PCB surface treatment process characteristics, application and development trend

- Sep 01, 2018-

1. Introduction with the continuous improvement of human requirements for living environment, the environmental problems involved in PCB production at present 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 ways.Although changes in the surface treatment process of PCB are not significant at present and seem to be relatively distant, it should be noted that the slow change in the long term will lead to great changes.PCB surface treatment technology is bound to change dramatically in the future as environmental concerns grow.

Purpose of surface treatment the most basic purpose of surface treatment is to ensure good weldability or electrical properties.Since natural copper tends to exist in the form of oxides in the air, it is unlikely to remain as raw copper for a long time, so additional treatment of copper is required.Although a strong flux can be used to remove most copper oxides in subsequent assembly, the strong flux itself is not easily removed, so strong flux is generally not used in the industry.

3. Five common surface treatment processes there are many PCB surface treatment processes, including hot air leveling, organic coating, chemical nickel/gold plating, silver and tin plating.

1. Hot air levelingThermally fluidized solder and copper form an intercalated copper-tin compound at the junction.The solder thickness of the protective copper surface is about 1-2mil.PCB should be immersed in molten solder during hot air conditioning.Blow-flat liquid solder before the solder solidifies;The wind knife minimizes and prevents weld bridging in the curved month of the solder on the copper surface.Hot air rectification is divided into two types: vertical type and horizontal type. Generally speaking, horizontal type is better, and horizontal hot air leveling coating is more uniform, which can realize automatic production.The general flow of hot air leveling process is as follows: micro etching, preheating, coating flux, spraying tin, cleaning.

2. Organic coating organic coating process is different from other surface treatment processes. It ACTS as a barrier between copper and air;Organic coating is simple and cheap, which makes it widely used in the industry.The early organic coated molecules were the antirust imidazole and benzotriazole, and the newest ones were mainly benzimidazole, which was the copper on the PCB that was bonded to nitrogen functional groups.In the subsequent welding process, if there is only one layer of organic coating on the copper surface, there must be many layers.This is why copper liquids are often added to chemical tanks.After coating the first layer, the coating layer adsorbs copper;The second layer of organic coating molecules are then bonded to copper until 20 or even hundreds of organic coating molecules are concentrated on the copper surface, thus ensuring multiple reflow soldering.The results show that the latest organic coating process can maintain good performance in many lead-free welding processes.The general process of organic coating technology is: degreasing, microetching, pickling, pure water cleaning, organic coating cleaning, process control is easier than other surface treatment processes.

3. Electroless nickel/gold electroless nickel/gold plating is not as simple as organic coating.In addition, the electroless nickel plating/gold impregnation process is also different from organic coating as rust barrier layer, which can be useful and achieve good electrical performance in the long-term use of PCB.Therefore, electroless nickel plating/gold impregnation consists of a thick, electrically sound nickel-gold alloy on the copper surface, which can protect the PCB for a long time.It also has environmental tolerance that other surface treatment processes do not.Nickel plating is due to the fact that gold and copper diffuse each other, and the nickel layer can prevent the diffusion of gold and copper;Without the nickel layer, the gold would have spread to the copper within hours.Another benefit of electroless nickel plating/gold impregnation is the strength of nickel, which is only five microns thick to limit Z expansion at high temperatures.In addition, electroless nickel/gold plating also prevents copper from dissolving, which would be beneficial for lead-free assembly.The general process of electroless nickel plating/gold impregnation process is as follows: acid cleaning, microerosion, preleaching, activation, electroless nickel plating and chemical impregnation. There are six chemical tanks, involving nearly 100 chemicals, so the process control is difficult.

4. Silver immersion silver process is between organic coating and chemical nickel plating/gold impregnation. The process is simple and rapid.It's not as complex as electroless nickel/gold plating, nor does it put a thick layer of armor on a PCB, but it still provides good electrical properties.Silver is the little brother of gold, and even when exposed to heat, humidity and pollution, it remains well weldable but tarnishes.Silver immersion does not have the physical strength of electroless nickel/gold impregnation because there is no nickel beneath the silver layer.In addition, the impregnation of silver has good storage, and the impregnation of silver after a few years of assembly will not have major problems.Silver impregnation is a displacement reaction that is almost submicron - grade pure silver coating.Sometimes there are some organics in the process of silver impregnation.It is difficult to measure this thin layer of organic matter, and analysis suggests that the organism weighs less than 1%.

5. Solder impregnation since all solders are based on tin at present, the solder layer can match any type of solder.From this point of view, the impregnation process has great development prospect.However, the adoption of the former PCB process is restricted because the tin whisker and tin migration during the soldering process will bring reliability problems.Later, organic additives were added in the solution, which made the tin layer structure to be granular structure, overcoming the previous problems, and also had good thermal stability and weldability.The process of tin impregnation can form a flat copper-tin intergeneric compound, which makes the impregnation as weldable as hot air leveling without the flatness that is a headache.Tin impregnation also does not have the problem of diffusion between chemically plated nickel and gold-impregnated metals - the copper-tin intermetallic compounds are firmly bonded together.The impregnation board cannot be stored for too long and must be assembled according to the order of impregnation.

