Details of selective welding technology for printed circuit board (PCB)

Looking back on the development process of electronic industry in recent years, it can be noted that reflow soldering technology is an obvious trend. In principle, traditional inserts can also be reflow welded, which is commonly known as through-hole reflow welding. The advantage is that it is possible to complete all the welding points at the same time, so as to minimize the production cost. However, the application of reflow soldering is limited by the temperature sensitive element, whether it is the insert or SMD. Then people turn to choose welding. In most applications, selective welding can be used after reflow welding. This will be an economical and effective welding method for the remaining inserts, and will be fully compatible with future lead-free welding.

 Process characteristics of selective welding

 The process characteristics of selective welding can be understood by comparing with wave soldering. The most obvious difference between them is that the lower part of PCB is completely immersed in the liquid solder in the wave soldering, while in the selective soldering, only some specific areas contact with the solder wave. Because PCB itself is a kind of bad heat conduction medium, it will not heat and melt the solder joints of adjacent components and PCB area during welding. Flux must also be applied in advance before welding. Compared with wave soldering, flux is only applied to the lower part of PCB to be welded, not the whole PCB. In addition, selective welding is only applicable to the welding of plug-in components. Selective welding is a new method. It is necessary to know the selective welding process and equipment thoroughly.

 Selective welding process

 Typical selective welding processes include flux spraying, PCB preheating, immersion welding and drag welding.

 Flux coating process

 In selective welding, flux coating process plays an important role. At the end of welding heating and welding, the flux shall have enough activity to prevent bridging and PCB oxidation. The X / y manipulator carries the PCB to the top of the flux nozzle, and the flux is sprayed to the PCB to be welded position. Flux has many ways, such as single nozzle spray, micro hole injection, synchronous multi-point / graphic spray. The most important of the microwave peak selection welding after reflow welder sequence is the accurate spraying of flux. Microporous spray type will never contaminate the area outside the solder joint. The minimum solder dot pattern diameter of micro spot spraying is greater than 2mm, so the position accuracy of the solder deposited on the PCB is ± 0.5mm, which can ensure that the flux is always covered on the welded part. The tolerance of the amount of spray flux shall be provided by the supplier. The technical specification shall specify the amount of flux used, and it is generally recommended to have a 100% safety tolerance range.

 Preheating technology

 The main purpose of preheating in the selective welding process is not to reduce the thermal stress, but to remove the solvent pre drying flux. Before entering the solder wave, the flux has the correct viscosity. During welding, the influence of the heat brought by preheating on the welding quality is not the key factor. The thickness of PCB material, the package specification of device and the type of flux determine the setting of preheating temperature. In selective welding, there are different theoretical explanations for preheating: some process engineers think that PCB should be preheated before flux spraying; another view is that PCB should be directly welded without preheating. Users can arrange the process flow of selective welding according to the specific situation.

 welding technology

 There are two different processes of selective welding: drag welding and immersion welding.

 The selective drag welding process is completed on a single solder wave with a small nozzle. The drag welding process is suitable for welding in a very tight space on PCB. For example: individual solder joints or pins, single row of pins can carry out drag welding process. The PCB moves on the solder wave of the nozzle at different speed and angle to achieve the best welding quality. In order to ensure the stability of welding process, the inner diameter of welding nozzle is less than 6mm. After the flow direction of solder solution is determined, the welding nozzle is installed and optimized in different directions for different welding needs. The manipulator can approach the solder wave from different directions, i.e. from 0 ° to 12 ° at different angles, so the user can weld various devices on the electronic components. For most devices, the recommended tilt angle is 10 °.

 Compared with the dip welding process, the solder solution of the drag welding process and the movement of PCB board make the heat conversion efficiency better than that of the dip welding process. However, the heat needed to form the weld connection is transferred by the solder wave, but the solder wave quality of a single nozzle is small. Only when the solder wave temperature is relatively high, can the requirements of the drag welding process be achieved. For example, the temperature of soldering tin is 275 ℃ ~ 300 ℃, and the dragging speed is generally acceptable from 10 mm / s to 25 mm / s. Nitrogen is supplied in the welding area to prevent the oxidation of solder wave. The oxidation of solder wave is eliminated, which makes the drag welding process avoid the occurrence of bridging defects. This advantage increases the stability and reliability of the drag welding process.

 The machine has the characteristics of high precision and high flexibility. The system of module structure design can be customized completely according to the special production requirements of customers, and can be upgraded to meet the needs of future production development. The movement radius of manipulator can cover 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 manipulator makes this selective welding have the characteristics of high precision and high quality welding. Firstly, the manipulator has a highly stable and accurate positioning ability (± 0.05mm), which ensures the parameters of each board are highly repeated and consistent; secondly, the manipulator’s five-dimensional movement enables PCB to contact the tin surface at any optimized angle and orientation, so as to obtain the best welding quality. Installation on manipulator clamping device