Deep Analysis of Difficulties in PCB Selective Welding

Deep Analysis of Difficulties in PCB Selective Welding
In the PCB electronic industry, more and more manufacturers begin to turn their attention to selective welding Selective welding can complete all welding points at the same time, reduce production costs, and overcome the temperature difference of reflow welding Due to the influence of sensitive components, selective welding is also compatible with lead-free welding in the future. These advantages make selective welding more and more widely used
PCB board

Process characteristics of selective soldering
The process characteristics of selective soldering can be understood by comparison with wave soldering.  The obvious difference between the two is wave soldering. The lower part of the printed circuit board is completely immersed in the liquid solder. During selective soldering, only some specific areas are in contact with the welding wave Since PCB itself is a medium with poor thermal conductivity, it will not heat and melt the area between adjacent components and PCB during welding The flux must also be pre coated before welding Compared with wave soldering, the flux is only applied to the lower part of the PCB to be soldered, not the entire PCB In addition, selective welding is only applicable to the welding of plug-in components Selective welding is a new method. Thorough understanding of selective welding process and equipment is a necessary condition for successful welding
The process of selective soldering
Typical selected soldering processes include: flux spraying, PCB preheating, dip soldering and drag soldering
Flux coating process
Flux coating plays an important role in selective welding During welding heat and at the end of welding, the flux shall have sufficient activity to prevent bridging and oxidation of PCB Flux spraying The x/y manipulator sprays the flux on the PCB board at the position to be welded through the flux nozzle through the PCB board The flux can be sprayed with a single nozzle, micro hole spray, and synchronous multi-point/pattern spraying In the selection of microwave peak value after reflow, it is important to spray flux accurately Microjet will not contaminate areas outside the solder joint The diameter of the micro spraying flux dot pattern is greater than 2mm. In addition, the position accuracy of the magnetic flux deposited on the surface is ± 0.5mm on the PCB to ensure that the flux always covers the welding part The tolerance of spraying flux shall be provided by the supplier. If the amount of flux to be used is specified, the technical specification shall be provided, and a safety tolerance range of 100% is generally recommended
Preheating process
The main purpose of preheating in a selective soldering process is not to reduce thermal stress During welding, the influence of heat brought by preheating on welding quality is not a key factor The thickness of PCB cloth, equipment package specification and flux type determine the preheating temperature setting In selective soldering, there are different theoretical explanations for preheating: some process engineers think that PCB should be preheated before flux spraying; Another point of view is to weld directly without preheating Users can arrange the process flow of selective welding according to specific conditions
Welding process
There are two different processes for selective soldering: drag soldering and dip soldering.  The selective drag welding process is completed on a single small tip welding wave Drag welding process is applicable to PCB welding in a very narrow space on the machine For example: single solder joint or pin, single row of pins can be soldered The welding quality is moved on the solder wave of the solder head by the PCB at different speeds and angles To ensure the stability of the welding process, the inner diameter of the welding head is less than 6mm After determining the flow direction of the solder solution, the welding joints are installed and optimized in different directions to meet different welding requirements The manipulator can approach the welding wave from different directions, that is, the angle between 0 ° and 12 ° is different. After all, users can weld various devices on electronic components For most equipment, the recommended tilt angle is 10 ° Compared with the immersion welding process, the solder solution and PCB board in the drag welding process make the heat conversion efficiency in the welding process better than that in the immersion welding process However, the heat required to form the solder joint is transferred through the welding wave, but the quality of the welding wave of a single solder joint is very small, only the temperature of the welding wave is relatively high, which can meet the requirements of the drag welding process Example: the welding temperature is 275 ℃, and the dragging speed is 10mm/s è ½ž 25mm/s is generally acceptable Nitrogen is provided in the welding area to prevent oxidation of the welding wave Welding wave eliminates oxidation, and drag welding process avoids bridging defects This advantage improves the stability and reliability of the drag welding process The machine is characterized by high accuracy and high flexibility The modular structure design system can be completely customized according to the special Production requirements of customers, and can be upgraded to meet the needs of future production development The moving radius of the robot can cover the flux nozzle, preheating and solder nozzle, and the same equipment can complete different welding processes The machine specific synchronization process can greatly shorten the board processing cycle The ability of the manipulator makes this selective welding have the characteristics of high precision and high quality welding The first is the highly stable positioning capability of the manipulator (± 0.05mm), which ensures that the parameters generated by each board are highly repeatable and consistent; The second is the 5-dimensional motion of the manipulator, which makes the PCB contact the tin surface at any optimized angle and direction to obtain welding Quality The tin wave high contact pin installed on the manipulator clamping plate device is made of titanium alloy Under the program control, tin wave height can be measured regularly. The tin wave height can be controlled by adjusting the speed of the tin pump to ensure the stability of the process Despite the above advantages, the single nozzle wave resistance welding process also has disadvantages: in the three processes of flux spraying, the welding time is long, preheating and welding Because the solder joints are dragged one by one, with the increase of the number of solder joints, the welding time will increase significantly, and the welding efficiency cannot be compared with the traditional wave soldering process However, the situation is changing. The multi nozzle design can maximize the delivery capacity. For example, the dual nozzle can be used to double the delivery capacity, and the flow can also be designed as a dual nozzle PCB