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PCB, FPC surface treatment technology
PCB, FPC surface treatment technology

With the continuous improvement of human requirements for the living environment, the environmental problems involved in the current PCB production process are particularly prominent. The topic of lead and bromine is the hottest, and lead-free and halogen-free will affect PCB development in many ways.
Although at present, the changes in the surface treatment process of PCB and FPC are not very large, it seems that it is still a distant matter, but it should be noted that long-term slow changes will lead to huge changes. In the case of increasingly high environmental protection calls, the surface treatment process of PCB and FPC will definitely change dramatically in the future.
The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since copper in nature tends to exist in the form of oxides in the air, it is unlikely to remain as raw copper for a long time, so other treatments of copper are required. Although in the subsequent assembly, strong flux can be used to remove most of the copper oxide, the strong flux itself is not easy to remove, so the industry generally does not use strong flux.
There are many PCB surface treatment processes, and the common ones are hot air leveling, organic coating, electroless nickel/immersion gold, immersion silver and immersion tin, which will be introduced one by one.
1, hot air leveling (spray tin) (lead-free tin), lead tin
Hot air leveling, also known as hot air solder leveling (commonly known as spray tin), is a process in which molten tin (lead) solder is applied to the surface of the PCB and is heated (compressed) by heating compressed air to form a layer that is resistant to copper oxidation. It also provides a good solderability coating. When the hot air is leveled, the solder and copper form a copper-tin intermetallic compound at the junction. The PCB is sunk in the molten solder during hot air leveling; the air knife blows off the liquid solder before the solder solidifies; the air knife minimizes the meniscus of the solder on the copper surface and prevents solder bridging.
2. Organic solderability protector (OSP)
OSP is a process that meets the requirements of the RoHS Directive for printed circuit board (PCB) copper foil surface treatment. OSP is the abbreviation of Organic Solderability Preservatives. Chinese translation is organic solder mask, also known as copper protector, also known as Preflux in English. Simply put, OSP chemically grows an organic film on a clean bare copper surface.
This film has anti-oxidation, thermal shock resistance and moisture resistance. It is used to protect the copper surface from rust (oxidation or vulcanization) in the normal environment. However, in the subsequent high temperature of welding, the protective film must be very It is easily removed by the flux, so that the exposed clean copper surface can be immediately combined with the molten solder into a firm solder joint in a very short time.
OSP organic solderability preservatives were called early heat preflux. In essence, it is an alkylbenzimidazole-based compound which has high heat resistance and generally requires a decomposition temperature of 300 ° C or higher. Therefore, it can well protect the fresh copper surface from oxidation and contamination. When soldering at high temperature, the OSP is removed by the action of the solder to reveal the fresh copper surface and quickly solder it to the solder. However, it is not suitable for multiple reflow soldering.
3, full plate nickel-plated gold (plated soft gold)
Nickel plated gold is plated with a layer of nickel on the surface conductor of the PCB and then plated with a layer of gold. The nickel plating is mainly to prevent the diffusion between gold and copper. There are two types of electroplated nickel gold: gold-plated gold (pure gold, the gold surface does not look bright) and hard gold (the surface is smooth and hard, wear-resistant, contains other elements such as cobalt, and the gold surface looks brighter). Soft gold is mainly used for gold wire bonding in chip packaging; hard gold is mainly used for electrical interconnection in non-welded parts.
4, Shen, nickel gold
Shen Jin is a thick, electrically good nickel-gold alloy wrapped on the copper surface, which can protect the PCB for a long time. In addition, it also has environmental tolerance that other surface treatment processes do not have. In addition, immersion gold can also prevent the dissolution of copper, which will benefit lead-free assembly.
5, Shen Xi
Since all current solders are based on tin, the tin layer can be matched to any type of solder. The immersion tin process can form a flat copper-tin intermetallic compound. This property makes the immersion tin have the same good solderability as the hot air leveling without the hot air leveling headache. The slab can not be stored for too long. The assembly must be carried out according to the order of the tin.
6, Shen Yin
The silver process is between organic coating and electroless nickel/immersion gold. The process is simple and fast. Even if exposed to heat, humidity and pollution, silver can maintain good solderability but lose luster. . Silver does not have the good physical strength of electroless nickel/immersion gold because there is no nickel under the silver layer.
