With the continuous improvement of living environment, the environmental problems involved in PCB production are especially prominent.Lead and bromine are the hottest topics at present; lead-free and halogen-free will affect PCB development in many ways.Although the PCB's surface treatment process changes are not very significant at the moment, it seems distant, it should be noted that long-term slow changes can lead to significant changes.Under the circumstances of increasing environmental protection, PCB surface treatment technology will surely change dramatically in the future.

Purpose of Surface Treatment

The basic purpose of surface treatment is to ensure good solderability or electrical properties.Since copper in nature tends to exist as an oxide in the air and is unlikely to remain as copper for a long time, other treatments are needed.Although strong flux can be used to remove most copper oxides in subsequent assemblies, strong flux itself is not easy to remove, so strong fluxes are not commonly used in the industry.

Five Common Surface Treatment Processes
There are many PCB surface treatment processes, the common are hot air leveling, organic coating, electroless nickel/gold plating, silver and tin immersion, which are described below.

Hot air leveling

Hot air solder leveling is a process of coating molten tin-lead solder on PCB surface with heated compressed air to form a coating that is both resistant to copper oxidation and provides good solderability.The copper-tin intermetallic compound is formed at the junction of solder and copper during hot air finishing.The thickness of the solder protecting the copper surface is about 1-2mil.The PCB is immersed in molten solder during hot air finishing; the air knife smoothes out liquid solder before solidification; the air knife minimizes the crescent shape of solder on copper surface and prevents solder bridging.Hot air leveling is divided into vertical type and horizontal type, the horizontal type is generally considered better, mainly horizontal type hot air leveling coating is more uniform, can realize automatic production.

Hot air leveling process for the general flow: micro-etching → preheating → coating flux → tin spray → cleaning.

2. Organic Coating Process

Unlike other surface treatment processes, it acts as a barrier between copper and air; organic coating is simple and inexpensive, making it widely available in the industry.The early organic coated molecules were imidazole and benzotriazole, the most recent of which were benzimidazole, copper chemically bonded to the PCB.During subsequent welding, if only one organic coating is not available on the copper surface, there must be many layers.This is why copper liquid is usually added to chemical tanks.After coating the first layer, the coating adsorbs the copper, and then the second layer of organic coating molecules binds to the copper until 20 or even hundreds of organic coating molecules are concentrated on the copper surface.
The results show that the latest organic coating technology can maintain good performance during multiple lead-free welding.
The general process of organic coating is degreasing → micro-etching → pickling → pure water → organic coating → cleaning. The process control is easier than other surface treatment processes.

3. Electroless Nickel Plating/Dip Gold Electroless Nickel Plating/Dip Gold Process

Unlike organic coatings, electroless nickel/gold plating is not as simple as thick armor for PCB, nor is electroless nickel/gold plating as an anti-rust barrier, which is useful and electrical for long-term use.Therefore, electroless nickel plating/gold impregnation is the coating of a thick, electrical nickel-gold alloy on the copper surface, which protects the PCB for a long time.

Reason for nickel plating: Because gold and copper diffuse, nickel layer can prevent diffusion between gold and copper.In addition, electroless nickel plating/gold impregnation prevents copper from dissolving, which is beneficial for lead-free assembly.
The general process of electroless nickel plating/gold leaching is acid cleaning → micro-etching → pre-immersion → activation → electroless nickel plating → chemical leaching.

4.Silver leaching process is intermediate between organic coating and electroless nickel plating/gold leaching

Silver immersion is a relatively simple and fast process between organic coating and electroless nickel/gold plating; it is not as complicated as electroless nickel/gold plating and does not coat PCB with thick armor, but it still provides good electrical properties.Silver is the younger brother of gold, and even when exposed to heat, moisture, and pollution, silver retains good solderability but loses its luster.Dip silver does not have the good physical strength of electroless nickel/dip gold because there is no nickel underneath the silver layer.In addition, soaking silver has good storage properties, so it will not be a big problem if put together for a few years after soaking silver.

