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SMT assembly technology

 SMT assembly technology



1. Solder


Currently, the most common wave soldering solder is eutectic tin-lead alloy: tin 63%; 37% lead, the solder pot should always grasp the solder temperature, the temperature should be higher than the temperature of the liquid alloy 183 , and temperature uniformity. In the past, the solder pot temperature is 250 is considered "standard."


With the innovative technology of flux throughout the solder pot, solder temperature uniformity under control, and the addition of a preheater, the trend is to use a lower temperature solder pot. Setting the solder pot temperature in the range of 230-240 is very common. Typically, the component does not have a uniform thermal mass, to ensure that all of the joints to reach a temperature sufficient to form acceptable solder joint is necessary. The important issue is to provide sufficient heat to raise the temperature of all leads and pads to ensure that the solder flow, solder wetting of both sides. Low temperature solder element and the substrate will reduce thermal shock and help reduce the formation of dross, at a lower intensity, carried out under the action of the flux coating operation and flux compounds can have a peak export sufficient flux, so it can reduce the generation of burrs and solder balls.


Solder composition of solder pot and time is closely related to that change over time, thus leading to the formation of dross, residues and other reasons, this is the removal of metal contaminants from the welding assembly and welding processes tin loss reasons. These factors may reduce the fluidity of the solder. In procurement, to the maximum limit specified trace metal solder dross and tin content in various standards, (such as the IPC / J-STD-006 are clearly defined). During the welding process, the solder purity requirements in ANSI / J-STD-001B standard also provided. In addition to restrictions on the scum, the tin for 63%; 37% tin lead alloy specified minimum content of not less than 61.5%. The organic layer was swimming gold copper concentration wave soldering assembly gathered more quickly than in the past. This gathering, coupled with a significant loss of tin, solder can lose liquidity, and the welding problems. The appearance of rough, grainy solder joint is often caused due to the solder dross. Since the solder pot gathering scum or residue components inherent dim, rough granular solder joints may also be a sign of low tin content, not local specialty joints, it is the result of a tin pot tin loss. This appearance may be in the solidification process, due to vibration or shock caused.




Solder joint appearance can directly reflect the workmanship or materials issues. In order to maintain solder "full pot" in accordance with state and process control solutions for checking the solder pot analysis it is very important. Since the solder and the dross pot "drained" pot solder flux, is usually not necessary, since the requirements of the application to the conventional tin solder pot add, tin solder pot is always full . In case of loss of tin, add pure tin helps maintain the desired concentration. In order to monitor the pot tin compound should be carried out routine analysis. If you add the tin, sampling and analysis should be to ensure that the solder composition ratio correct. Excessive dross is a very difficult problem. Undoubtedly, there is always the solder pot scum exists when welding in the atmosphere in particular. Using the "chip crest" This high density welding assembly is helpful due to exposure to the atmosphere of the solder surface is too large, leaving the oxidation of the solder, it will produce more scum. Solder pot covered with solder surface scum layer, the oxidation rate will slow down.


In welding, since the tin pot peak flow and turbulence and produce more scum. The recommended conventional method is to skim the scum, if frequent write-cut, it would produce more dross, solder and consume more. Scum may also be mixed in the crest, the crest cause instability or turbulence, thus requiring liquid composition of the solder pot to give more maintenance. If allowed to reduce the amount of tin solder pot, then dross solder surface will enter the pump, this phenomenon is likely to occur. Sometimes, the particulate will be mixed solder dross. Initially found dross, may be caused by rough peaks, and may plug the pump. Shall be equipped with low volume solder sensor and alarm devices can be adjusted on the tin pot.


2. peaks


In the wave soldering process, the crest is the core. May be preheated, is coated with a flux, metal free from contamination by the belt to the welding station, having a certain temperature, which contacts the solder, and then heated so that the solder will produce a chemical reaction, the solder alloy is formed by the interconnected power peaks, which It is the most critical step. At present, the commonly used symmetric peak is called the main peak, set pump speed, wave height, depth of invasion, transmission angle and transmission speed, to achieve good welding characteristics provide a full range of conditions. Data should be appropriately adjusted, leaving behind the peak (the outlet end) should make the solder run slow down, and slowly stopped. With the final run to the crest PCB solder pushed to the outlet. In the case of the hanging, the peak running surface tension of the solder and optimized plate between the component and the outlet end of the peak can achieve zero relative motion. The shelling region is to achieve the removal of solder board. It should provide adequate inclination, no bridge, burrs, drawing and balls and other defects. Sometimes, the peak export need to have the hot air stream in order to ensure the formation of bridging rule out the possibility. After the bottom plate mounted on the surface-mount components, sometimes before compensation flux or "harsh spikes" region formed behind the bubble, and the peaks were leveled using turbulence chip peaks. Turbulence crest high vertical speed helps to ensure that the contact with the leads and solder pads. In laminar flow vibrating portion behind the crest of leveling also be used to eliminate air bubbles and ensure the solder to achieve satisfactory contact assembly. Basically welding station shall: High purity solder (standard), the peak temperature (230 ~ 250 ), the total exposure time of peak (3-5 seconds), the printed circuit board is immersed in the peak depth (50 ~ 80 %) to achieve parallel conveyor track and tin content solder pot at a peak and a track parallel to the state.


3.cooled after wave


Typically additional cooling station at the end of the wave soldering machine. In order to limit the formation of copper-tin intermetallic compounds trend joints, it is another reason to accelerate the cooling components in the solder is not completely cured, to avoid shifting the board. Rapid cooling assembly, to limit the sensitive element is exposed to high temperatures. However, it should take into account the dangers of aggressive cooling system components and solder heat shocks. A well-controlled "soft and stable" and forced gas cooling system should not damage the majority component. The reason for using this system has two: fast processing board without holding hands, and ensure component temperatures lower than the temperature of the cleaning solution. It is concerned with a cause, which may be caused by some reasons foaming flux residue. Another phenomenon is the phenomenon sometimes react with certain solder dross, which makes the residue "cleansing can not afford." Welding stations set up to ensure data satisfy all the machines, all design, materials and process materials all the conditions and requirements adopted no fixed formula can meet these requirements. We must understand the whole process every step of the operation. 4 Conclusion In summary, to get the best welding quality to meet the needs of users, it must be controlled before soldering, each process step, because each step of the entire SMT assembly process are interrelated, interaction, there are any further Movies are problems within the overall reliability and quality. Welding operation, too, so that all of the parameters should be strictly controlled, the time / temperature, the amount of solder, flux composition and the transmission rate and so on. Welding defects generated, should try to identify the causes, analyze and take appropriate measures, various defects will affect the quality of being nipped in the bud. Thus, in order to ensure the production of the products meet the technical specifications.



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