What is selective welding? Introduction to technical features and process flow

Date：2022-02-24 17:21:00Views：1519

What is selective welding? Selective welding is one of the processes used in the manufacture of various electronic components (usually circuit boards). Typically, the process involves welding specific electronic components to a printed circuit board without affecting other areas of the circuit board. This is different from various reflow processes, exposing the entire circuit board to molten solder. In fact, selective welding can refer to any welding method, from manual welding to special welding equipment, as long as the method is accurate enough to use solder only in the required area.

1、 Process characteristics

Compared with wave soldering, the most obvious difference between the two is that in wave soldering, the lower part of PCB is completely immersed in liquid solder, while in selective soldering, only some specific areas are in contact with solder wave. For selective welding, flux must also be applied in advance before welding; Compared with wave soldering, its flux is only applied to the part to be welded at the lower part of the PCB, not the whole PCB. In addition, selective welding is only applicable to the welding of plug-in components.

2、 Technological process

Selective welding process flow: flux spraying, PCB preheating, immersion welding and drag welding.

1. Flux coating process

In selective welding, flux coating process plays an important role. At the end of welding heating and welding, the flux shall have sufficient activity to prevent bridging and PCB oxidation. The flux spraying is carried by the X / y manipulator, and the PCB passes above the flux nozzle, and the flux is sprayed on the position of the PCB to be welded. Flux has many ways, such as single nozzle spray, micro hole injection, synchronous multi-point / graphic spray.

For microwave peak selective welding after reflow welding sequence, the most important thing is the accurate spraying of flux. Microporous jet will never stain the area outside the solder joint. The minimum flux dot pattern diameter of micro dot spraying is greater than 2mm, so the position accuracy of flux sprayed and deposited on PCB is ± 0.5mm, so as to ensure that the flux always covers the welded part.

2. Preheating process

The main purpose of preheating is not to reduce thermal stress, but to remove the solvent, pre dry the flux and make the flux have the correct viscosity before entering the solder wave. During welding, PCB material thickness, device packaging specification and flux type determine the setting of preheating temperature.

3. Welding process

There are two kinds of selective welding processes: drag welding process and immersion welding process.

The drag welding process is completed on the solder wave of a single small nozzle, which is suitable for welding in a very tight space on PCB. During welding, PCB moves on the solder wave of the welding nozzle at different speeds and angles to achieve the best welding quality.

In order to ensure the stability of the welding process, the inner diameter of the welding nozzle is less than 6mm. After the flow direction of solder solution is determined, the welding nozzle is installed and optimized in different directions for different welding needs.

Compared with immersion welding process, the solder solution of drag welding process and the movement of PCB board make the heat conversion efficiency better than immersion welding process. However, the heat required to form the weld connection is transferred by the solder wave, but the quality of the solder wave of a single nozzle is small. Only when the temperature of the solder wave is relatively high can it meet the requirements of the drag welding process. Nitrogen is supplied in the welding area to prevent solder wave oxidation and make the drag welding process avoid bridging defects. This advantage increases the stability and reliability of the drag welding process.

Single nozzle solder wave drag welding process also has shortcomings: the welding time is the longest in the three processes of flux spraying, preheating and welding. And because the solder joints are dragged one by one, the welding efficiency can not be compared with the traditional wave soldering process. However, the situation is changing. The multi nozzle design can maximize the output.

In a word, selective welding can complete all solder joints at the same time, minimize the production cost, and overcome the impact of reflow welding on temperature sensitive components. Selective welding can also be compatible with future lead-free welding. These advantages make the application range of selective welding more and more extensive.