5. Soaking of tin

Since all solder is currently tin-based, the tin layer can match any type of solder.From this point of view, the technology of soaking tin has a great prospect.However, the previous PCB had tin-soaking process, tin-soaking process was restricted because tin-soaking and tin migration would cause reliability problems during welding.Afterwards, organic additives were added to the solution to make the tin layer granular, overcome the previous problems, and have good thermal stability and solderability.

Tin impregnation technology can form flat copper-tin intermetallic compounds, which have the same solderability as hot air leveling without the headache of hot air leveling.Dip tin plate can not be stored for too long, assembly must be according to the order of dip tin.

6.Other surface treatment processes

Other surface treatment techniques are less applicable, but the following are relatively more applicable electro-nickel-gold plating and electroless palladium plating.
Nickel-gold plating is the origin of PCB surface treatment technology, it has appeared since PCB appearance, and gradually evolved into other ways.It is PCB surface conductor first coated with nickel then coated with gold, nickel plating is mainly to prevent diffusion between gold and copper.

There are two types of nickel plating today: soft gold (pure gold with a dull surface) and hard gold (smooth and hard surface, abrasion resistant, cobalt and other elements, with a brighter surface).Soft gold is mainly used for punching gold wires in chip packaging; hard gold is mainly used for electrical interconnections at non-welded locations.Considering the cost, the industry often uses image transfer to selectively electroplating to reduce the use of gold.Electroless palladium plating is similar to electroless nickel plating.The main process is to reduce palladium ions to palladium by reducing agents such as sodium hypophosphite. The new palladium can be used as catalyst to promote the reaction.The advantages of electroless palladium plating are good welding reliability, thermal stability and surface flatness.

Selection of Surface Treatment Process

The choice of the surface treatment process depends on the type of final assembly unit. The surface treatment process will affect PCB production, assembly and final use. The five common surface treatment processes are described below.

1. Hot air leveling used to be the dominant technology in PCB surface treatment.In the 1980s, more than three-quarters of PCB use hot air leveling, but industry has been reducing the use of hot air leveling over the past decade. It is estimated that about 25-40% of PCB use hot air leveling today. Hot air leveling is never a favorite process because it is dirty, unpleasant and dangerous, but hot air leveling is an excellent process for larger components and wires with larger spacing.

2. Organic coating is estimated to be about 25-30% of PCB use organic coating processes and this proportion has been increasing (it is likely that organic coating has now overtaken hot-air leveling as the number one).

Organic coating technology can be used in low-tech PCB or high-tech PCB, such as PCB for single-sided television, high-density chip packaging board.For BGA, organic coating is also widely used.Organic coating is the ideal surface treatment process for PCB without surface bonding functionality requirements or storage period limitations.

3. Unlike organic coating, electroless nickel/gold plating is mainly used on boards with functional requirements and long storage periods, such as cell phone key area, router housing edge connection area and chip processor elastic connection electrical contact area.Considering that the solder joints become brittle when copper-tin intermetallic compounds are removed, there will be many problems in the relatively brittle Ni-tin intermetallic compounds.

4. Silver immersion is cheaper than electroless nickel/gold plating. Silver immersion is a good option if PCB has connectivity requirements and needs to reduce costs.There are many applications in communication products, automobiles and computer peripherals, as well as in high-speed signal design.Silver impregnated with other surface treatments has excellent electrical properties, it can also be used in high frequency signals.

It has been ten years since tin was introduced into the surface treatment process. The appearance of this process is the result of the requirement.The impregnated tin does not bring any new elements into the weld, especially for communication backboards.It is estimated that about 5%-10% of PCB currently uses tin leaching.

5 Concluding remarks

With the increasing demands of customers, the environment is becoming more and more demanding, and surface treatment technology is becoming more and more difficult to choose.It is impossible to predict exactly where PCB surface treatment technology will go in the future.In any case, meeting customer requirements and protecting the environment must be done first!