Selective soldering technology for printed circuit board (PCB)

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


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 in the wave soldering is completely immersed in liquid solder, while in selective welding only partial regions are in contact with solder wave. Compared with wave soldering, the flux is only coated on the lower part of the PCB, but not the whole PCB. In addition, selective soldering is only suitable for soldering of cartridge elements. Selective soldering is a completely new method, and thorough understanding of selective soldering process and equipment is essential for successful welding.


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


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


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


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. Only the temperature of Han Xibo is relatively high, can the requirements of the welding process be satisfied.


The machine has the characteristics of high precision and high flexibility, and the module structure design system can be customized according to the special production requirements of customers, and can be upgraded to meet the needs of future production development. The machine specific synchronization process can greatly shorten the single board process cycle. The ability of the manipulator to make this selective welding has the characteristics of high precision and high quality welding. The tin wave height measuring needle installed on the clamping device of the manipulator is made of titanium alloy. The tin wave height can be regularly measured under the program control, and the tin wave height can be controlled by regulating the speed of the tin pump, so as to ensure the process stability.


Despite the above advantages, the single nozzle solder wave soldering process is also inadequate: welding time is the longest in the three processes of flux spraying, preheating and welding. And because the solder joint is one of the drag welding, with the increase of the number of solder joints, the welding time will increase greatly, and the welding efficiency can not be compared with the traditional wave soldering process. But the situation is changing, multi nozzle design can maximize the yield, for example, the use of double welded nozzles can increase production doubled, the flux can also be designed double nozzles.


 

(Source: internal information)

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