A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. PCB's can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductor on different layers are connected with plated-through holes called vias. Advanced PCB's may contain components - capacitors, resistors or active devices - embedded in the substrate.
Summary
The basic purpose of the surface treatment is to ensure good solderability or electrical properties. Due to the nature of the copper oxide tends to exist in the form of air, the copper is not likely to maintain long-term, so the need for additional processing of copper. Although the subsequent assembly can be used to remove most of the strong flux of copper oxide, but the strong flux itself is difficult to remove, so the industry is generally not used strong flux. Common PCB surface treatment process are: hot air leveling (HASL, hot air solder leveling), organic coating (OSP), electroless nickel / immersion gold, immersion silver, immersion tin.
Outline
Common PCB surface treatment process are: hot air leveling (HASL, hot air solder leveling), organic coating (OSP), electroless nickel / immersion gold, immersion silver, immersion tin.
Background
The basic purpose of the surface treatment is to ensure good solderability or electrical properties. Due to the nature of the copper oxide tends to exist in the form of air, the copper is not likely to maintain long-term, so the need for additional processing of copper. Although the subsequent assembly can be used to remove most of the strong flux of copper oxide, but the strong flux itself is difficult to remove, so the industry is generally not used strong flux.
Technology Introduction
1 HASL
HASL, hot air solder leveling HASL also known as hot air solder leveling , it is in the molten tin-lead solder and PCB surface coating by heating the compressed air leveling ( blown flat ) technology, to form a layer of copper oxide has both anti- can provide good solderability coating . HASL solder usually copper and copper -tin intermetallic compounds formed at the junction , which has a thickness of about 1 ~ 2mil.
HASL PCB usually to be immersed in the molten solder , solder solidified before the wind blowing knife flat liquid solder , and the ability to solder copper crescent-shaped surface is minimized and prevent solder bridging.
HASL is divided into vertical and horizontal type two, is generally considered horizontal better, mainly horizontal HASL coating is uniform , automated production . The general process is: microetch - > Preheat - > Coating Flux - > HAL - > cleaning.
2 . Organic coating
OSP differs from the other surface treatment processes are: its role is to serve as a barrier layer between the copper and the air ; simply , OSP is the clean bare copper surface to grow a layer of organic film chemical methods . This film having oxidation resistance, thermal shock resistance, moisture resistance, and to protect the copper surface does not continue in the normal environment of rust ( oxide or sulfide , etc. ) ; while high temperatures must follow in welding , can be easily co- the rapid removal of flux for welding.
Organic coating process is simple, low cost, so that it is widely used in the industry . Early organic molecules are coated with a rust prevention effect of imidazole and benzotriazole latest molecular mainly benzimidazole. In order to guarantee multiple reflow , only one layer of organic copper surface coating layer is not enough , there must be a lot of layers , which is why you need to add chemical bath usually copper solution . After coating the first layer , the coating layer adsorption of copper ; then copper-binding organic molecules and the second coating layer is applied until the organic molecule or even hundreds of times on the copper surface of the build .
The general process is: Skim - > microetch - > pickling - > water cleaning - > organic coating - > cleaning , process control shows that compared to other treatment processes easier.
3 Electroless Nickel / Immersion Gold
Electroless nickel / immersion gold surface coated with copper is really thick, good electrical properties of nickel and gold alloys can be long-term protection PCB. OSP not only as a barrier layer as rust, which can be useful to achieve good electrical properties, and the PCB during long-term use. In addition, it also has other surface treatment processes do not have the patience for the environment.
The reason is due to be nickel interdiffusion between gold and copper, and nickel layer can prevent the diffusion between the nickel barrier layer, if not, the gold will spread within a few hours to go to the copper. Electroless nickel / immersion gold is another advantage of the strength of nickel, nickel-5um only can control the thickness of the high-temperature expansion of the Z-direction. Also electroless nickel / immersion gold can also prevent the dissolution of copper, which will benefit the lead-free soldering.
The general process is: clean off pickling -> microetch -> preg -> activate -> Electroless Nickel -> chemical leaching of gold; their course has six chemical tank, involving hundreds of species chemicals, the process is more complicated.