6. Other surface treatment processes other surface treatment processes have less application. Next, the electroless nickel and gold plating and chemical palladium plating processes have more application.Electroplating nickel gold is the originator of surface treatment of PCB.It is used to coat the surface conductor of PCB with a layer of nickel and then a layer of gold.There are two types of electroplated nickel gold: soft gold (pure gold, with an unshiny surface) and hard gold (smooth and hard, wear-resistant, and containing other elements such as cobalt, with a shiny surface).Soft gold is mainly used for chip packaging with gold wire.Hard gold is mainly used for electrical interconnections in non-welds.Given the cost, the industry often USES image transfer to make selective plating to reduce the use of gold.Currently, the use of selective electroplating continues to increase in the industry, mainly due to the difficulty in controlling the process of electroless nickel plating/impregnation.Under normal circumstances, welding will result in brittle electroplating, which will shorten the service life, thus avoiding welding on electroplating.But chemical nickel plating/gold impregnation is rare because the gold is thin and consistent.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 through a reducing agent (such as sodium dihydrogen hydride). The new palladium can be used as a catalyst to promote the reaction and thus any thickness of palladium coating can be obtained.The advantages of chemical palladium plating are good welding reliability, thermal stability and surface flatness.

The choice of surface treatment process mainly depends on the type of final assembled components.The surface treatment process will affect the production, assembly and final use of the PCB. The following is a detailed introduction of the five commonly used surface treatment processes.

1. Hot air levelingIn the 1980s, more than three-quarters of PCBS used hot air levelling, but the industry has been reducing the use of hot air levelling over the past decade.Hot air leveling is a dirty, unpleasant, and dangerous process and has never been a desirable process, but hot air leveling is an excellent process for larger components and larger conductors.In high-density PCB, the flatness of hot air leveling will affect subsequent assembly.Therefore, the HDI plate generally does not use hot air leveling process.With the progress of technology, the industry has now developed the hot air leveling process suitable for assembling the smaller spacing of QFP and BGA, but the actual application is less.At present, some factories use organic coating and chemical nickel plating/gold impregnation process instead of hot air leveling process.Technological developments have also led some factories to adopt the process of tin and silver impregnation.Combined with the trend of lead free in recent years, the use of hot air leveling is further limited.While so-called lead-free hot air leveling has occurred, this could involve equipment compatibility issues.

2. It is estimated that about 25-30% of PCBS currently use organic coating processes, which has been on the rise (it is likely that organic coating now takes precedence over hot air levelling).Organic coating technology can be applied to low-tech PCB or high-technology PCB, such as single-sided TV PCB and high-density chip packaging board.For BGA, organic coating application is also more.Organic coating would be ideal for surface treatment if there is no functional requirement for surface connection or storage period.

3. Electroless nickel/gold electroless nickel/gold plating process is different from organic coating. It is mainly used on boards with functional connection requirements and long storage life on the surface, such as the button area of mobile phone, the edge connection area of router shell and the electrical contact area of the elastic connection of chip processor.The use of chemical nickel plating/impregnation was widespread in the 1990s due to the flatness of hot air leveling and the removal of organic coated flux;The use of electroless nickel plating/impregnation processes was later reduced due to the advent of black plates and brittle ni-p alloys, although nearly every high-technology PCB plant currently has electroless nickel plating/impregnation lines.Considering that solder joints will be brittle when copper-tin intergeneric compounds are removed, many problems will arise in relatively brittle nickel-tin intergeneric compounds.Therefore, almost all portable electronic products (such as mobile phones) use organic coated, silver-impregnated or tin-impregnated copper-tin interchemical solder joints, while electroless nickel/impregnation is used to form key areas, contact areas and EMI shielding areas.It is estimated that approximately 10-20% of PCBS currently use electroless nickel plating/gold impregnation processes.

4. Silver impregnation is cheaper than electroless nickel/gold impregnation, and silver impregnation is a good choice if PCB has connection functional requirements and needs to reduce costs;Added to the good flatness and contact of the silver immersion, that should choose the silver immersion process.There are many applications in communication products, automobiles, computer peripherals and in high-speed signal design.It can also be used in high frequency signals because of its excellent electrical properties, which cannot be matched by other surface treatments.EMS recommends the silver dip process because it is easy to assemble and has good inspection.But its growth has been slow (but not falling) because of defects such as tarnishing and spot voids.It is estimated that about 10-15% of PCBS currently use silver impregnation.

5. It has been nearly a decade since the impregnation of solder tin was introduced to the surface treatment process as a result of the requirement of production automation.The solder soak does not bring in any new elements at the solder, which is especially suitable for the communication back plate.The solderability will be lost outside the storage period of the board, and impregnation requires better storage conditions.In addition, the impregnation process is restricted due to the presence of carcinogenic substances.It is estimated that about 5%-10% of PCBS currently use tinning processes.With the increasing requirements of customers, stricter environmental requirements and more surface treatment processes, the choice of a promising and more versatile surface treatment process seems to be a little confusing and confusing at present.It is impossible to predict exactly where PCB surface treatment will go in the future.However, meeting customer requirements and protecting the environment must be done first!