Recalling the course of its development in the electronic industry, you can notice a very obvious trend is the soldering technique. In principle, the traditional cartridge cases can also be used reflow process, which is commonly referred to as through-hole reflow. The advantage is likely to be completed at the same time, all the joints, so that production costs to a minimum. However, it limits the temperature-sensitive components of the application of reflow soldering, both cartridge cases or SMD. Then people turn select welding. Most applications can be used in the selection after reflow soldering welding. This will become an economic and efficient completion of the remaining parts of the welding cartridge, but also in the future lead-free solder compatible.

Characteristics of selective soldering process
By comparison with the wave soldering process to understand the characteristics of selectivity. The most obvious difference between the two is the lower part of the wave soldering PCB completely immersed in liquid solder, and in the selective soldering, only some specific areas and touch the solder wave. As the PCB itself is a bad heat conduction medium, so it does not heat melt welding components and PCB area near the joints. Prior to soldering flux must be pre-coating. Compared with the wave soldering flux coating only the lower part of the PCB parts to be welded, rather than the entire PCB. Other selective soldering components only for plug welding. Selective soldering is a new approach, a thorough understanding of welding and selective soldering equipment is necessary for success.

Selective soldering process
Selective soldering process typically include: flux coating, PCB preheat, dip soldering and drag soldering.

Flux Coating Process
In selective soldering, the flux coating process plays an important role. Welding the end of heating and welding, flux activity should be sufficient to prevent the bridge formation and to prevent oxidation of PCB production. Spray flux from the X / Y PCB manipulator carrying the nozzle through the flux above the flux coating to the PCB to be soldered position. Flux with a single spray nozzle, micro jet, simultaneous multi-point / graphics spray a variety of ways. Peak reflow process after the election microwave welding, the most important thing is accurate spray flux. Micro jet solder joints will not soiled beyond the region. Micro-point spray flux point graphics diameter greater than the minimum 2mm, so spray deposition flux on the PCB location accuracy of ± 0.5mm, can guarantee the flux is always covered in the welded parts of the above, the tolerance dose of spray welding provided by the supplier, technical specifications should provide for the use of flux, usually 100% of the proposed safety tolerances.

Preheating process
In the selective soldering process is not the main purpose of preheating to reduce heat stress, but to remove the solvent pre-drying flux, before entering the solder wave so that the correct viscosity flux. In welding, preheat the heat brought on the welding quality is not the key factor, PCB thickness, device packaging specifications and the type of flux preheat temperature settings. In selective soldering, there are different theories on preheating explain: Some process engineers that the PCB flux spray should be carried out before preheating; another point of view directly without preheating welding. Users can be arranged according to the specific situation of selective soldering process.

Welding Technology
Selective soldering process, there are two different processes: drag welding process and dip soldering process.
Selective soldering process is dragging on a single small welding done on wave solder nozzle. Drag soldering process is suitable for PCB is very close in space to weld. For example: the individual solder joints or pin, single row pin welding process can be delayed. PCB at different speeds and angles of the solder in the solder wave at the mouth movement to achieve the best quality of welding. To ensure the stability of welding process, welding nozzle diameter of less than 6mm. Solder flow solution is determined, for different welding needs, welding nozzle installed and optimized for different directions. Manipulator can be from different directions, that is 0 ° ~ 12 ° between the different angles close to the solder wave, so the user can solder in the electronic components of various devices, for most devices, the proposed tilt angle is 10 °.
Compared with the dip soldering process, dragging welding process solder bath and PCB board sports, making the heat during welding efficiency better than the dip soldering process. However, the formation of weld required to connect the heat transfer from the solder wave, but a single wave solder nozzle of the solder mass is small, only the temperature of the solder wave is relatively high, to achieve the delay requirements of welding process. Example: Soldering temperature 275 ℃ ~ 300 ℃, drag speed 10mm / s ~ 25mm / s is usually acceptable. Nitrogen in the welding area to prevent oxidation of the solder wave, solder wave eliminates oxidation, making the bridge to avoid dragging welding defects in the production, the advantages of increased drag welding process stability and reliability.
Machines with high precision and high flexibility of features, the module structure design of the system in full accordance with customer specific production requirements to customize, and can be upgraded to meet future production development of Xu Qiu. Manipulator movement radius can cover flux nozzle, preheat and solder nozzle, thus the same machine can perform different welding processes. Machine-specific synchronization process can greatly reduce the veneer process cycle. Manipulator has the ability to make that choice with high precision and high quality welding welding characteristics. First manipulator highly stable precision positioning capability (± 0.05mm), ensuring each plate production parameters of a high degree of duplication of the same; followed by the manipulator 5-dimensional movement allows PCB to any optimization of the angle and direction contact with tin face, get the best good welding quality. Mechanical hand splint device installed tin wave height test needle is made from the titanium alloy, under program control can be regularly measured in the tin wave height, by adjusting the pump speed to control the tin tin wave height, in order to ensure process stability.
Despite so many advantages with the single wave solder nozzle welding process there are also less drag: spray welding time in flux, preheat and welding processes in the three longest. And because the drag of a solder joint is a weld, with the increase of welding points, welding time, a large increase in the welding efficiency can not be compared with the traditional wave soldering process. But the situation are changing, multi-welding nozzle design to maximize yield, for example, dual welding nozzle can double its production of the flux also can be designed dual nozzle.
Select welding system has a number immersion solder mouth, and with the PCB solder joint is one to one to be designed, although less flexible robot type, but the output is equivalent to the traditional wave soldering equipment, equipment cost and lower the relative robot-style. According to PCB size, can be transmitted in parallel veneer or boards and all joints had to be in parallel at the same time will be completed within the flux coating, preheat and soldering. However, the distribution of solder joints with different PCB, different, and therefore need to produce different specific solder PCB mouth. Welding nozzle size as large as possible to ensure the stability of welding process does not affect the PCB adjacent to the peripheral device, it is important to the design engineers say, and it is difficult, because the process of stability may depend on it.
Select welding processes using immersion can be welded 0.7mm ~ 10mm of solder joints, short pin and the small size of pads and more stable welding process, the bridge is also the possibility of small, adjacent to the edge of solder joints, components and solder the distance between the mouth should be larger than 5mm.