elements such as cobalt, and the gold surface looks brighter). Soft gold is mainly used for gold wire in chip packaging; hard gold is mainly used for electrical interconnection in non-soldering places. Considering the cost, the industry often uses image transfer to reduce the use of gold by selective electroplating. At present, the use of selective gold plating in the industry continues to increase, mainly due to the difficulty in controlling the electroless nickel plating/immersion gold process. Under normal circumstances, welding will cause electroplated gold to become brittle, which will shorten the service life, so avoid welding on electroplated gold; but electroless nickel plating/immersion gold is very thin and consistent, and 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 by a reducing agent (such as sodium dihydrogen hypophosphite), and the new palladium can become a catalyst to promote the reaction, so that a palladium coating of any thickness can be obtained. The advantages of electroless palladium plating are good soldering reliability, thermal stability, and surface flatness.

4. The choice of surface treatment technology
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 the PCB board. The following will specifically introduce the use of five common surface treatment processes.
1) Hot air leveling
Hot air leveling once dominated the PCB surface treatment process. In the 1980s, more than three-quarters of the PCB boards used the hot air leveling process, but the industry has been reducing the use of the hot air leveling process over the past decade. It is estimated that there are currently about 25%-40% of the PCB boards. Use a hot air levelling process. The dirty, smelly, and dangerous process of hot air leveling has never been a favorite process, but it is an excellent process for larger components and larger pitch wires. In the PCB board with higher density, the flatness of the hot air leveling will affect the subsequent assembly; therefore, the HDI board generally does not use the hot air leveling process. With the advancement of technology, the industry has now developed a hot air leveling process suitable for QFP and BGA with smaller assembly pitch, but there are few practical applications. At present, some factories use organic coating and electroless nickel plating/immersion gold process to replace the hot air leveling process; technological development also makes some factories adopt immersion tin and immersion silver process. Coupled with the trend of lead-free in recent years, the use of hot air leveling is further restricted. Although so-called lead-free hot air leveling has appeared, this may involve equipment compatibility issues.

2) Organic coating
It is estimated that about 25%-30% of PCB boards currently use an organic coating process, and this proportion has been rising (it is likely that organic coating is now in place over hot air leveling). The organic coating process can be used on low-tech PCB boards as well as high-tech PCB boards, such as single-sided TV PCB boards and high-density chip packaging boards. For BGA, there are also many applications of organic coating. If the PCB board does not have surface connection functional requirements or shelf life restrictions, organic coating will be an ideal surface treatment process.

3) Electroless Nickel/Immersion Gold
The electroless nickel/immersion gold process is different from organic coating. It is mainly used on boards with surface connection functional requirements and long shelf life, such as mobile phone key areas, edge connection areas of router casings, and chip processor elasticity. The electrical contact area of the connection. Electroless nickel/immersion gold was widely used in the 1990s due to the flatness problem of hot air leveling and the removal of organically coated flux; The application of immersion gold/immersion gold process has decreased, but almost every high-tech PCB board factory has electroless nickel/immersion gold wire. Considering the brittleness of the solder joints when removing the copper-tin intermetallics, there are many problems with the relatively brittle nickel-tin intermetallics. Therefore, almost all portable electronic products (such as mobile phones) use copper-tin intermetallic compound solder joints formed by organic coating, immersion silver or immersion tin, and use electroless nickel plating/immersion gold to form key areas, contact areas and EMI shielding areas . It is estimated that about 10%-20% of PCB boards currently use electroless nickel/immersion gold process.

4) Immersion silver
Immersion silver is cheaper than electroless nickel plating/immersion gold. If the PCB board has connection functional requirements and needs to reduce costs, immersion silver is a good choice; plus the good flatness and contact of immersion silver, then immersion silver should be chosen more Silver craft. There are many applications of immersion silver in communication products, automobiles, and computer peripherals, and immersion silver is also used in high-speed signal design. Because immersion silver has good electrical properties unmatched by other surface treatments, it can also be used in high frequency signals. EMS recommends the immersion silver process because of its ease of assembly and better inspectability. However, the growth of immersion silver is slow (but not decreased) due to defects such as tarnishing and solder joint voids. It is estimated that about 10%-15% of PCB boards currently use the immersion silver process.

5) Immersion Tin
The introduction of tin into the surface treatment process is a matter of nearly ten years, and the emergence of this process is the result of the requirement of production automation. Immersion tin does not bring any * elements into the welding place, especially suitable for communication backplanes. Tin will lose solderability beyond the shelf life of the board, so immersion tin requires better storage conditions. In addition, the use of tin immersion process is restricted due to the presence of carcinogens. It is estimated that about 5%-10% of PCB boards currently use the immersion tin process.

5. Conclusion
As customer requirements are getting higher and higher, environmental requirements are getting stricter, and surface treatment processes are becoming more and more numerous, it seems a bit dazzling and confusing which surface treatment process to choose which has development prospects and more versatility. . Where the PCB board surface treatment process will go in the future cannot be accurately predicted now.