PCB Technical - PCB surface treatment process characteristics, uses and development trends

PCB surface treatment process characteristics, uses and development trends

The PCB factory noticed that with the continuous improvement of science and technology, the environmental problems involved in the current PCB production process appear to be particularly prominent. At present, the topic of lead and bromine is the most popular; 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 very big, it seems to be a relatively remote thing, but it should be noted that long-term slow changes will lead to huge changes. With the increasing demand for environmental protection, the surface treatment process of PCB will definitely undergo tremendous changes in the future.

Purpose of surface treatment

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 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.

There are many PCB surface treatment processes, the common ones are hot air leveling, organic coating, electroless nickel/immersion gold, immersion silver and immersion tin, which will be introduced one by one below.

1. Hot air leveling

Hot-air leveling is also known as hot-air solder leveling. It 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 is resistant to copper oxidation and provides good solderability. Sexual coating. During hot air leveling, the solder and copper form a copper-tin intermetallic compound at the joint. The thickness of the solder to protect the copper surface is about 1-2 mils.

The PCB should be immersed in molten solder during hot air leveling; 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. There are two types of hot air leveling: vertical and horizontal. Generally, the horizontal type is considered to be better. The main reason is that the horizontal hot air leveling is more uniform and can realize automated production. The general flow 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 in that it acts as a barrier between copper and air; the organic coating process is simple and low in cost, which makes it widely used in the industry. The early organic coated molecules were imidazole and benzotriazole, which played a role in rust prevention, and the newest molecules were mainly benzimidazole, which was copper that chemically bonded nitrogen functional groups to the PCB.

In the subsequent soldering process, if there is only one organic coating layer on the copper surface, it will not work, there must be many layers. This is why copper liquid 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 twenty or even hundreds of organic coating molecules gather on the copper surface, which can ensure that multiple cycles are performed. Flow welding.

Tests have shown that the latest organic coating process can maintain good performance during multiple lead-free soldering processes. The general flow 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/immersion gold

The electroless nickel/immersion gold process is not as simple as the organic coating. The electroless nickel/immersion gold seems to put a thick armor on the PCB; in addition, the electroless nickel/immersion gold process is not like the organic coating as an anti-rust barrier layer, It can be useful in the long-term use of PCB and achieve good electrical performance.

Therefore, electroless nickel/immersion gold is to wrap a thick, good electrical nickel-gold alloy 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 will diffuse each other, and the nickel layer can prevent the diffusion between gold and copper; if there is no nickel layer, gold will diffuse into the copper within a few hours.

Another benefit of electroless nickel/immersion gold is the strength of nickel. Only 5 microns of nickel can limit the expansion in the Z direction at high temperatures. In addition, electroless nickel/immersion gold can also prevent the dissolution of copper, which will benefit lead-free assembly. The general process of electroless nickel plating/gold immersion 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 is compared 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 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 will lose its luster.

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 several years of immersion silver. Immersion silver is a displacement reaction, it is almost submicron pure silver coating. Sometimes the immersion silver process also contains some organic matter, 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 the organism is less than 1%.

5. Immersion tin

Since all current solders are based on tin, the tin layer can be matched with any type of solder. From this point of view, the immersion tin process is extremely promising. However, tin whiskers appear in the previous PCB after the immersion tin process, and the migration of tin whiskers and tin during the soldering process will cause reliability problems, so the use of the immersion tin process is limited.

Later, organic additives were added to the tin immersion solution to make the tin layer structure in 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. This feature makes immersion tin have the same good solderability as hot air leveling without the headache flatness problem of hot air leveling; there is no electroless nickel plating for immersion tin /Diffusion between immersion gold metals-copper-tin intermetallic compounds can be firmly bonded together. The immersion tin plate cannot be stored for too long, and the assembly must be carried out according to the order of immersion tin.

6. Other surface treatment processes

There are fewer applications of other surface treatment processes. Let’s look at the relatively more application of nickel-gold electroplating and electroless palladium plating processes. Electroplating of nickel and gold is the originator of PCB surface treatment technology. It has appeared since PCB appeared, and it has gradually evolved into other methods. It is to plate a layer of nickel on the PCB surface conductor first and then a layer of gold. The 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, 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 brighter).