4. Immersion silver
OSP and immersion silver process between electroless nickel / immersion gold between, the process is relatively simple and fast. Immersion silver PCB not to wear thick armor, even when exposed to heat, humidity and pollution of the environment, can still provide good electrical properties and good weldability, but will lose their luster. Because there is no nickel-silver layer below, so do not have the immersion silver electroless nickel / immersion gold all good physical strength.
Immersion Silver is a replacement reaction, it is almost pure silver coated sub-micron level. Sometimes immersion silver process also contains some organic matter, mainly to prevent corrosion and eliminate silver silver migration issues, generally difficult to measure out the thin layer of organic material, the analysis shows that less than 1% by weight of the organism.
5. Immersion tin
Immersion silver process between OSP and electroless nickel/immersion gold, the craft is simple, rapid. Immersion silver not for PCB put on thick armor, even if exposure to the hot, wet and pollution of the environment, still can provide good performance and good weldability, but will lose its luster. Because there is no nickel silver layer below, so the immersion silver does not have the chemical plating nickel/gold leaching all good physical strength.
Immersion tin process can be formed between the flat copper-tin intermetallic compounds, this feature makes the immersion tin and hot air leveling with the same good weldability without HASL headaches flatness problem; nor Electroless Nickel / Immersion proliferation issues between gold metal; just dip tin plate can not be stored for long.
Other surface treatment process
Other applications showed fewer treatment process, in which applications have more gold plating palladium nickel plating process.
1 nickel plating
Nickel plating PCB surface treatment process is the originator of the product since it appears PCB appears, slowly evolved into other processes. Electroless nickel gold plated PCB is on a first surface of the conductor layer after layer of nickel-plated and then 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 show that does not look bright) and gold plated (surface smooth, hard, wear-resistant, contain other elements such as cobalt, the surface looks brighter). Gold is mainly used to fight soft chip packaging gold; hard gold is mainly used in non-welding electrical interconnect (such as Goldfinger).
Normally, the welding will cause brittle gold plating, which will shorten the life, and thus to avoid welding on IPG; the electroless nickel / immersion gold because gold is very thin and uniform, brittle rarely occurs.
2 electroless palladium
Palladium plating electroless nickel plating process and the process was similar. The main process by reducing (hypophosphorous acid, sodium dihydrogen) palladium ions to the surface of the palladium-catalyzed, the newborn can be referred to the palladium catalyst to promote the reaction, the palladium plating layer thus obtained desired thickness. Advantages of palladium plating good welding reliability, thermal stability, surface smoothness. Palladium is a disadvantage of relatively rare noble metal, and thus the cost will increase.
Process Selection
Other applications showed fewer treatment process, in which applications have more gold plating palladium nickel plating process.
Surface treatment choice depends on the type of final assembly components; surface treatment process will affect the production of PCB, assembly and end use, the following describes the specific surface treatment process five common usage scenarios.
1. HASL
HASL PCB had a dominant position in the surface treatment processes. In the 1980s, more than three-quarters of PCB hot air leveling process, but in the past decade have 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 process. HASL process dirty, smelly, dangerous, and thus the process is never pleasing, but HASL For larger components and larger wire spacing is concerned, it is an excellent process. In the high-density PCB, the hot air leveling flatness will affect subsequent assembly; therefore HDI board (High Density Intrerconnection Technology high-density circuit boards) generally do not use hot air leveling process. With advances in technology, the industry has now emerged for assembling smaller pitch QFP and BGA hot air leveling process, but less practical. At present, some plants using organic coating and electroless nickel / immersion gold process to replace the hot air leveling process; technological developments have led some factories using immersion tin, immersion silver process. Together with the recent trend of lead-free, hot air leveling use subject to further restrictions. Although there have been so-called lead-free HASL, but this may be related to equipment compatibility issues.
2. Organic coating
Estimated that about 25% -30% of the PCB using organic coating process, the ratio has been rising (probably organic coating over hot air leveling now living in the first one). 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 with the board. For BGA, the organic coating applications is greater. If you do not connect PCB surface functional requirements or limited storage period, the organic coating would be ideal surface treatment processes.