7. Chemical nickel palladium gold
Compared with immersion gold, chemical nickel-palladium gold has an additional layer of palladium between nickel and gold. Palladium can prevent corrosion caused by displacement reaction and is fully prepared for immersion gold. Gold is tightly covered on top of the palladium to provide a good contact surface.
8, electroplated hard gold
In order to improve the wear resistance of the product, the number of insertions and removals is increased and the hard gold is plated. The wear resistance of the electric gold is relatively high. As the integration degree of the IC becomes higher and higher, the IC foot is more and more dense. The vertical tin-spraying process is difficult to flatten the thinned pads, which makes the placement of the SMT difficult; in addition, the shelf life of the tin-plated board is very short. The gold-plated plate solves these problems: 1 For the surface mount process, especially for the 0603 and 0402 ultra-small surface mounts, because the flatness of the pad is directly related to the quality of the solder paste printing process, the quality of the reflow soldering from the back To the decisive impact, the entire plate is plated in high density and ultra-small surface mount processes.
9, organic solderability protection agent (OSP) + gold finger
As ICs become more integrated, the IC pins are more dense. The vertical tin-spraying process is difficult to flatten the thinned pads, which makes the placement of the SMT difficult; in addition, the shelf life of the tin-plated board is very short. The gold-plated plate solves these problems: 1 For the surface mount process, especially for the 0603 and 0402 ultra-small surface mounts, because the flatness of the pad is directly related to the quality of the solder paste printing process, the quality of the reflow soldering from the back To the decisive impact, the entire plate is plated in high density and ultra-small surface mount processes. 2 In the trial production stage, due to the factors such as component procurement, it is often not the board to be welded immediately, but it is often necessary to wait for a few weeks or even a month to use, the shelf life of the gold plate is better than the lead tin. Hehe. The gold is many times longer, so everyone is willing to adopt it. Besides, the cost of the gold-plated PCB in the sample stage is almost the same as that of the lead-tin alloy plate. However, as the wiring becomes denser and denser, the line width and spacing have reached 3-4 MIL, thus bringing about the problem of short-circuiting of the gold wire; as the frequency of the signal is getting higher and higher, the signal is transmitted in the multi-plating layer due to the skin effect. The effect of the situation on the signal quality is more obvious. In order to improve the wear resistance of the finger position product, the number of insertions and removals is increased and the hard gold is plated. The electric gold wear resistance is relatively high, the hardness is relatively good, the gold is thicker than the conventional gold plating, and most of them are made of thick gold.
10, organic solderability protectant (OSP) + nickel-plated gold
Compared with other products, organic coating liquid has the following characteristics:
1) The solution is clarified and stable, low temperature operation, mild chemicality, and ensures the stability of the printed circuit board structure. 2) The OSP film is uniform and compact, smooth and non-tacky, and has good apparent quality. 3) Excellent heat resistance and compatible with lead-free SMT process. It can withstand more than three times of reflow soldering to maintain excellent solderability. 4) Good compatibility with flux and solder paste. It is satisfactory in the through-hole soldering performance of the board and the solder wettability on the SMT pad. 5) Excellent moisture resistance, which can protect the copper surface from oxidation for about 6-12 months. 6) Water-soluble solution of non-volatile solvent, does not contain halogen ions such as chlorine and bromine, meets the environmental requirements of RoHs directive, and has high safety.
11, electroplating tin
1 Overview Electroplating belongs to the field of electrochemistry, which is a process for surface processing of products by electrochemical methods. Electroplated tin plate, also known as tinplate, is a high-tech and high value-added deep-processing product. It is used to make packaging containers and as non-packaging materials. It has high strength, good weldability, corrosion resistance and non-toxicity. .
It is favored by people and is widely used in jewelry, watches, glasses, buttons and other industries. The author introduces the process of electroplating tin-cobalt alloy and the effects of various components, bath maintenance and common fault handling.
Attempts have been made to replace the Sn-Pb plating which has been used all the time with Sn-Zn, Sn-Bi, Sb-Ag and Sn-Cu plating. However, these lead-free plating technologies are also short and long, not perfect.
As the requirements of users are getting higher and higher, the environmental requirements are getting more and more strict, and the surface treatment process is getting more and more. It is a bit dazzling and confusing to choose the kind of surface treatment process with promising prospects and versatility. . Where the PCB surface treatment process will go in the future will not be accurately predicted. In any case, to meet user requirements and protect the environment must first be done!
 

Pub Time : 2006-06-01 12:32:12 >> News list
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