How To Choose PCB Surface Treatment Process

1. Introduction

With the continuous improvement of human requirements for the living environment, the environmental problems involved in the PCB production process are particularly prominent. Lead and bromine are the hottest topics at the moment; Lead-free and halogen-free will affect the development of PCB in many aspects.

Although at present, the change of PCB surface treatment technology is not very big, it seems to be a relatively distant thing, but it should be noted that: long-term slow change will lead to great changes. In the case of increasing environmental protection, PCB surface treatment technology will certainly change dramatically in the future.

2. The purpose of surface treatment

The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since natural copper tends to exist in the air as an oxide, it is unlikely to remain raw copper for long periods of time, so additional treatment of copper is required. Although a strong flux can be used to remove most copper oxides in subsequent assembly, it is not easy to remove the flux itself, so the industry generally does not use strong flux.

3. Common five kinds of surface treatment technology

Now there are many PCB surface treatment processes, the common are hot air leveling, organic coating, electroless nickel plating/immersion gold, immersion silver, and immersion tin, the following will be introduced one by one.

The choice of surface treatment technology mainly depends on the type of final assembly components. Surface treatment technology will affect the production, assembly, and final use of PCB, the following will be the specific introduction of the five common surface treatment technology use occasions.

• Hot air leveling(HASL)

Hot air leveling used to play a leading role in PCB surface treatment technology. In the 1980s, hot air leveling was used on more than three-quarters of PCBs, but the industry has been reducing its use over the past decade, and it is estimated that 25 to 40 percent of PCBs now use hot air leveling. Hot air leveling has never been a favorite process because it is dirty, smelly, and dangerous, but it is excellent for larger components and wider-spaced wires. In high-density PCB, the flatness of hot air leveling will affect subsequent assembly; Therefore, HDI PCB generally does not use hot air leveling process.

With the development of technology, hot air leveling processes for QFP and BGA with smaller assembly spacing are now available in the industry, but there are few practical applications. At present, some factories use organic coating and electroless nickel plating/immersion gold process to replace the hot air leveling process.

The development of technology also makes some factories use immersion tin, immersion silver process. Coupled with the trend of lead-free in recent years, the use of hot air leveling is further restricted. Although the so-called lead-free hot air leveling has appeared at present, it will involve the compatibility of the equipment.

• Organic coating

It is estimated that around 25% to 30% of PCBs are now organic coated, and this proportion has been increasing (it is likely that organic coating has now overtaken hot air leveling as the number one coating). The organic coating process can be used on low-tech PCB as well as high-technology PCB, such as single-sided TV PCB, high-density chip packaging board. For BGA, organic coating is also widely used. If there is no surface connection functional requirements or shelf-life restrictions on PCB, organic coating is the ideal surface treatment process.

• Electroless nickel/immersion gold (ENIG) and Electroless nickel-palladium/immersion gold (ENEPIG)

Unlike organic coating, electroless nickel/immersion gold is mainly used on surfaces with functional connection requirements and long shelf life, such as button areas of mobile phones, edge connection areas of router shells, and electrical contact areas of flexible connection of chip processors. Electroless nickel/immersion gold was widely used in the 1990s due to the flatness of hot air leveling and the removal of organic coating flux. The application of electroless nickel plating/immersion gold process has been reduced since the emergence of black, brittle nickel-phosphorus alloys, but now almost every high-tech PCB plant has electroless nickel plating/immersion gold line.

Considering that the solder joints become brittle when the Intergeneric compounds of copper and tin are removed, there are many problems with the relatively brittle Ni-Si intergeneric compounds. Therefore, portable electronic products (such as mobile phones) almost all use organic coating, silver dip or dip tin formed copper-tin intermetallic compound solder joint, and the use of electroless nickel plating/gold dip to form key area, contact area, and EMI shielding area. It is estimated that approximately 10% to 20% of PCB’s use the electroless nickel plating/gold leaching process.

• Immersion silver

Silver immersion is cheaper than electroless nickel plating/gold immersion. Silver immersion is a good choice if the PCB has connection functional requirements and needs to reduce costs. Plus silver immersion good flatness and contact, it should choose more silver immersion process. In communication products, automobiles, computer peripherals, many applications of silver immersion, silver immersion in high-speed signal design has also been used. Silver immersion can also be used in high-frequency signals because it has excellent electrical properties unmatched by other surface treatments. EMS recommends the silver immersion process because it is easy to assemble and has good inspection performance. But imperfections such as tarnishing and hollow solder joints make it slow to grow (but not decline). It is estimated that about 10% to 15% of PCB’s use the immersion silver process today.

• Immersion tin

Tin was introduced into the surface treatment process in the last ten years, the emergence of the process is the result of production automation requirements. Soldering is not brought into any * elements, especially suitable for communication backboard. The tin will lose solderability beyond the shelf life of the board, so better storage conditions are required for dipping in. In addition, the use of the tin leaching process is limited due to the carcinogens contained in it. It is estimated that about 5-10% of PCBs are currently using the tin immersion process.

4. Conclusion

It is also impossible to predict exactly where PCB surface treatment technology will go in the future. With more and more high customer requirements, more and more stringent environmental requirements, surface treatment process, in the end, the choice of the kind of development prospects, more versatile surface treatment process, it seems that a little dazzling, confusing. Anyway, meeting customer requirements and protecting the environment must be done first!