Selective Welding Technology For Printed Circuit Board

- Jan 30, 2018-

In retrospect of the process of Technological Development in the electronic industry in recent years, it can be noted that a very obvious trend is the reflow welding technology. In principle, traditional inserts can also be used in reflow soldering, which is commonly referred to as through hole reflow. The advantage is that it is possible to finish all the solder joints at the same time so that the cost of production will be reduced to a minimum. However, the temperature sensitive element limits the application of reflow soldering, whether it is a cartridge or a SMD. Then people turned their eyes to the choice of welding. In most applications, selective welding can be used after reflow soldering. This will be an economical and effective method of welding residual inserts, and is fully compatible with future lead-free soldering.

Technological characteristics of selective welding

The characteristics of the selective welding process can be understood by comparing with the wave soldering. The most obvious difference between them is that the lower part of PCB in wave soldering is completely immersed in liquid solder, while in selective welding, only a few specific areas are contacted with solder wave. Since PCB itself is a bad heat conduction medium, it does not heat and melts solder joints of adjacent components and PCB regions during welding.  Before welding, it is necessary to apply the flux in advance. Compared with the wave soldering, the flux is only coated on the lower part of the PCB, not the whole PCB. In addition, the selective welding is only suitable for the welding of the intercalation element. Selective welding is a new method. It is necessary to thoroughly understand the selective welding process and equipment.

In selective welding, the coating process plays an important role. At the end of welding heat and welding, the flux should be sufficiently active to prevent the formation of bridging and prevent PCB from oxidizing. The flux spraying is carried by the X/Y manipulator with the PCB through the flux nozzle, and the flux is sprayed to the PCB pending position. The flux has a variety of ways of single mouth spray, microporous spray, synchronous multi point / graphic spray. The most important thing is to spray the flux accurately after the reflow welder preface the microwave peak soldering. But never with microbore type solder joint area. Micro spray flux minimum point graphics diameter greater than 2mm, so the spray flux position accuracy of deposition of PCB on + 0.5mm, in order to ensure the flux always covers on the welding part above the spray flux tolerance provided by the supplier, technical specifications shall be prescribed flux usage, usually recommended safe tolerance range 100%.

Preheating process

The main purpose of preheating in selective welding process is not to reduce thermal stress, but to remove solvent pre drying flux, and to get the proper viscosity of the flux before entering the solder wave. During welding, the influence of preheating heat on welding quality is not a key factor. PCB material thickness, device package specification and type of flux decide the setting of preheating temperature. In selective welding, there are different theoretical explanations for preheating. Some process engineers believe that PCB should be preheated before spraying, and the other view is that direct welding is not necessary. The user can arrange the process of the selective welding according to the specific circumstances.

The machine has the characteristics of high accuracy and high flexibility. The module structure design system can be customized according to the special production requirements of customers, and it can be upgraded to meet the needs of future production development. The motion radius of the manipulator can cover the flux nozzles, preheating and solder joints, so the same equipment can perform different welding processes. The machine - specific synchronization process can greatly shorten the period of the single plate process. The ability of the manipulator to make this choice of welding with high precision and high quality welding characteristics. The first is the highly accurate positioning ability of manipulator (+ 0.05mm), which ensures the highly consistent and consistent parameters of every plate production. Next is the 5 dimensional movement of the manipulator, enabling PCB to contact the tin surface with any optimized angle and orientation to get the best welding quality. The tin wave height measuring needle installed on the splint device of the manipulator is made of titanium alloy. Under the program control, the tin wave height can be measured regularly, and the tin wave height can be controlled by adjusting the speed of the tin pump, so as to ensure the stability of the process.

Despite all the above advantages, the single nozzle soldering process is also insufficient: the welding time is the longest in the three processes of flux spraying, preheating and welding. And because the solder joint is a trailing welding, with the increase of the number of solder joints, the welding time will increase significantly, and the welding efficiency can not be compared with the traditional wave soldering process. But the situation is changing. The design of multi nozzle can maximize the output. For example, double welding nozzles can double the output, and the flux can also be designed as two nozzles.

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