Printed circuit board (PCB) selective soldering technology in detail

Reviewing the development of electronic industry in recent years, one of the obvious trends is the reflow soldering technology. In principle, conventional inserts can also be reflow processes, which are commonly referred to as through hole reflow soldering. The advantage is that it is possible to complete all solder joints at the same time and minimize production costs. However, the temperature sensitive component limits the application of reflow soldering, whether it is the plug-in or the SMD. Then people turned their attention to selective welding. In most applications, selective welding may be performed after reflow soldering. This will be the economical and effective way to complete the welding of the remaining inserts and be fully compatible with future lead-free soldering.


The process characteristics of selective welding can be understood by comparison with wave soldering. The most obvious difference is that the lower part of the PCB in the wave soldering is completely immersed in the liquid solder, whereas in selective soldering only some specific regions are in contact with solder wave. Since PCB itself is a poor heat transfer medium, it does not heat up the solder joints that fuse nearby components and PCB regions.


A typical process includes: selective soldering flux spraying, preheating, dip and drag welding PCB.


In selective welding, flux coating process plays an important role. At the end of the welding, heating and soldering, the flux shall be sufficiently active to prevent bridging and prevent oxidation of the PCB. Flux has a single mouth spray, micro jet, synchronous multi-point / graphic spray in a variety of ways. Microwave selective welding of reflow soldering sequence, and most important of all, accurate spraying of flux. But never with microbore type solder joint area.

The main purpose of preheating in selective soldering process is not to reduce thermal stress, but to remove the solvent pre drying flux into the solder wave front so that the flux has the proper viscosity. The effect of preheating on the welding quality is not the key factor in the welding process. The thickness of the PCB material, the package specification and the type of soldering flux decide the setting of the preheating temperature.


Selective welding process has two different processes: drag welding and dip process.


Selective solder process is accomplished on a single soldering nozzle wave. The welding process is suitable for welding in very tight spaces on the PCB. For example: individual solder joints or pins, single row pins can carry out the welding process. PCB moves at different speeds and angles on the solder wave of the solder head to achieve the best welding quality. In order to ensure the stability of welding process, the inner diameter of the welding nozzle is less than 6mm.


Compared with the soldering process, soldering welding process and PCB solution drag plate movement, the heat conversion efficiency when welding than soldering process. However, the heat required to form the weld joint is transmitted by Han Xibo, but the soldering wave quality of the single welding nozzle is small, and only the temperature of Han Xibo is relatively high, so as to meet the requirements of the welding process.


The machine has the characteristics of high precision and high flexibility. The system of modular structure design can be customized 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 soldering nozzle. Therefore, the same equipment can accomplish different welding processes. Machine specific synchronization process can greatly shorten the process cycle of single board.


 

(Source: internal information)

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