Surface treatment purpose
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 of copper are required. Although in the subsequent assembly, strong flux can be used to remove most of the copper oxide, the strong flux itself is not easy to remove, so the industry generally does not use strong flux.
Common surface treatment processes
1.Hot air leveling
Hot air leveling, also known as hot air solder leveling, is a process of applying molten tin-lead solder to the surface of the PCB and leveling it with heated compressed air to form a coating that is both resistant to copper oxidation and provides good solderability. When the hot air is leveled, the solder and copper form a copper-tin intermetallic compound at the junction. The thickness of the solder that protects the copper surface is approximately 1-2 mils.
The PCB is immersed in molten solder during hot air leveling; the air knife blows liquid solder before the solder solidifies; the air knife minimizes the meniscus of the solder on the copper surface and prevents solder bridging. Hot air leveling is divided into vertical type and horizontal type. It is generally considered that the horizontal type is better, mainly because the horizontal hot air leveling coating is relatively uniform and can realize automatic production. The general process of the hot air leveling process is: micro-etching→preheating→coating flux→spraying→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 inexpensive, which makes it widely used in the industry. The early organically coated molecules were imidazole and benzotriazole, which act as rust inhibitors. The most recent molecule is mainly benzimidazole, which is a copper that chemically bonds nitrogen functional groups to the PCB.
In the subsequent soldering process, it’s not ok if there is only one layer of organic coating on the copper surface, there must be many layers. This is why it is often necessary to add copper to the chemical bath. After coating the first layer, the coating layer adsorbs copper; then the second layer of organic coating molecules is combined with copper until twenty or even hundreds of organic coating molecules are concentrated on the copper surface, thus ensuring multiple times flow soldering.
Tests have shown that the latest organic coating process maintains good performance during multiple lead-free soldering processes. The general process of the organic coating process is: degreasing → micro-etching → pickling → pure water cleaning → organic coating → cleaning. Process control is relatively easy compared to other surface treatment processes.
3. Electroless nickel plating / immersion gold
The electroless nickel/immersion gold process is not as simple as organic coating. 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 a rust barrier. It can be useful and achieve good electrical performance during long-term use of the PCB.
Therefore, electroless nickel/immersion gold is coated with 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 not available in other surface treatment processes. Sex. Nickel plating is caused by the mutual diffusion of gold and copper, and the nickel layer prevents 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, which is limited to nickel in the Z direction at high temperatures. In addition, electroless nickel/immersion gold can also prevent copper dissolution, which will benefit lead-free assembly. The general process of electroless nickel/immersion gold process is: acid cleaning → micro-etching → pre-dip → activation → electroless nickel plating → chemical immersion gold, mainly 6 chemical tanks, involving nearly 100 kinds of chemicals, so process control comparison difficult.
4. Immersion of silver
The immersion silver process is between organic coating and electroless nickel/immersion gold. The process is simple and fast. Unlike the electroless nickel/immersion gold, it is not a thick armor for the PCB, but it Still able to 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 it 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 is no big problem in assembly after immersion in silver for several years. Immersion silver is a displacement reaction that is almost sub-micron coated with pure silver. 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 the analysis shows that the weight of the organism is less than 1%.
5. Immersion of 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 development prospects. However, tin whiskers have appeared in the previous PCB after the immersion tin process, and the migration of tin whiskers and tin during the soldering process brings reliability problems, so the use of the immersion tin process is limited.
Later, an organic additive was 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 weldability. The immersion tin process can form a flat copper-tin intermetallic compound. This property makes the immersion tin have the same good solderability as the hot air leveling without the flatness problem of hot air leveling; the immersion tin also has no electroless nickel plating. /Diffuse metal diffusion problems - copper-tin intermetallic compounds can be firmly combined. The immersion tin plate cannot be stored for too long, and must be assembled according to the order of immersion tin.
6. Other surface treatment processes
Other surface treatment processes are less used. The following is a relatively high application of electroplated nickel gold and electroless palladium plating. Electroplated nickel gold is the originator of the PCB surface treatment process. It has appeared since the PCB appeared, and has gradually evolved into other ways. It is coated with a layer of nickel on the surface conductor of the PCB and then plated with 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: gold-plated gold (pure gold, the gold surface does not look bright) and hard gold (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 bonding in chip packaging. Hard gold is mainly used for electrical interconnection in non-welded parts. Considering the cost, the industry often uses selective image plating to reduce the use of gold. At present, the use of selective electroplating gold continues to increase in the industry, mainly due to the difficulty in electroless nickel/immersion gold process control.
Under normal circumstances, the welding will cause the electroplating gold to become brittle, which will shorten the service life, so it is necessary to avoid welding on the electroplated gold; but the electroless nickel/immersion gold is very thin due to the thinness of gold, and the brittle phenomenon rarely occurs. . The process of electroless palladium is similar to the process of electroless nickel plating. The main process is to reduce the palladium ions to palladium on the catalytic surface by a reducing agent (such as sodium dihydrogen phosphate). The new palladium can be used as a catalyst to promote the reaction, so that a palladium plating layer of any thickness can be obtained. The advantages of electroless palladium plating are good soldering reliability, thermal stability, and surface flatness.