3. Electroless Nickel / Immersion Gold
Electroless nickel / immersion gold process with 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 mobile phone keypad, the edge router connected to the housing area and chip processor flexibility electrical contact area connection. As the hot air leveling flatness problem and organic coating flux clear the problem, in the 1990s electroless nickel / immersion gold using a very wide; later because of the black disk, appears brittle nickel-phosphorus alloy, electroless nickel plating / immersion gold process applications has decreased, but now almost every high-tech PCB plant has electroless nickel / immersion gold. Taking into account the removal of copper-tin intermetallic compound becomes brittle joints, many of the compounds of the problem will occur between the relatively brittle metal nickel-tin. Therefore, the portable electronic products (such as mobile phones) are used almost solder copper-tin intermetallic compounds organic coating, dip or immersion tin silver formed, while the use of electroless nickel / immersion gold form a key area, the contact area and EMI shielding area . Estimated that about 10% -20% of the PCB using electroless nickel / immersion gold process.
4. Immersion silver
Than electroless nickel immersion silver / immersion gold cheap, if PCB has connectivity requirements and the need to reduce costs, immersion silver is a good choice; plus immersion silver good flatness and contact, it should choose immersion silver processes. In communications, automotive, computer peripherals for many applications areas immersion silver, immersion in the design of high-speed signal silver has also been applied. As the immersion silver surface treatment with other unmatched good electrical properties, it can also be used in high-frequency signal. EMS recommend using immersion silver process because it is easy to assemble and can be checked with a good nature. However, due to problems such as immersion silver tarnish, solder voids and other defects makes its slow growth (but not decreased). Estimated that about 10% -15% of the PCB using immersion silver process.
5. Immersion tin
Surface treatment of tin is introduced in the last decade of use, there is the result of the process automation requirements. Immersion into the tin welding without any new element, especially for communications backplane. In addition to the board storage tin will lose weldability, immersion tin and thus the need for better storage conditions. Another immersion tin process because it contains carcinogens were restricted. Estimated that there are about 5% -10% of the PCB using the immersion tin process.
PCB surface treatment characteristics and uses
With the continuous improvement of the living environment of human requirements, the current PCB production process involved in environmental issues is particularly prominent. Currently on lead and bromine is the hottest topic; lead-free and halogen-free will affect the development of PCB's in many ways. Despite the current situation, change aspects of PCB surface treatment technology is not great, it seems quite distant thing, but it should be noted: a long slow change will lead to huge changes. In the case of environmental voice rising, PCB surface treatment process will certainly be changes happen in the future.
The purpose of surface treatment
The basic purpose of the surface treatment is to ensure good solderability or electrical properties. Due to the nature of the copper oxide tends to exist in the form of air, the copper is not likely to maintain long-term, so the need for additional processing of copper. Although the subsequent assembly can be used to remove most of the strong flux of copper oxide, but the strong flux itself is difficult to remove, so the industry is generally not used strong flux.
With the rising demand of customers, increasingly stringent environmental requirements, surface treatment process more and more, in the end choose the kind of promising, more versatile surface treatment technology, now seems like a little confused, confusing . PCB surface treatment process will go in the future, it also can not be accurately predicted. Anyway, meet customer requirements and to protect the environment must first be done!
About PCBway
Since 2003 PCBWAY has been the leading PCB quick turn manufacturer specializing in both Prototype and Production quantities, Initially produced single-sided and double-sided printed circuit boards for the consumer electronics market. PCBWAY is ranked among the top 4 board fabricators in asia and is well-known for its expedited turn time capabilities and its reliable best on-time shipping record.
Today, we have over 450 operators with high modern facilities to manufacture multi-layer PCB up to 12 layers. Backing up with a group of professional engineers, and well established quality system. PCBWay has grown to become a major PCB manufacturer in Asia to serve in diverse customers base such as electronics appliance, communication, educational electronics, power supplies, Automationsetc.
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