Detailed explanation: What are the PCB surface treatment processes?

With the continuous improvement of human requirements for the living environment, the environmental problems involved in the current PCB production process are particularly prominent. Currently the topic of lead and bromine is the hottest; lead-free and halogen-free will affect the development of PCB in many ways. Although at present, the changes in the surface treatment process of PCB are not very big, it seems to be a relatively distant thing, but it should be noted that long-term slow changes will lead to huge changes. With the growing demand 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 copper in nature tends to exist in the form of oxides in the air, it is unlikely to remain as raw copper for a long time, so other treatments are required for copper. Although strong flux can be used to remove most copper oxides in subsequent assembly, strong flux itself is not easy to remove, so the industry generally does not use strong flux.

Now there are many PCB surface treatment processes, 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 leveling (blowing) it with heated compressed air to form a layer that resists copper oxidation and provides good solderability. permanent coating. Solder and copper form a copper-tin intermetallic compound at the joint during hot air leveling. The thickness of the solder to protect the copper surface is about 1-2mil,

When the PCB is hot air leveled, it must be immersed in the molten solder; the air knife blows the liquid solder before the solder solidifies; the air knife can minimize the meniscus of the solder on the copper surface and prevent solder bridging. Hot air leveling is divided into vertical type and horizontal type. It is generally believed 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 the hot air leveling process is: micro-etching→preheating→coating flux→spraying tin→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. Early organic coating molecules were imidazoles and benzotriazoles for rust prevention, and the latest molecules are mainly benzimidazoles, which are copper chemically bonded nitrogen functional groups to the PCB. In the subsequent soldering process, if there is only one layer of organic coating on the copper surface, it will not work, and there must be many layers. This is why chemical baths usually require copper addition. After coating the first 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 cycles. flow soldering. Tests have shown that the latest 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. The process control is easier than other surface treatment processes.

3. Electroless nickel/immersion gold

The electroless nickel/immersion gold process is not as simple as organic coating. The electroless nickel/immersion gold seems to put thick armor on the PCB; in addition, the electroless nickel/immersion gold process is not like organic coating as an anti-rust barrier layer , it can be useful and achieve good electrical performance during the long-term use of PCB. Therefore, electroless nickel plating/immersion gold is to coat the copper surface with a thick layer of nickel-gold alloy with good electrical properties, which can protect the PCB for a long time; in addition, it also has environmental tolerance that other surface treatment processes do not have sex. The reason for the nickel plating is that the gold and copper will diffuse into each other, and the nickel layer can prevent the diffusion between the gold and the copper; without the nickel layer, the gold will diffuse into the copper within a few hours. Another benefit of electroless nickel/immersion gold is the strength of nickel, which can limit the expansion in the z direction at high temperature with only 5 micron thickness of nickel. In addition, electroless nickel/immersion gold can also prevent copper dissolution, which will benefit lead-free assembly.

The general process of 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 tanks, involving nearly 100 kinds of chemicals, so the process control is relatively 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 is it to put a thick layer of armor on the PCB, but it Still able to provide good electrical performance. Silver is the little brother of gold, and even when exposed to heat, humidity and pollution, silver still maintains good solderability, but it will lose its luster. Immersion silver does not have the good physical strength of electroless nickel/immersion gold because there is no nickel underneath 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, which is almost a submicron coating of pure silver. Sometimes some organic matter is also included in the silver immersion process, 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 organic matter is less than 1%.

5. Immersion tin

Since all current solders are tin-based, the tin layer matches any type of solder. From this point of view, the immersion tin process has great development prospects. However, tin whiskers appeared on the previous PCB after the immersion tin process, and tin whiskers and tin migration during the soldering process would cause reliability problems, so the adoption of the immersion tin process was limited. Later, organic additives were added to the immersion tin solution to make the tin layer structure 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 troublesome flatness problem of hot air leveling; immersion tin also has no electroless nickel plating / Immersion gold intermetallic diffusion problem - copper-tin intermetallic compounds can be firmly bonded together. Tin dipping boards cannot be stored for too long, and must be assembled according to the order of tin dipping.

