Understanding the characteristics of selective soldering can be gleaned through a comparison with wave soldering. The most significant difference between the two lies in how the PCB is treated during soldering. In wave soldering, the bottom part of the PCB is entirely immersed in liquid solder, whereas in selective soldering, only specific areas of the PCB come into contact with the solder wave. Since PCBs are poor conductors of heat, they do not heat up to melt nearby components or solder points during selective soldering. Moreover, the application of flux is necessary before soldering begins. In contrast to wave soldering, where flux is applied uniformly, in selective soldering, flux is applied only to the areas where soldering is required, not to the entire PCB. Additionally, selective soldering is only applicable to through-hole components soldering. Understanding the selective soldering process and equipment thoroughly is essential for successful soldering. Typical steps in selective soldering include flux application, PCB preheating, soldering, and drag soldering.
Flux Application Process
The flux application process plays a crucial role in selective soldering. During heating and soldering, the flux must remain sufficiently active to prevent bridging and prevent oxidation of the PCB. Flux application is performed by an XY manipulator carrying the PCB through a flux nozzle, which applies flux to the desired soldering positions. Flux application methods include single-nozzle spray, micro-hole jetting, and synchronous multi-point/graphics spraying.
Preheating Process
In the selective soldering process, preheating is primarily aimed at removing solvent and pre-drying the flux to ensure proper viscosity before entering the solder wave. The heat provided during soldering is not a critical factor affecting soldering quality. The preheating temperature is determined by the PCB material thickness, device package size, and flux type. There are different theories regarding preheating in selective soldering: some process engineers believe preheating should be done before flux application, while others argue that preheating is unnecessary, and soldering can proceed directly. Users can arrange the selective soldering process flow according to specific circumstances.
Soldering Process
There are two different processes in selective soldering: drag soldering and dip soldering. Selective drag soldering is performed on a single small solder nozzle. It is suitable for soldering in very tight spaces on the PCB, such as individual solder points or pins, and single-row pins can be drag-soldered. The PCB moves at different speeds and angles over the solder nozzle to achieve optimal soldering quality. To ensure process stability, the inner diameter of the solder nozzle should be less than 6mm. Different nozzle directions are installed and optimized for different soldering requirements after determining the flow direction of the solder solution. The manipulator can approach the solder wave from different directions, usually at angles of 0° to 12°, allowing users to solder various devices on electronic components. For most devices, a tilt angle of 10° is recommended.
Compared with dip soldering, the movement of the solder solution and PCB in drag soldering results in better heat transfer efficiency during soldering. However, the heat required to form solder joints is transferred by the solder wave. Since the quality of the solder wave from a single solder nozzle is limited, only relatively high temperatures of the solder wave can meet the requirements of drag soldering. For example, solder temperatures ranging from 275°C to 300°C and drag speeds of 10mm/s to 25mm/s are generally acceptable. Nitrogen is supplied to the soldering area to prevent oxidation of the solder wave, and the solder wave removes oxide, preventing bridging defects and increasing the stability and reliability of the drag soldering process.