Despite the above-mentioned advantages, the single-nozzle solder wave drag soldering process also has its shortcomings. The soldering time is the longest among the three processes of flux spraying, preheating and soldering. And because the solder joints are dragged one by one, as the number of solder joints increases, the soldering time will increase significantly, and the welding efficiency cannot be compared with the traditional wave soldering process. But the situation is changing. The design of multiple welding nozzles can maximize the output. For example, the use of dual welding nozzles can double the output, and the flux can also be designed as double nozzles.
The immersion selective soldering system has multiple solder nozzles and is designed one-to-one with the PCB to be soldered. Although the flexibility is not as good as the robot type, the output is equivalent to the traditional wave soldering equipment, and the equipment cost is relatively low compared to the robot type. According to the size of the PCB, single board or multiple boards can be transferred in parallel, and all the points to be soldered will be sprayed, preheated and soldered in parallel at the same time. However, due to the different distribution of solder joints on different PCBs, special solder nozzles need to be made for different PCBs. The size of the soldering tip is as large as possible to ensure the stability of the soldering process without affecting the next components on the PCB. This is important and difficult for the design engineer, because the stability of the process may depend on it.
Using the immersion selective soldering process, solder joints of 0.7mm to 10mm can be soldered. The soldering process of short leads and small-sized pads is more stable, and the possibility of bridging is small. The distance between the edges of adjacent solder joints, devices and soldering tips should be more than 5mm. The main purpose of preheating in the selective soldering process is not to reduce thermal stress, but to remove the solvent and pre-dry the flux, so that the flux has the correct viscosity before entering the solder wave. During soldering, the influence of heat from preheating on soldering quality is not a key factor. PCB material thickness, device packaging specifications and flux type determine the setting of 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 the preheating is not required and the soldering should be performed directly. The user can arrange the selective welding process according to the specific situation.
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