Selection of Surface Treatment Process

The choice of surface treatment process depends mainly on the type of component that is finally assembled. And the surface treatment process will affect the production, assembly and end use of the PCB. The use occasion of five common surface treatment processes will be described below.

Hot Air Solder Level(HASL)

HASL has been dominant in PCB surface treatment processes. In the 1980s, more than three-quarters of PCBs used HASL, but the industry has been reducing the use of hot air leveling processes for the past decade. It is estimated that about 25%-40% of PCBs currently use HASL.

HASL are dirty, unpleasant, and dangerous, and have never been a favorite process, but it’s 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 the hot air leveling process. With the advancement of technology, the industry has now developed a HASL suitable for assembling QFP and BGA with smaller pitches, but the practical application is less.

At present, some factories use organic coating and electroless nickel plating/immersion gold process to replace the hot air leveling process. The technical development also makes some factories use immersion tin and immersion silver. Coupled with the trend of lead-free in recent years, the use of hot air leveling is further limited. Although so-called HASL lead free has occurred, this may involve compatibility issues with the equipment.

Organic coating

It is estimated that about 25%-30% of the PCBs currently use the organic coating process, and the ratio has been rising (it is likely that the organic coating has now surpassed the hot air leveling in the first place). The organic coating process can be used on low-tech PCBs or on high-tech PCBs, such as single-sided TV PCBs and high-density chip packaging boards. For BGA, there are 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.

Electroless nickel plating / Immersion gold

The electroless nickel/immersion gold process is different from the organic coating. It is mainly used on boards with connection functional requirements and long shelf life, such as the keypad of the mobile phone, the edge connection area of the router housing, and electrical contact area of the chip processor elastically connected.

Due to the flatness of hot air leveling and the removal of organic coating fluxes, electroless nickel/immersion gold was widely used in the 1990s. Later, due to the appearance of black disks and brittle nickel-phosphorus alloys, the application of electroless nickel plating/ immersion gold process has been reduced. But currently almost every high-tech PCB factory has an electroless nickel/immersion gold line. In view of the fact that the solder joints become brittle when the copper-tin intermetallic compound is removed, many problems occur in the relatively brittle nickel-tin intermetallic compound.

Therefore, almost all portable electronic products (such as mobile phones) use copper-tin intermetallic solder joints formed by organic coating, immersion silver or immersion tin, and electroless nickel/immersion gold to form the shielding area of ??the button area, contact area and EMI. . It is estimated that about 10%-20% of PCBs currently use electroless nickel/immersion gold.

Immersion silver

Immersion silver is cheaper than electroless nickel/immersion gold. If the PCB has connectivity requirements and needs to reduce costs, immersion silver is a good choice. Plus good immersion silver flatness and contact, then you should choose immersion silver process. There are a lot of immersion silver applications in communication products, automobiles, and computer peripherals. Immersion silver is also used in high-speed signal design. Since immersion silver has good electrical properties that are unmatched by other surface treatments, it can also be used in high frequency signals.

EMS recommends the use of an immersion silver process because it is easy to assemble and has good checkability. However, due to defects such as tarnishing, solder joint voids, etc., the growth is slow (but not decreased). It is estimated that about 10%-15% of PCBs currently use immersion silver.

Immersion tin

The introduction of tin into the surface treatment process has been a matter of the past decade, and the emergence of this process is the result of the requirements of production automation. The immersion tin does not bring in any new elements at the weld, making it 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 immersion tin process is restricted in use due to the inclusion of carcinogenic substances. It is estimated that about 5%-10% of PCBs currently use immersion tin.

As customer requirements are getting higher and higher, environmental requirements are becoming stricter. It is a bit dazzling and confusing to choose the kind of surface treatment process with promising prospects and versatility. . Where the PCB surface treatment process will go in the future will not be accurately predicted. In any case, meeting customer requirements and protecting the environment is the most important!