PCB surface treatment process

The environmental problems involved in the PCB production process are particularly prominent. The topic of lead and bromine is the hottest topic; lead-free and halogen-free will affect PCB development in many ways. Although it seems that the changes in the surface treatment process of PCBs 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. Under the circumstance that the voice of environmental protection is getting higher and higher, the surface treatment process of PCB will definitely undergo great changes in the future

Basic classification

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Common PCB surface treatment processes are: hot air leveling (HASL, hot air solder leveling), organic coating (OSP), electroless nickel plating/immersion gold, immersion silver, immersion tin, etc.
Hot air leveling HASL, hot air solder 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) with heated compressed air to form a layer that resists copper oxidation and provides good solderability. Sexual coating. During hot air leveling, the solder and copper form a copper-tin metal compound at the junction with a thickness of about 1-2 mils.
The PCB is immersed in molten solder for hot air leveling. The air knife blows out the liquid solder before the solder solidifies and can minimize the meniscus of the solder on the copper side and prevent solder 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 is: micro-etching --> preheating --> flux coating --> tin spraying --> cleaning.

organic coating

The difference between OSP and other surface treatment processes is that it acts as a barrier layer between copper and air; in short, OSP is to chemically grow an organic film on the clean bare copper surface. This film has anti-oxidation, thermal shock, moisture resistance, and is used to protect the copper surface from rusting (oxidation or vulcanization, etc.) in the normal environment; at the same time, it must be easily assisted in the subsequent high temperature of welding. Flux is quickly removed for soldering.
The simple process and low cost of organic coating make it widely used in the industry. The early organic coating molecules are imidazoles and benzotriazoles that play a role in rust prevention , and the latest molecules are mainly benzimidazoles. In order to ensure that multiple reflow soldering can be performed, only one organic coating layer on the copper surface is not acceptable, there must be many layers, which 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 combine with copper until twenty or even hundreds of times the organic coating molecules accumulate on the copper surface.
The general process is: degreasing-->micro-etching-->pickling-->pure water cleaning-->organic coating-->cleaning, the process control is relatively easy compared to other processes.

chemical gold

Electroless nickel/immersion gold is a thick layer of nickel-gold alloy with good electrical properties wrapped on the copper surface and can protect the PCB for a long time. Unlike OSP, which only acts as a rust barrier, it can be useful and achieve good electrical performance during long-term use of the PCB. 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 them. Without the barrier of 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, as only 5um thick nickel can control the Z-direction expansion at high temperature. In addition, electroless nickel/immersion gold also prevents copper from dissolving, which will benefit lead-free soldering.
The general process is: de-pickling cleaning --> micro-etching --> pre-dipping --> activation --> chemical nickel plating --> chemical immersion gold; there are 6 chemical tanks in the process, involving nearly 100 species Chemicals, the process is more complicated.

Immersion silver

The immersion silver process is between OSP and electroless nickel/immersion gold, and the process is relatively simple and fast. Immersion silver is not to put a thick armor on the PCB, even when exposed to heat, humidity and pollution, it can still provide good electrical properties and maintain good solderability, but it will tarnish. Because there is no nickel under the silver layer, immersion silver does not have all the good physical strength of electroless nickel/immersion gold.
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. Generally, it is difficult to measure the organic matter in this thin layer. Analysis shows that the weight of the organic matter is less than 1%.

Immersion Tin

Since all solders are based on tin, the tin layer can be matched with any type of solder, and from this point of view, the immersion tin process is very promising. However, the previous PCB is prone to tin whiskers after the tin immersion process, and the migration of tin whiskers and tin during the soldering process will bring reliability problems, thus limiting the use of the tin immersion process. After adding organic additives to the tin immersion solution, the tin layer has a granular structure, which overcomes the previous problems and 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 ; no electroless nickel / immersion Diffusion problem between gold metals; just immersion tin plate can not be stored for too long,

Surface Technology

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The application of other surface treatment processes is less, among which the electroplating nickel gold and electroless palladium plating process are more widely used.

Electroplated nickel gold

Electroplating nickel-gold is the originator of PCB surface treatment technology. It has appeared since the appearance of PCB, and other processes have gradually evolved. Nickel-gold electroplating is to first electroplate a layer of nickel on the surface conductor of the PCB and then electroplate a layer of gold. 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 other elements such as cobalt, and the surface looks brighter). Soft gold is mainly used for gold wires in chip packaging; hard gold is mainly used for electrical interconnections (such as gold fingers) in non-soldering places.
Under normal circumstances, welding will cause electroplated gold to become brittle, which will shorten the service life, so avoid welding on electroplated gold; and electroless nickel / immersion gold, because the gold is thin and uniform, embrittlement rarely occurs.

Electroless Palladium Plating

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). The newly formed palladium can be called a catalyst to promote the reaction, so 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. The disadvantage is that palladium is a relatively rare precious metal, so the cost will increase.