How to choose PCB surface treatment

- Aug 08, 2016-


The purpose of the surface treatment

The most basic purpose of the surface treatment is to ensure good solderability or electrical properties. Due to the nature of the copper oxide form tend to be in the air, it is unlikely to maintain the original long copper, copper requiring additional processing. Although the subsequent assembly can be removed using a strong flux of most copper oxide, strong but flux itself difficult to remove, so the industry generally do not use strong flux.


Five common surface treatment process

There are many PCB surface treatment, it is common HASL, organic coating, electroless nickel / immersion gold, immersion silver and immersion tin process five, will be introduced one by one.


1. HASL

Also known as hot air solder leveling, it is in the molten tin-lead solder and PCB surface coated with whole (blowing) with heated compressed air process level, to form a layer of both copper anti-oxidation, but also provide good solderability coating cladding. HAL usually copper-tin solder and copper intermetallic compound formed at the junction. Protect the copper surface of the solder thickness of about 1-2mil. Usually to the PCB HAL immersed in molten solder; air knife blow before solidification of the solder liquid level solder; air knife can be crescent-shaped solder copper surface is minimized and to prevent solder bridging. HASL is divided into two kinds of vertical and horizontal, is generally believed that horizontal well, mainly horizontal HASL more uniform coating can be achieved automated production. Hot air leveling process is the general process: micro-corrosion coating flux → preheating → → → spray-cleaning.

2. Organic coating process

Unlike other surface treatment processes, it is to act as a barrier layer between the copper and air; organic coating process is simple, low cost, which makes it possible to widely used in the industry. Early organic molecules from rust coating is the role of imidazole and benzotriazole, latest molecular primarily benzimidazole, which is chemically bonded to the nitrogen functional groups on the PCB copper. In the subsequent soldering process, if only the surface of the copper layer of organic coating layer is not enough, there must be a lot of layers. This is why you need to add a chemical bath usually copper solution. After the first layer of the coating, the coating layer adsorption of copper; then coated with organic molecules binding with a second layer of copper, twenty or even a hundred times until coated organic molecules assembled in the copper surface, which can be many times back guarantee flow soldering. Test showed that: the latest organic coating process to maintain good performance in a number of lead-free soldering process. The general process of organic coating process is as follows: degreasing → → → acid etching micro-water washing → → washing the organic coating, process control relative to other surface treatment processes easier.

3. Electroless Nickel / Immersion Gold

Electroless Nickel / Immersion Gold Process not as simple organic coating, electroless nickel / immersion gold PCB like to wear thick armor; additional electroless nickel / immersion gold process unlike organic coating as rust-proof barrier it can be useful electrical properties and achieve good long-term use in PCB process. Therefore, electroless nickel / immersion gold surface is wrapped in a thick layer of copper, nickel electrical good gold alloy, which can be long-term protection PCB; in addition it also has other surface treatment processes do not have the patience for the environment sex. Nickel reason is due to interdiffusion between gold and copper will be, and the nickel layer can prevent the spread between gold and copper; if there is no layer of nickel, gold will spread to the copper in a few hours to go. Electroless Nickel / Immersion Gold Another benefit is the strength of nickel, nickel only 5 microns thick can limit expansion at high temperature in the Z direction. In addition electroless nickel / immersion gold can also prevent dissolution of copper, which will benefit the lead-free assembly. Electroless Nickel / Immersion Gold Process is the general process: micro-corrosion acid cleaning → → prepreg → activation → → Electroless Nickel Immersion Gold, there are six chemical tank, involving nearly 100 chemicals, process control and therefore relatively difficult.

4. immersion silver process

Between the organic coating and electroless nickel / immersion gold between, the process is relatively simple, fast; not as complicated electroless nickel / immersion gold, nor is it to the PCB put on a thick layer of armor, but it is still able to provide good electrical properties. Silver is gold little brother, even when exposed to heat, humidity and pollution of the environment, the silver is still able to maintain good weldability, but will lose their luster. Immersion Silver does not have good physical strength of electroless nickel / immersion gold because silver has the layer below no nickel. Also good storability immersion silver, put a few years there will not be a big problem assembled after immersion silver. Immersion Silver is a substitution reaction, it is almost pure silver coated submicron. Sometimes immersion silver process also contains some organic matter, mainly to prevent the corrosion of silver and silver eliminate migration issues; this is generally difficult to measure out a thin layer of organic matter, the analysis showed less than 1% by weight of the organism.

5. Soldering

Since all current solders are tin-based, so the tin layer can be matched with any type of solder phase. From this point of view, immersion tin process extremely development prospects. But after a previous PCB by immersion tin process appears whisker in the welding and tin migration Chengzhong Xi must bring reliability problems, so the use of immersion tin process is limited. Later, immersion tin solution was added to the organic additives can make tin layer structure granular structure, to overcome the previous problems, but also has good thermal stability and weldability. Immersion tin process can be formed between a flat copper-tin intermetallic compound, this feature makes the immersion tin and having a hot air leveling as good weldability without the hot air leveling headaches flatness problem; there is no dip tin plating Nickel / immersion gold metal diffusion problems between - copper-tin intermetallic compound capable of firmly together. Dip tin plate can not be stored for too long, it must be carried out according to the order Soldering assembly.

