PCB surface treatment characteristics, uses and development trends

1. Introduction

As human demands for continuous improvement of living environment, the current PCB production process involved in environmental issues is particularly prominent. Currently the subject of lead and bromine are the most popular; lead-free and non-halogenated will influence many aspects of PCB development. Although the present situation, PCB's surface treatment changes in not too great, as is quite a long way off, but it should be noted: the slow long-term changes will lead to dramatic changes. Voice rising in environmental cases, PCB surface treatment technology changes the future will certainly happen.

2. The purpose of surface treatment

Surface basic aim is to ensure good solderability or electrical performance. As the nature of copper oxides in the air tend to exist in the form, probably not long remain of the original copper, copper needed for other processing. Although follow-up assembly, the strong flux can be used to remove most of the copper oxides, but the strong flux itself is not easily removed, the industry generally do not use strong flux.

3. Common surface treatment of five

There are many PCB surface treatment, commonly HASL, organic coating, electroless nickel / Immersion Gold, Immersion Silver, and the five dip tin process, the following will go through.

1. HASL

Hot Air Leveling also known as hot air solder leveling, which is coating the surface of molten tin in the PCB and the whole heated compressed air (blowing) peace process, both to form a resistance to copper oxidation, but also provide good solderability of the coating layer. HASL solder and copper, usually in the form of copper binding sites tin intermetallic compound. To protect the copper surface of solder thickness about 1-2mil.

HASL PCB should always be immersed in molten solder; air knives level the solder solidification of liquid solder before the wind; air knife can be crescent-shaped solder copper surface to minimize and prevent solder bridging. HASL is divided into two kinds of vertical and horizontal, is generally believed that horizontal well, mainly horizontal HASL coating is uniform, can be automated production. Hot Air Leveling Technology General procedures: The micro-corrosion coating flux → → → preheated tin → cleaning spray.

2. Organic coating

Organic coating process is different from other surface treatment, it is in the air between copper and act as a barrier layer; organic coating process is simple, low cost, making it widely used in the industry. Early molecular organic coating is rust from the role of imidazole and benzotriazole, the latest molecular mainly benzimidazole, which is the chemical bonding of nitrogen functional groups to the copper on the PCB. In the follow-up of the welding process, if the copper surface layer of the organic coating layer only is not enough, there must be a lot of layers. This is why the chemical tank usually need to add copper solution. After the first layer in the coating,
Adsorption of copper coating layer; then coating the second layer of organic molecules and the copper binding, up to 20 or even hundreds of organic molecules assembled in the copper coated surface, so you can guarantee that many times reflow. Results show that: the latest in a number of organic coating process to lead-free soldering process to maintain good performance.

Organic coating process of the general process: degreasing → Etching → acid → water washing → cleaning → organic coating, process control, compared with other surface treatment processes more easily.

3. Electroless Nickel / Immersion Gold

Electroless Nickel / Immersion Gold Process not as simple as the organic coating, electroless nickel / Immersion Gold PCB like to wear thick armor; other Electroless Nickel / Immersion Gold Process is also not as rust-proof barrier layer of organic coating It can be useful during long-term use PCB and achieve good electrical properties. Therefore, electroless nickel / Immersion gold is wrapped in a thick layer of copper surface, good electrical properties of nickel gold alloy, which can be long-term protection PCB; In addition, it also has other surface treatment not available in the environment of the patient sex. Nickel is due to be mutual diffusion between gold and copper,
The nickel coating to prevent the spread between gold and copper; If there is no nickel, gold will be in a few hours to spread to the copper. Electroless Nickel / Immersion Gold, another advantage is the strength of nickel, nickel thickness of only 5 microns high temperatures can limit the expansion of Z direction. In addition, electroless nickel / Immersion Gold also can prevent the dissolution of copper, which will benefit the lead-free assembly.

Electroless Nickel / Immersion Gold Process general procedures: acid cleaning → Etching → prepreg → activation → → Electroless Nickel Immersion Gold, there are six major chemical tanks, involving nearly 100 chemicals, so comparison of process control difficult.

4. Baptist Silver

Immersion Silver process between the organic coating and electroless nickel / Immersion gold between the process is relatively simple, rapid; not Electroless Nickel / Immersion Gold as complex, nor is it to put a thick layer of armor PCB, but it still be able to provide good electrical properties. Silver is golden little brother, even if exposed to heat, humidity and pollution, environment, banks are still able to maintain good solderability, but will lose their luster. Silver does not have the dip Electroless Nickel / Immersion Gold possess good physical strength because there is no nickel-silver layer below. Also a good storage of silver dip, dip up a few years after the assembly of silver would not be a big problem.

Immersion Silver is the replacement reaction, it is almost pure silver coated sub-micron level. Sometimes the process of leaching of silver also contains organic compounds, mainly silver corrosion prevention and elimination of silver migration; generally difficult to measure out the organic layer, analysis shows that less than 1% of the weight of the organism.

5. Soldering

Because all of the current tin-based solders are based, so the layer of tin solder with any type of match. From this point of view, most have a dip tin process development prospects. But by the previous PCB dip tin after tin process must, in the welding and tin migration through Cheng Zhongxi to bring reliability, so dip tin process used is limited. Tin solution was joined in the dip of organic additives can be made of tin structure of granular layer structure to overcome the previous problem, but also has good thermal stability and weldability.

Soldering process can form a flat copper tin intermetallic compounds, this feature makes the dip and the hot air solder leveling with the same good weldability with no hot air leveling flat headache problem; dip tin electroless nickel plating does not / Immersion Gold proliferation between metal - copper tin intermetallic compounds can be strong together. Dip tin plate can not be stored for too long, assembly must be carried out according to the order dip tin.