Soft gold is mainly used for gold wire during chip packaging; hard gold is mainly used for electrical interconnection in non-welded areas. Considering the cost, the industry often adopts the method of image transfer to perform selective electroplating to reduce the use of gold. At present, the use of selective electroplating gold in the industry continues to increase, which is mainly due to the difficulty of controlling the electroless nickel/immersion gold process.

Under normal circumstances, welding will cause the electroplated gold to become brittle, which will shorten the service life, so avoid welding on electroplated gold; but the electroless nickel/immersion gold is very thin and consistent, so brittleness rarely occurs . The process of electroless palladium plating is similar to that of electroless nickel plating. The main process is to reduce the palladium ions to palladium on the catalytic surface through a reducing agent (such as sodium dihydrogen hypophosphite). 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 welding reliability, thermal stability, and surface smoothness.

Selection of surface treatment process

The choice of surface treatment process mainly depends on the type of final assembly components; the surface treatment process will affect PCB production, assembly and final use. The following will specifically introduce the use of five common surface treatment processes.

1. Hot air leveling

Hot air leveling was once in a dominant position in the PCB surface treatment process. In the 1980s, more than three-quarters of PCBs used hot air leveling processes, but the industry has been reducing the use of hot air leveling processes in the past ten years. It is estimated that about 25%-40% of PCBs currently use hot air. Leveling process.

The hot air leveling process is dirty, unpleasant, and dangerous, so it has never been a favorite process, but hot air leveling is an excellent process for larger components and wires with larger spacing. In high-density PCBs, the flatness of hot air leveling will affect subsequent assembly; therefore, HDI boards generally do not use hot air leveling processes. With the advancement of technology, the industry now has a hot air leveling process suitable for assembling QFPs and BGAs with smaller pitches, but there are fewer practical applications.

At present, some PCB factories use organic coating and electroless nickel/immersion gold processes to replace the hot air leveling process; technological developments have also led some factories to use immersion tin and silver immersion processes. Coupled with the lead-free trend in recent years, the use of hot air leveling has been 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 now surpassed hot air leveling in the first place). The organic coating process can be used on low-tech PCBs or high-tech PCBs, such as PCBs for single-sided TVs and boards for high-density chip packaging. For BGA, there are also more applications of organic coating. If PCB does not have functional requirements for surface connection or a limitation of storage period, organic coating will be the most ideal surface treatment process.

3. Electroless nickel plating/immersion gold

The electroless nickel/immersion gold process is different from organic coating. It is mainly used on boards that have functional requirements for connection and a long storage period, such as mobile phone keypads, edge connection areas of router housings, and chip processor flexibility. The electrical contact area of the connection.

Due to the flatness problem of hot air leveling and the removal of organic coating flux, electroless nickel/immersion gold was widely used in the 1990s; later, due to the appearance of black disks and brittle nickel-phosphorus alloys, electroless nickel plating /The application of immersion gold process has decreased, but at present almost every high-tech PCB factory has electroless nickel plating/immersion gold wire. Considering that the solder joint will become brittle when removing the copper-tin intermetallic compound, there will be many problems in the relatively brittle nickel-tin intermetallic compound.

Therefore, portable electronic products (such as mobile phones) almost all use organic coating, immersion silver or immersion tin formed copper-tin intermetallic compound solder joints, while electroless nickel/immersion gold is used 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 functional requirements for connection and needs to reduce costs, immersion silver is a good choice; coupled with the good flatness and contact of immersion silver, it is better to choose immersion silver Craft. There are many immersion silver applications in communication products, automobiles, and computer peripherals, and immersion silver also has applications in high-speed signal design. Because immersion silver has good electrical properties that other surface treatments cannot match, it can also be used in high-frequency signals.

EMS recommends the immersion silver process because it is easy to assemble and has better checkability. However, due to defects such as tarnishing and solder joint voids, the growth of immersion silver has been slow (but not decreased). It is estimated that about 10%-15% of PCBs currently use the immersion silver process.

5. Immersion tin

Tin was introduced into the surface treatment process in the past ten years, and the emergence of this process is the result of the requirements of production automation. Immersion tin does not bring any new elements into the solder joints, which is especially suitable for backplanes for communications. Tin will lose its solderability beyond the storage period of the board, so immersion tin requires better storage conditions. In addition, the immersion tin process has been restricted in its use due to the carcinogenic substances contained in it. It is estimated that about 5%-10% of PCBs currently use the immersion tin process.