Introduction to Selective Welding Process in PCB Design

PCB manufacturers, PCB designers and PCBA manufacturers explain the selective welding process in PCB design

In PCB design and electronic industry welding process, more and more manufacturers begin to pay attention to selective welding. Selective welding can complete all welding points at the same time, minimizing production costs. At the same time, it overcomes the impact of reflow welding on temperature sensitive components. Selective welding can also be compatible with lead-free welding in the future. These advantages make selective welding more and more widely used.

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 the liquid solder in wave soldering, while in selective soldering, only some specific areas are in contact with the solder wave. Because PCB itself is a bad 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 coated before welding. Compared with wave soldering, the flux is only applied to the lower part of the PCB to be welded, not the entire PCB. In addition, selective welding is only applicable to the welding of plug-in components. Selective welding is a new method, and a thorough understanding of the selective welding process and equipment is necessary for successful welding.

PCB board

Flux coating process plays an important role in selective welding. During welding heating and at the end of welding, the flux shall have enough activity to prevent bridging and PCB oxidation. Flux spraying The X/Y manipulator carries the PCB through the upper part of the flux nozzle, and the flux is sprayed to the PCB to be welded position. Flux has a variety of ways, including single nozzle spray, micropore spray, and synchronous multi-point/pattern spray. The most important thing for microwave peak selection welding after reflow welding sequence is accurate spraying of flux. The micro hole spray type will never contaminate the area outside the solder joint. The minimum flux spot pattern diameter of micro spot spraying is greater than 2mm, so the position accuracy of the flux sprayed and deposited on the PCB is ± 0.5mm, which can ensure that the flux always covers the welded part. The tolerance of the amount of sprayed flux is provided by the supplier. The technical specification should specify the amount of flux used, and it is generally recommended that the 100% safety tolerance range.

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, so that the flux has the correct viscosity before entering the solder wavefront. During welding, the influence of the heat carried by preheating on the welding quality is not a key factor. The PCB material thickness, device package specification and flux type determine the preheating temperature setting. In selective welding, there are different theoretical explanations for preheating: some process engineers believe that PCB should be preheated before flux spraying; Another view is that welding is carried out directly without preheating. Users can arrange the process flow of selective welding according to specific conditions.

The selective drag welding process is completed on a single small tip solder wave. Drag welding process is suitable for welding in very close space on PCB. Compared with the immersion welding process, the solder solution of the drag welding process and the movement of the PCB board make the heat conversion efficiency in welding better than that of the immersion welding process. However, the heat needed to form the weld connection is transferred by the solder wave, but the quality of the solder wave of a single nozzle is small. Only the temperature of the solder wave is relatively high can it meet the requirements of the drag welding process.

The machine has the characteristics of high precision and high flexibility. The system designed with module structure 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 the manipulator can cover the flux nozzle, preheating and solder nozzle, so the same equipment can complete different welding processes. The unique synchronous manufacturing process of the machine can greatly shorten the single board manufacturing process cycle. The ability of the manipulator makes this selective welding have the characteristics of high precision and high quality welding. Firstly, the highly stable precise positioning capability of the manipulator ensures that the parameters produced by each plate are highly repeated and consistent; Secondly, the 5-dimensional motion of the manipulator enables the PCB to contact the tin surface at any optimized angle and orientation to obtain the best welding quality. The tin wave height measuring needle installed on the manipulator splint device is made of titanium alloy. Under the program control, the tin wave height can be measured regularly. The tin wave height can be controlled by adjusting the speed of the tin pump to ensure process stability.

Despite the above advantages, the single nozzle solder wave drag welding 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 welding time will increase significantly with the increase of the number of solder joints, and the welding efficiency cannot be compared with the traditional wave soldering process. However, the situation is changing. The design of multiple welding nozzles can maximize the output. For example, the use of double welding nozzles can double the output, and the flux can also be designed as double nozzles. PCB manufacturers, PCB designers and PCBA manufacturers will explain the selective welding process in PCB design.