6. Other surface treatment

Less other surface treatment applications, the following applications run relatively large number of palladium nickel plating gold plating process. Nickel plating is the originator of PCB surface treatment, since it appears PCB appears that later evolved into other ways. It is the first conductor in the PCB surface coated with a layer of nickel 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 nickel plating: soft gold plating (gold, gold surface does not look bright) and gold plated (surface smooth and hard, wear-resistant, contain other elements such as cobalt, gold surface look more shiny). Gold is mainly used for playing soft gold wire chip packaging; Hard Gold is mainly used in non-welding electrical interconnection at. Taking into account the cost, the industry often selective plating method of transferring an image by reducing the use of gold. Selective gold plating currently used in the industry continues to increase, mainly due to the electroless nickel / immersion gold process control more difficult. Under normal circumstances, the welding can cause embrittlement gold plating, which will shorten the life, and thus to avoid welding on gold plating; but electroless nickel / immersion gold Because gold is very thin, and very consistent, rarely occurs brittle . Palladium plating process and electroless nickel plating process was similar. The main process is performed by a reducing agent (such as hypophosphorous acid, sodium dihydrogen) palladium ions to the surface of the palladium-catalyzed, the newborn can be a palladium catalyst to promote the reaction and gave a palladium coating of any thickness. Advantages palladium plating for good reliability welding, thermal stability and surface smoothness.


Selective Surface Treatment Technology

Surface treatment choice depends on the type of final assembly of components; surface treatment will affect the PCB production, assembly and final use, the following describes the specific surface treatment process five common usage scenarios.

1, hot air leveling

HASL PCB had a dominant position in the surface treatment process. 1980s, more than three-quarters of PCB hot air leveling process, but in the past decade has been in the industry to reduce the use of hot air leveling process, it is estimated there are about 25% -40% of the PCB using a hot air leveling technology. Hot air leveling process is dirty, smelly, dangerous, and therefore is never pleasing process, but hot air leveling for large size components and larger wire spacing is concerned, it is an excellent process.

In the higher density PCB, HASL flatness will affect subsequent assembly; it HDI boards generally do not use hot air leveling process. As technology advances, the industry has now appeared less suitable for assembly pitch QFP and BGA hot air leveling process, but less practical use. At present, some 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 using immersion tin, immersion silver process. Together with the recent trend of lead-free, hot air leveling use is further limited. Although there have been so-called lead-free HASL, but it may be related to compatibility issues with the device.

2. Organic coating

Estimated that about 25% -30% of the PCB using an organic coating process, the ratio has been rising (probably organic coating now has more than hot air leveling living in the first).

The organic coating process can be used in low-tech PCB, can also be used in high-tech PCB, such as television sets with single-sided PCB, high-density chip package board. For BGA aspect, organic coating applications is greater. If there is no connection PCB surface functional requirements or limited storage period, the organic coating is the ideal surface treatment process.

3. Electroless Nickel / Immersion Gold

Electroless nickel / immersion gold process and different organic coating, it is mainly used on the surface of the board to connect the functional requirements and a longer shelf life, such as the phone keypad, the edge router connected to the housing area and the elastic chip processor electrically connected to the contact zone. Since the hot air leveling and flatness problems organic coating flux remove problems in the 1990s electroless nickel / immersion gold wide use; Later, because black disk appears brittle nickel-phosphorus alloy, chemical nickel plating / immersion gold process applications has decreased, but at the moment almost every high-tech PCB plant has electroless nickel / immersion gold.

Taking into account the removal of copper-tin intermetallic compound when the joints become brittle compound at a lot of issues will arise between the relatively brittle nickel metal tin. Therefore, the portable electronic products (such as mobile phones) are used almost solder copper-tin intermetallic compound organic coating, immersion silver or immersion tin is formed, while the use of electroless nickel / immersion gold forming a keypad, the contact area and EMI shielding areas . Is estimated that about 10% -20% of the PCB using electroless nickel / immersion gold process.

4. Immersion Silver

Immersion silver than electroless nickel / immersion gold cheap, if the PCB connectivity requirements and the need to reduce costs, immersion silver is a good choice; plus immersion silver good flatness and contact, you should choose immersion silver process. In communications, automotive, computer peripherals immersion silver for many applications in high-speed signal design immersion silver has been applied. Since the immersion silver surface treatment have other unmatched good electrical properties, it can also be used in high-frequency signal. EMS is recommended to use immersion silver process because it is easy to assemble and has better checking of. However, due to problems such as immersion silver tarnish, solder voids and other defects such that it is slow growth (but not decreased). Is estimated that about 10% -15% of the PCB use immersion silver process.

5. Soldering

Is introduced into the surface treatment process is nearly a decade of thing, there is the result of the process automation requirements. Soldering welding at not bring any new element, especially for communication backplane. In addition to the board of the storage period will lose weldability tin, immersion tin and thus the need for better storage conditions. Further immersion tin process because it contains carcinogenic substances is restricted. Is estimated that about 5% -10% of the PCB using the immersion tin process.


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