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Five common PCB surface treatment processes

Five- PCB-surface-treatment

PCB surface treatment technology refers to the process of artificially forming a surface layer different from the mechanical, physical and chemical properties of the substrate on the PCB components and electrical connection points. Its purpose is to ensure good solderability or electrical performance of the PCB. Since copper tends to exist in the form of oxides in the air, which seriously affects the solderability and electrical performance of the PCB, surface treatment of the PCB is required.

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 PCB are relatively distant, 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.
purpose of PCB surface treatment

Purpose of PCB surface treatment

The most basic purpose of PCB 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 and is unlikely to remain as raw copper for long periods of time, additional treatment of the copper is required. Although in subsequent assembly, strong flux can be used to remove most copper oxides, but strong flux itself is not easy to remove, so the industry generally does not use this.

There are many PCB surface treatment processes. The common ones are HASL,OSP, Electronicless Ni/Immerse Au (ENIG), Immersion Silver(I-Ag) and Immersion Tin (I-Sn). The following will introduce them one by one , and how to choose them in PCB manufacturing .

1.HASL (Hot Air Solder Leveling)

HASLHASL 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 coating that resists copper oxidation and provides good solderability. Solder and copper form a copper-tin intermetallic compound at the junction during HASL.

The thickness of the solder to protect the copper surface is about 1-2 mils. The PCB is immersed in molten solder for HASL, and the air knife blows the liquid solder before the solder solidifies and minimizes the meniscus of the solder on the copper surface and prevents solder bridging. HASL is divided into two types: vertical type and horizontal type. It is generally considered that the horizontal type is better, mainly because the horizontal HASL is more uniform and can realize automatic production. The general process of the hot air leveling process is: micro-etching → preheating → flux coating → tin spraying → cleaning.

HASL once dominated the PCB surface treatment process. In the 1980s, more than three-quarters of PCB used this technology, but the industry has been reducing its use over the past decade. It is estimated that about 25%-40% of PCB currently use HASL process. The dirty, smelly, and dangerous process of HASL has never been a favorite process, but it is an excellent process for larger components and larger pitch wires.

In a PCB with a higher density, the flatness of the HASL will affect the subsequent assembly. Therefore, the HDI board generally does not use this process. With the advancement of technology, the industry has now developed a HASL process suitable for QFP and BGA with smaller assembly pitch, but there are few practical applications. At present, some factories use OSP and ENIG process to replace hot air leveling process. Technological development also makes some factories use I-Sn and I-Ag process. Besides,with the trend of lead-free in recent years, the use of HASL is further restricted.

2.OSP (Organic solderability preservative)

OSP-Organic-solderability-preservativeOSP (Organic solderability preservative) is an organic coating used to prevent oxidation of copper before soldering, that is, to protect the solderability of PCB pads from damage. Unlike other surface treatment processes, it acts as a barrier layer between copper and air. The OSP process is simple and inexpensive, which makes it widely used in the industry.

After the surface of the PCB is treated with OSP, a thin layer of organic compounds is formed on the surface of the copper to protect the copper from being oxidized. The thickness of Benzotriazoles type OSP is generally 100 A°, while the thickness of Imidazoles type OSP is thicker, generally 400 A°.

OSP film is transparent, its existence is not easy to be identified by naked eyes, and detection is difficult. During the assembly process (reflow soldering), the OSP is easily melted into the solder paste or the acidic Flux, and at the same time, the copper surface with strong activity is exposed, and finally a Sn/Cu intermetallic compound is formed between the component and the pad. Therefore, OSP has very good characteristics for processing the welding surface. By the way, OSP does not have the problem of lead pollution, so it is environmentally friendly.

The early organic coating molecules are imidazoles and benzotriazoles that play a role in rust prevention, and the latest molecules are mainly benzimidazoles, which are chemically bonded nitrogen functional groups to copper on the PCB. In the subsequent soldering process, if only one organic coating layer on the copper surface is not acceptable, there must be many layers. This is why copper liquids are usually added to chemical tanks.

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 are assembled on the copper surface, which can ensure multiple flow soldering processes. Tests have shown that the latest organic coating process can maintain 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, and the process control is easier than other surface treatment processes.

It is estimated that about 25%-30% of PCB currently use an organic coating process, and this proportion has been rising (it is likely that OSP has now surpassed HASL in the first place).

The organic coating process can be used on low-tech PCB as well as high-tech PCB, such as single-sided TV PCB and high-density chip packaging boards. For BGA, there are also many organic coating applications. If the PCB has no surface connection functional requirements or storage period limitations, organic coating will be the most ideal surface treatment process.

3.ENIG (Electronicless Ni/Immerse Au)

ENIGThe electroless nickel/immersion gold process is not as simple as the organic coating, and it seems to put a thick armor on the PCB. In addition, the ENIG process is not like an organic coating as a rust barrier, it can be useful in the long-term use of the PCB and achieve good electrical performance. Therefore, ENIG is to wrap a thick layer of nickel-gold alloy with good electrical properties 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. sex. The reason for nickel plating is that gold and copper interdiffuse, and the nickel layer prevents the diffusion between gold and copper; without the nickel layer, the gold would diffuse into the copper within hours. Another benefit of this process is the strength of nickel, as only 5 microns of nickel can limit Z-direction expansion at high temperatures. In addition, electroless nickel/immersion gold also prevents copper dissolution, which will benefit lead-free assembly.

