During the development of circuit boards, an obvious trend is reflow soldering technology. Basically, the traditional plug-in parts can also use reflow soldering process, which is generally referred to as through-hole reflow soldering. The advantage is to complete all solder joints at the same time, minimizing production costs. However, temperature-sensitive components limit the use of reflow soldering, whether it is an interposer or an SMD. People have turned their attention to welding. In many applications, selective soldering is used after reflow soldering. This is a very effective way.
Selective welding process characteristics:
The process characteristics of selective soldering can be understood by comparing with wave soldering. The most obvious difference between the two is that in wave soldering, the lower part of the PCB is completely immersed in liquid solder, while in selective soldering, only some specific areas are in contact with the solder wave. Since the PCB is a poor heat conduction medium, it will not heat and melt the solder joints of adjacent components and the PCB area during soldering. Flux must also be pre-applied before soldering. Compared with wave soldering, the flux is only applied to the lower part of the PCB to be soldered, rather than the entire PCB. In addition, selective soldering is only applicable to the soldering of plug-in components. Selective welding is a brand new method. A thorough understanding of selective welding processes and equipment is necessary for successful welding.
Selective welding process：
The typical selective soldering process includes: flux spraying, PCB preheating, dip soldering and drag soldering.
Preheating process and flux coating process:
In selective soldering, the flux coating process plays an important role. When the soldering heating and soldering are finished, the flux should have sufficient activity to prevent the generation of bridges and prevent the PCB from oxidizing. Flux spraying is carried by the X/Y manipulator to carry the PCB through the flux nozzle, and the flux is sprayed onto the PCB to be soldered. Flux has multiple methods such as single nozzle spray, micro-hole spray, and synchronous multi-point/pattern spray. The most important thing for microwave peak selective soldering after the reflow soldering process is the accurate spraying of the flux. The micro-hole jet will never contaminate the area outside the solder joints. The minimum flux point pattern diameter of micro-point spraying is greater than 2mm, so the position accuracy of the flux deposited on the PCB is ±0.5mm to ensure that the flux is always covered on the welded part. The spray flux tolerance is provided by the supplier, and the technical specification should To specify the amount of flux used, a 100% safety tolerance range is usually recommended.
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