Analysis of Difficulties in PCB Selective Soldering Process-EEWORLD

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In the PCB electronic industry welding process, more and more manufacturers are beginning to turn their attention to selective welding. Selective welding can complete all solder joints at the same time, so as to minimize production costs. At the same time, it overcomes the problem of reflow soldering affecting temperature-sensitive components. Selective welding can also be compatible with future lead-free soldering. These advantages make the application scope of selective welding wider and wider. The process
    characteristics
    of selective welding can be understood by comparing with wave soldering. The most obvious difference between the two is that the lower part of the PCB is completely immersed in liquid solder in wave soldering, while in selective soldering, only some specific areas are in contact with the solder wave. Since the PCB itself is a poor heat conduction medium, it will not heat and melt the solder joints of adjacent components and PCB areas during welding. Flux must also be pre-applied before welding. Compared with wave soldering, flux is only applied to the lower part of the PCB to be welded, not the entire PCB. In addition, selective soldering is only applicable to the welding of plug-in components. Selective soldering is a new method, and a thorough understanding of the selective soldering process and equipment is necessary for successful welding.      The typical selective soldering process includes: flux spraying, PCB preheating, dip soldering and drag soldering.
     Flux      coating process      In selective soldering, the flux coating process plays an important role. At the end of soldering heating and soldering, the flux should be active enough to prevent the formation of bridges and prevent the PCB from oxidation. The flux spraying is carried by the X/Y manipulator over the flux nozzle, and the flux is sprayed on the PCB to be soldered. The flux has multiple modes such as single nozzle spray, micro-hole spray, and synchronous multi-point/graphic spray. The most important thing for microwave peak selective soldering after the reflow process is accurate flux spraying. The micro-hole spray type will never contaminate the area outside the solder joint. The minimum flux dot pattern diameter of micro-point spraying is greater than 2mm, so the flux position accuracy of the sprayed deposited on the PCB is ±0.5mm to ensure that the flux always covers the soldered part. The tolerance of the sprayed flux amount is provided by the supplier. The technical specification should specify the amount of flux used, and a 100% safety tolerance range is usually recommended.     Preheating process     The main purpose of preheating in the selective soldering process is not to reduce thermal stress, but to remove solvents and pre-dry the flux so that the flux has the correct viscosity before entering the solder wave. During soldering, the effect of the heat brought by preheating on the soldering quality is not a key factor. The thickness of the PCB material, the device packaging specifications and the type of flux determine the setting of the preheating temperature. In selective soldering, there are different theoretical explanations for preheating: some process engineers believe that the PCB should be preheated before the flux is sprayed; another view is that preheating is not required and soldering can be carried out directly. Users can arrange the process flow of selective soldering according to specific circumstances.     Soldering process     There are two different processes in the selective soldering process: drag soldering and dip soldering.     The selective drag soldering process is completed on a single small solder nozzle solder wave. The drag soldering process is suitable for soldering in very tight spaces on the PCB. For example: individual solder joints or pins, single-row pins can be drag soldered. The PCB moves on the solder wave of the solder nozzle at different speeds and angles to achieve the best soldering quality. To ensure the stability of the soldering process, the inner diameter of the solder nozzle is less than 6mm. After the flow direction of the solder solution is determined, the solder nozzle is installed and optimized in different directions for different welding needs. The manipulator can approach the solder wave from different directions, that is, at different angles between 0° and 12°, so that users can solder various devices on electronic components. For most devices, the recommended tilt angle is 10°.      Compared with the dip soldering process, the movement of the solder solution and the PCB board in the drag soldering process makes the heat conversion efficiency during soldering better than that of the dip soldering process. However, the heat required to form the weld connection is transferred by the solder wave, but the mass of the solder wave of a single solder nozzle is small. Only when the temperature of the solder wave is relatively high can the requirements of the drag soldering process be met. Example: The solder temperature is 275℃～300℃, and the drag speed is usually acceptable at 10mm/s～25mm/s. Nitrogen is supplied in the welding area to prevent the oxidation of the solder wave. The solder wave eliminates oxidation, so that the drag soldering process avoids the generation of bridging defects. This advantage increases the stability and reliability of the drag soldering process.      The machine has the characteristics of high precision and high flexibility. The modular structure design system can be customized according to the special production requirements of customers and can be upgraded to meet the needs of future production development. The movement radius of the manipulator can cover the flux nozzle, preheating and solder nozzle, so the same equipment can complete different welding processes. The unique synchronous process of the machine can greatly shorten the single board process cycle. The ability of the manipulator makes this selective welding have the characteristics of high precision and high quality welding. First, the manipulator's highly stable and precise positioning ability (±0.05mm) ensures that the parameters of each board production are highly repetitive; second, the manipulator's 5-dimensional movement enables the PCB to contact the tin surface at any optimized angle and orientation to obtain the best welding quality. The tin wave height probe installed on the manipulator clamping device is made of titanium alloy. Under program control, the tin wave height can be measured regularly, and the tin wave height can be controlled by adjusting the tin pump speed to ensure process stability.      Despite the above advantages, the single nozzle solder wave drag soldering process also has shortcomings: the welding time is the longest among the three processes of flux spraying, preheating and welding. And because the solder joints are dragged one by one, the soldering time will increase significantly as the number of solder joints increases, and the soldering efficiency cannot be compared with the traditional wave soldering process. But the situation is changing, and the multi-nozzle design can maximize the output. For example, the use of double soldering nozzles can double the output, and the flux can also be designed with double nozzles.

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