6. Other surface treatment

Other surface treatment is rarely applied, the following terms apply relatively more gold plating Nickel Palladium technology.

Nickel gold is the originator of PCB surface treatment, since the PCB there it appears that later evolved into other way. It is the first conductor in the PCB surface layer of nickel-plated and then coated with a layer of gold, nickel is mainly to prevent the spread between gold and copper. Now there are two types of Ni-Au plating: plating of soft gold (pure gold, a gold surface does not look bright) and hard gold plating (surface smooth and hard, wear-resistant, containing cobalt and other elements, a gold surface looks more bright). Soft money is mainly used for chip packaging line when the goldsmith; hard gold is mainly used in non-welding of the electrical interconnection.

Taking into account the cost of the industry through the image transfer method often selective gold plating to reduce the use. Selective gold plating currently used in the industry continue to increase, mainly due to electroless nickel / Immersion gold process control more difficult.

Normally, the welding can cause brittle gold plating, which will shorten the service life, and thus to avoid gold plating on soldering; but Electroless Nickel / Immersion Gold as Gold thin, and very consistent, crisp rare phenomenon .

Palladium plating and electroless nickel plating process was similar to the process. Mainly through the process of reducing agent (sodium dihydrogen phosphate summarized as follows) to the surface of palladium ions in the catalyst reduced to palladium, palladium newborn can be a catalyst to promote the reaction, thereby enabling the arbitrary thickness of the palladium coating. The advantages of palladium plating welded to the good reliability, thermal stability, surface smoothness.

IV. Surface Treatment Choice

Surface treatment of choice depends on the type of final assembly of components; surface treatment will affect the PCB manufacturing, assembly and end use, the following detailed description will be common surface treatment of the use of five occasions.

1. HASL

HASL PCB surface treatment had in a dominant position. Twentieth century, the eighties, more than three-quarters of HASL PCB use technology, but the industry has been over the past decade in reducing the use of hot air leveling process, it is estimated there are about 25% -40% of the PCB using the hot air Leveling Technology. HASL process the dirty, smelly, dangerous, and therefore has never been very popular technology, but the hot air leveling for large size components and large wire spacing, the process is excellent. In the higher density PCB in
The flatness of hot air leveling up will affect the assembly; so HDI board is generally not used hot air leveling process. As technology advances, the industry now appears more suitable for assembly pitch QFP and BGA small hot air leveling process, but little practical application. At present a number of factories using organic coating and electroless nickel / Immersion gold process to replace the hot air leveling process; technological development also makes a number of factories tin dipping, dip Silver. Together with the recent trend of lead-free, hot air leveling use subject to further restrictions. Although there has been a so-called lead-free HASL, but this can be related to equipment compatibility problems.

2. Organic coating

It is estimated that about 25% -30% of PCB using organic coating process, the ratio has been rising (probably organic coating more than hot air leveling is now living in the first). Organic coating process can be used in low-tech PCB, also can be used in high-tech PCB, such as one-sided TV with PCB, high-density chip package with the board. For BGA, the organic coating is also used widely. If there is no surface connection PCB functional requirements or the limited storage period, the organic coating will be the best surface treatment.

3. Electroless Nickel / Immersion Gold

Electroless Nickel / Immersion Gold Process and organic coating different, it is mainly used in surface connectivity requirements and a longer storage period of the board, such as mobile phone keypad, the shell's edge router to connect district and flexible chip processor connected to electrical contact area. As the hot air leveling the flatness problem and the flux of organic coating removal problems, the twentieth century nineties Electroless Nickel / Immersion Gold wide use; Later on black plates, crisp appearance of nickel-phosphorus alloy, nickel-plating / Immersion Gold Process applications decreased, but at the moment almost every high-tech PCB factory has Electroless Nickel / Immersion Gold Line.
Taking into account the removal of copper tin intermetallic compound when the solder joints will become brittle, relatively brittle nickel-tin intermetallic compounds will be many problems in the office. Therefore, the portable electronic products (such as mobile phones) almost all organic coating, dip dip tin-silver or tin formation of intermetallic compounds of copper solder joints, while the use of electroless nickel / Immersion Gold formed keypad, contact area and EMI shielding zone . Estimated that about 10% -20% of the PCB using electroless nickel / Immersion gold process.

4. Baptist Silver

Immersion Silver Plating than Ni / Immersion Gold cheap, if there is connectivity PCB requirements and the need to reduce costs, Immersion Silver is a good choice; with Immersion Silver good flatness and contact, it should select the Baptist Silver process. In communications, automotive, computer peripherals, the application of many aspects of Immersion Silver, dip in the high-speed signal design, silver has also been applied. As with other surface treatment of silver leaching unmatched good electrical properties, it also can be used in high-frequency signals. EMS silver leaching process is recommended because it is easy to assemble and can have a good check of.
However, due to the existence of silver leaching, such as loss of gloss, empty solder defects such as slow growth make it (but not decreased). Estimated that about 10% -15% of the PCB using dip Silver.

5. Soldering

Surface treatment of tin was introduced in the last decade things, the emergence of this technology is the result of production automation. Soldering welding does not bring in any new elements, especially for communications backplane. In addition to tin-board storage will be lost on the weldability, so dip tin the need for better storage conditions. Another dip tin process due to carcinogenic substances is limited. Estimated that about 5% -10% of the PCB using dip tin process.

5. Conclusion

With customers demanding higher and higher, ever more stringent environmental requirements, the surface treatment process more and more, in the end the choice that the prospects for development, a more general surface treatment process, now seems a little confused, confusing . PCB surface treatment where the future will go, and now can not be accurately predicted. In any case, customer satisfaction and protection of the environment must first be done!