6. Other surface treatment processes

The application of other surface treatment processes is less, let's look at the relatively more applied nickel-gold plating and electroless plating processes.

Nickel-gold electroplating is the originator of PCB surface treatment technology. It has appeared since the appearance of PCB, and then slowly evolved into other methods. It is to plate a layer of nickel on the surface of the PCB and then plate a layer of gold. The 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 making gold wires in chip packaging; hard gold is mainly used for electrical interconnection at non-welding places.

Considering the cost, the industry often performs selective plating by image transfer to reduce the use of gold. The use of selective electroplating gold continues to increase in the industry, mainly due to the difficulty of electroless nickel/immersion gold process control.

Under normal circumstances, soldering will cause the electroplated gold to become brittle, which will shorten the service life, so soldering on electroplated gold should be avoided; however, electroless nickel/immersion gold is very thin and consistent, and brittleness rarely occurs .

The process of electroless plating is similar to that of electroless nickel plating. The main process is to reduce  ions to on the catalytic surface through a reducing agent (such as sodium dihydrogen hypophosphite), and the nascent can become a catalyst to promote the reaction, so  coatings of any thickness can be obtained. The advantages of electroless plating are good soldering reliability, thermal stability, and surface smoothness.

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. The following will specifically introduce the use of five common surface treatment processes.

1. Hot air leveling

Hot air leveling used to play a dominant role in the PCB surface treatment process. In the 1980s, more than three-quarters of PCBs used the hot air leveling process, but the industry has been reducing the use of the hot air leveling process in the past decade. It is estimated that about 25%-40% of the PCBs currently use hot air Leveling process. The hot air leveling process is dirty, smelly, dangerous and has never been a favorite process, but it is an excellent process for larger components and finer pitch wires. In higher-density PCBs, the flatness of hot air leveling will affect subsequent assembly; therefore, HDI boards generally do not use hot air leveling. With the advancement of technology, the hot air leveling process suitable for assembling QFP and BGA with smaller spacing has appeared in the industry, but there are few practical applications. At present, some factories adopt organic coating and chemical nickel plating/immersion gold process instead of hot air leveling process; technical 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 the so-called lead-free hot air leveling has appeared, this may involve the compatibility of equipment.

2. Organic coating

It is estimated that about 25%-30% of PCBs currently use organic coating technology, and this proportion has been rising (it is likely that organic coating has surpassed hot air leveling and ranks first). The organic coating process can be used on low-tech PCBs as well as high-tech PCBs, such as PCBs for single-sided TVs and boards for high-density chip packaging. For BGA, there are many organic coating applications. If the PCB has no functional requirements for surface connection or limited storage period, organic coating will be the most 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 connection functional requirements and long storage period on the surface, such as the keypad area of mobile phones, the edge connection area of ​​router shells and the flexibility of chip processors. Connected electrical contacts. Electroless nickel/immersion gold was widely used in the 1990s due to flatness issues with hot air leveling and removal of organically coated fluxes; The application of /immersion gold process has been reduced, but at present, almost every high-tech PCB factory has electroless nickel plating/immersion gold wire. Considering that the solder joint becomes brittle when the copper-tin intermetallics are removed, many problems will arise at 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 the key area, contact area and EMI shielding area. . It is estimated that about 10%-20% of PCBs currently use electroless nickel/immersion gold processes.

4. Immersion silver

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

5. Immersion tin

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. Immersion tin does not bring any elements into the soldering place, especially suitable for communication backplanes. Tin will lose its solderability beyond the storage period of the board, so immersion tin requires better storage conditions. In addition, due to the carcinogens contained in the immersion tin process, its use is restricted. It is estimated that about 5%-10% of PCBs currently use the immersion tin process.

It is impossible to accurately predict where the PCB surface treatment process will go in the future. As the requirements of customers are getting higher and higher, the requirements of the environment are getting stricter, and there are more and more surface treatment processes. It seems a bit confusing and confusing to choose the kind of surface treatment process that has development prospects and is more versatile. . In any case, meeting customer requirements and protecting the environment must be done first.