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SMT ASSEMBLY PROCESS AND WELDING Date:【2018-01-05】 Read【2349】 【Back

SMT assembly process is closely related to each process step before welding, including capital investment, PCB design, component solderability, assembly operations, flux selection, temperature / time control, solder and crystal structure.
    1, solder
    At present, the most commonly used solder soldering solder is eutectic tin lead alloy: tin 63%; lead 37%, should always grasp the solder pot solder temperature, the temperature should be higher than the alloy liquid temperature 183 ℃, and the temperature uniformity. In the past, the solder pot temperature at 250 ° C was considered "standard".
    With the innovation of flux technology, the uniformity of solder temperature throughout the solder pot has been controlled and the preheater has been added. The development trend is to use a lower temperature solder pot. It is common to set the temperature of the solder pot in the range of 230-240 ° C. Normally, the components do not have uniform thermal mass, and it is necessary to ensure that all solder joints reach a sufficient temperature in order to form a qualified solder joint. The important question is to provide enough heat to raise the temperature of all leads and pads, thus ensuring solder flow, wetting both sides of the solder joint. The lower the temperature of the solder will reduce the thermal shock to the component and the substrate, help to reduce the formation of scum, at a lower intensity, the flux coating operation and flux compound under the combined effect of the peak outlet can have Sufficient flux, so that can reduce the burr and the production of solder balls.
    The solder composition in the solder pot is closely related to the time, that is, over time, which results in the formation of scum, which is the reason for removing the residue and other metal impurities from the welded assembly and in the welding process Tin loss reasons. These factors can reduce the flow of solder. In the procurement, the maximum limits of the tin content of the metal trace dross and the solder are specified, as defined in the respective standards (eg, as defined in IPC / J-STD-006). In the welding process, the requirements for solder purity are also specified in the ANSI / J-STD-001B standard. In addition to the restrictions on scum, 63% tin; 37% lead alloy in the provisions of the minimum tin content shall not be less than 61.5%. The gold and organic baths on the wave soldering assembly gather more copper concentrations than in the past. This aggregation, coupled with significant tin loss, can cause solder loss of liquidity and produce welding problems. Rough, granular solder joints are often caused by scum in the solder. Due to the accumulation of dross in the solder pot or the inherent remnants of the components themselves, the rough granular solder joints may also be signs of low tin content, not the local special solder joints, which is the result of tin loss in tin pots. This appearance may also be caused by vibration or shock during the solidification process.
    The appearance of the solder joint can directly reflect the process or material problems. It is important to keep the solder "full pot" state and to analyze the solder pot analysis according to the process control scheme. It is usually not necessary to "drain" the solder in the solder pot because of the scum in the solder pot, and the solder in the tin pot is always full because it is required to add solder to the tin pot in conventional applications The In the case of loss of tin, the addition of pure tin helps to maintain the desired concentration. In order to monitor the compounds in the tin pot, routine analysis should be performed. If tin is added, the analysis should be sampled to ensure that the solder composition is in proportion. Excessive scum is a tricky problem. There is no doubt that there is always a scum in the solder pot, especially when welding in the atmosphere. The use of "chip crests" is helpful for welding high-density components, resulting in more scum due to the solder surface being exposed to the surface of the solder that is too large to oxidize the solder. Solder pot surface with a scum covered with solder, the oxidation rate to slow down.
    In the welding, due to the tin pot in the turbulence and flow will produce more scum. The recommended method is to skim the scum, if it is often written, then it will produce more scum, and the use of more solder. The scum may also be mixed in the crests, resulting in instability or turbulence of the crests, thus requiring more maintenance of the liquid components in the solder pot. If the amount of solder in the tin pot is allowed to be reduced, the scum on the surface of the solder will enter the pump. This phenomenon is likely to occur. Sometimes, the granular solder joints will be mixed with scum. The initially found scum may be caused by rough crests and may block the pump. The tin pot should be equipped with an adjustable low-capacity solder sensor and alarm device.
    2, crest
    In the wave soldering process, the peak is the core. Can be preheated, coated with flux, dirt-free metal through the conveyor belt to the welding station, contact with a certain temperature of the solder, and then heated, so that the flux will produce a chemical reaction, the solder alloy through the formation of interconnection of peak power, which Is the most critical step. At present, the commonly used symmetrical peak is called the main peak, set the pump speed, crest height, depth of infiltration, transmission angle and transmission speed, in order to achieve good welding characteristics to provide a full range of conditions. The data should be properly adjusted and the solder run should be decelerated and slowly stopped running behind the exit (exit). The PCB will eventually push the solder to the outlet as the crest runs. In the case of the most hanging, the surface tension of the solder and the optimized crest run of the plate can achieve zero relative movement between the component and the peak at the exit end. This hulled area is achieved by removing the solder on the board. Should provide adequate inclination, does not produce bridges, burrs, wire drawing and solder balls and other defects. Sometimes, crests are required to have hot airflow to ensure that the possible bridging is eliminated. After the surface mount element is mounted on the bottom of the board, the flux is sometimes compensated for the flux or the bubbles in the "harsh peak" area formed later, while the wave crests are used before the turbulence chip peaks are used. The high vertical speed of the turbulence peak helps to ensure contact of the solder with the lead or pad. The vibrating part behind the leveling laminar peak can also be used to eliminate air bubbles and ensure satisfactory solder contact components. The welding station should basically be: high purity solder (according to standard), crest temperature (230 ~ 250 ℃), the total time of exposure to the peak (3 to 5 seconds), the depth of the printed board immersed in the peak %), To achieve parallel transmission of the track and in the wave and the track parallel to the tin pot flux content.
    3, after the wave soldering cooling
    Usually in the tail of the wave soldering machine additional cooling station. In order to limit the tendency of copper-tin intermetallic compounds to form solder joints, another reason is to accelerate the cooling of the components, the solder is not fully cured, to avoid the board shift. Quickly cool the assembly to limit the exposure of the sensitive element to high temperatures. However, the risk of thermal shock to components and solder joints of the aggressive cooling system should be taken into account. A well controlled "soft and stable", forced gas cooling system should not damage most components. There are two reasons for using this system: the board can be quickly processed without hand clamping, and the component temperature can be guaranteed to be lower than the temperature of the cleaning solution. What people care about is the latter reason, which may be the cause of some solder residue foaming. Another phenomenon is sometimes the phenomenon of reaction with some flux scum, which makes the residue "clean". It is possible to meet these requirements by ensuring that the data set by the welding station meets all the machines, all designs, all materials and process materials used and requirements. You must understand every step in the entire process. 4 Conclusion In summary, to obtain the best quality of welding to meet the needs of users, it is necessary to control each process step before welding, because each step of the entire assembly process of SMT is interrelated and interacts with each other. The problem will be shadow to the overall reliability and quality. Welding operation is also the case, it should be strictly controlled all the parameters, time / temperature, the amount of solder, flux composition and transmission speed and so on. The defects in the welding, should be identified as soon as possible, the analysis, to take appropriate measures to affect the quality of the various defects eliminated in the bud. In this way, to ensure that the products are produced in line with technical specifications.