PCB Surface Treatment Process Characteristics and Development Trends (2)

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 a 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 properties during long-term use of the PCB. Therefore, electroless nickel/immersion gold is to wrap a thick, good electrical 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 due to the mutual diffusion between gold and copper, 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, and it does not put a thick layer on the PCB armor, but it can still provide good electrical performance. Silver is like 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, the immersion silver has good storage properties, and there will be no major problems in assembly after a few years of immersion. Immersion silver is a displacement reaction, it is almost sub-micron pure silver coating. 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 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 match 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 immersion tin 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 board cannot be stored for too long, and the assembly must be carried out according to the order of immersion tin.

Other Surface Treatment Processes:
The application of other surface treatment processes is less, let’s look at the relatively more application of nickel-gold electroplating and electroless palladium plating processes.
Electroplating nickel gold is the originator of PCB surface treatment technology. It has appeared since the appearance of PCB, 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 plating to reduce the use of gold. At present, the use of selective electroplating 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 the 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 a palladium coating of any thickness can be obtained. The advantages of electroless palladium plating are good welding reliability, thermal stability, and surface smoothness.