Spray tin (Hot Air Solder Leveling, HASL hot air leveling)
It 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 coating that resists copper oxidation and provides good solderability. During hot air leveling, solder and copper form a copper-tin metal compound at the junction, and its thickness 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, and can minimize the meniscus of the solder on the copper surface and prevent solder bridging. Hot air leveling can be divided into vertical type and horizontal type. The hot air leveling type is divided into vertical type and horizontal type. Generally, the horizontal type is better, and its coating is relatively uniform, which is convenient for automatic production.
The advantages of the HASL process are: lower price, good welding performance.
②The disadvantage of the HASL process is that it is not suitable for welding pins with fine gaps and too small components, because the surface flatness of the tin plate is poor, and it is easy to produce solder beads in the subsequent assembly process. Fine pitch components are more likely to cause short circuits.
Organic coating (OSP anti-oxidation)
OSP is different from other surface treatment processes in that its function is to act as a barrier layer between copper and air; simply put, OSP is to chemically grow an organic film on a clean bare copper surface. This layer of film has anti-oxidation, thermal shock resistance, and moisture resistance, and is used to protect the copper surface from rusting (oxidation or vulcanization, etc.) in a normal environment; at the same time, it must be easily assisted in the subsequent welding high temperature The flux is removed quickly for soldering.
The organic coating process is simple and low in cost, making it widely used in the industry. Early organic coating molecules were imidazole and benzotri
azoles, the newest molecules are mainly benzimidazoles. In order to ensure that multiple reflow soldering can be performed, only one layer of organic coating on the copper surface is not enough, there must be many layers, which is why copper liquid is usually added to the chemical bath. 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.
The general process is: degreasing --> micro-etching --> pickling --> pure water cleaning --> organic coating --> cleaning, the process control is relatively easy compared to other display processes.
Chemical nickel/immersion gold
It 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 is only used as an anti-rust barrier layer, it can be useful and achieve good electrical performance during the 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 that the gold and copper will diffuse into each other, 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. Only 5um thickness of nickel can control the expansion in Z direction at high temperature. In addition, electroless nickel/immersion gold can also prevent copper dissolution, which will benefit lead-free soldering.
The general process is: depickling cleaning --> micro-etching --> pre-dipping --> activation --> chemical nickel plating --> chemical immersion gold; there are 6 chemical tanks in the process, involving nearly a hundred kinds of Chemicals, the process is more complicated.
sinking silver
The immersion silver process is between OSP and electroless nickel/immersion gold, and the process is relatively simple and fast. Instead of putting thick armor on the PCB, immersion silver can still provide good electrical performance and maintain good solderability even when exposed to heat, humidity and polluted environments, but it will lose its luster. Because there is no nickel underneath the silver layer, immersion silver does not have the good physical strength of electroless nickel/immersion gold.
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 the organic matter in this thin layer, and the analysis shows that the weight of the organic matter is less than 1%.
Shen tin
Since all 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 has great development prospects. However, the previous PCBs were prone to tin whiskers after the immersion tin process, and tin whiskers and tin migration during the soldering process would cause reliability problems, thus limiting the adoption of the immersion tin process. Finally, organic additives are 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. This feature makes immersion tin have the same good solderability as hot air leveling without the troublesome flatness problem of hot air leveling; there is also no electroless nickel plating/immersion tin The problem of diffusion between gold and metal; it's just that the immersion tin plate cannot be stored for too long.
Electroplated hard gold
The nickel-gold electroplating process needs to electroplate a layer of nickel on the conductor on the surface of the PCB first, and then electroplate a layer of gold. Nickel plating is mainly to prevent the diffusion between gold and copper. This process is usually used at the gold finger of the PCB. .
Chemical Nickel Palladiu Gold
The main process of electroless palladiu plating is to reduce palladiu ions to palladiu on the catalytic surface through a reducing agent. The nascent palladiu can be called a catalyst to promote the reaction, so a palladiu plating layer of any thickness can be obtained. Its advantages include good welding, smooth surface and good thermal stability.
Nowadays, hot air leveling is used more in the surface treatment process. The hot air leveling process is very good for larger components and wires with larger spacing, but it is not practical for PCBs with relatively high density. The flatness of hot air leveling will affect the subsequent assembly, so generally HDI boards do not use this process.
The range of organic coating is relatively wide. Because the price is relatively cheap, it is widely used in low-density and high-density PCBs. Therefore, organic coating is the most ideal surface treatment process when surface connection functionality is required. .
Electroless nickel plating is usually applied to memory boards with relatively long connection functional requirements. Contrary to our organic coating, it is generally used in mobile phones and routers
Finally, immersion silver is usually a process between organic coating and electroless nickel plating. If the PCB has certain requirements for connectivity and needs to reduce costs, then immersion silver is usually the best choice.
There is also a significant difference in price. Take immersion gold and organic HASL as well as lead-free HASL and OSP processes as examples. Under the same conditions, immersion gold processes are generally more expensive, followed by OSP and lead-free HASL. , the price of HASL is the cheapest