The general process of ENIG 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 comparison difficulty.

The ENIG process is different from organic coating. It is mainly used on boards with surface connection functional requirements and long shelf life, such as mobile phone key areas, edge connection areas of router casings, and chip processor elasticity. The electrical contact area of the connection. It was widely used in the 1990s due to the flatness problem of hot air leveling and the removal of organically coated flux. The application of immersion gold / immersion gold process has decreased, but almost every high-tech PCB factory has electroless nickel / immersion gold wire.

Considering the brittleness of the solder joints when removing the copper-tin intermetallics, there are many problems with the relatively brittle nickel-tin intermetallics. Therefore, almost all portable electronic products (such as mobile phones) use copper-tin intermetallic compound solder joints formed by organic coating, immersion silver or immersion tin, and use electroless nickel plating/immersion gold to form key areas, contact areas and EMI shielding areas . It is estimated that about 10%-20% of PCBs currently use ENIG processes.

4.I-Ag (Immersion Silver)

I-Ag-Immersion-SilverBetween OSP and ENIG, the process is relatively simple and fast, not as complicated as ENIG, nor does it put a thick layer of armor on the PCB, but it can still provide good electrical properties. Silver is gold’s little brother and retains good solderability even when exposed to heat, humidity and pollution, but tarnishes. Immersion silver does not have the good physical strength of ENIG 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 a few years of immersion silver. 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; it is generally difficult to measure this thin layer of organic matter, and the analysis shows that the weight of the organic matter is less than 1%.

Immersion silver is cheaper than electroless nickel plating/immersion gold. If the PCB has connection functional requirements and needs to reduce costs, immersion silver is a good choice. Besides, the good flatness and contact of immersion silver, then immersion silver should be the best choice.

There are many applications of immersion silver in communication products, automobiles, and computer peripherals, and immersion silver is also used in high-speed signal design. Because immersion silver has good electrical properties unmatched by other surface treatments, it can also be used in high frequency signals.

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

5. I-Sn (Immersion Tin)

I-SnSince 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 prospects for development. However, tin whiskers appeared in the previous PCB after the immersion tin process, and the migration of tin whiskers and tin during the soldering process would bring reliability problems, so the adoption of the tin immersion process was limited.

Later, organic additives were added to the tin immersion solution, which can make the tin layer structure have 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, which makes immersion tin have the same good solderability as hot air leveling without the headache of hot air leveling.

immersion tin also does not have electroless nickel plating / Diffusion problem between immersion gold metals – copper-tin intermetallic compounds can be firmly bonded together. The tin immersion board cannot be stored for too long, and the assembly must be carried out according to the order of tin immersion.

It is almost a decade since the introduction of the surface treatment process, which emerged as a result of the requirements for automation of production. Immersion tin does not bring any new elements into the soldering place, and is especially suitable for communication backplanes.

Tin will lose solderability beyond the shelf life of the board, so immersion tin requires better storage conditions. In addition, the use of tin immersion process is restricted due to the presence of carcinogens. It is estimated that about 5%-10% of PCBs currently use the immersion tin process.

Other surface treatment processes

other-PCB-surface-treatment-technologyThe application of other surface treatment processes is less. In addition to the five commonly used surface treatment processes mentioned above, there are also electroplating nickel-gold and electroless palladium plating processes that are relatively widely used.

● Electroplating nickel-gold is the originator of the PCB surface treatment process. It has appeared since the appearance of PCB, and has gradually evolved into other methods. It is to first coat a layer of nickel and then a layer of gold on the conductor on the surface of the PCB.

Nickel plating is mainly to prevent the diffusion between gold and copper. Now 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 gold surface looks brighter).

Soft gold is mainly used for gold wire in chip packaging; hard gold is mainly used for electrical interconnection in non-soldering places. Considering the cost, the industry often uses image transfer to reduce the use of gold by selective electroplating.

At present, the use of selective gold plating in the industry continues to increase, mainly due to the difficulty in controlling the electroless nickel plating/immersion gold process. Under normal circumstances, welding will cause electroplated gold to become brittle, which will shorten the service life, so avoid welding on electroplated gold; but electroless nickel plating/immersion gold is very thin and consistent, and embrittlement rarely occurs.

● 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), and 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 soldering reliability, thermal stability, and surface flatness.

Conclusion

The choice of surface treatment process depends primarily on the type of final assembled component. The surface treatment process will affect the production, assembly and end use of the PCB. Each surface treatment process has its own unique features, and the scope of application is not the same. Different surface treatment requirements are required according to the application of different boards. Under the limitation of the manufacturing process, we will have a reasonable choice of surface treatment according to the characteristics of the board and the application of the product.

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