PCB Blog - In-depth analysis of the difficulties of selective soldering of PCB board

In the soldering process of PCB board electronics industry, more and more manufacturers have begun to turn their attention to selective soldering. Selective soldering can complete all solder joints at the same time, reduce production costs, and overcome the temperature difference of reflow soldering. The impact of sensitive components, selective soldering is also compatible with future lead-free soldering, these advantages make selective soldering more and more widely used.

Process characteristics of selective soldering
The process characteristics of selective soldering can be understood by comparison with wave soldering. The obvious difference between the two is that in wave soldering, the lower part of the PCB is completely immersed in the liquid solder, while in selective soldering, only some specific areas are in contact with the solder wave. Since the PCB board itself is a poor thermal conduction medium, it will not heat and melt the solder joints in the adjacent components and PCB board area during soldering. Flux must also be pre-applied before soldering. Compared with wave soldering, the flux is only applied to the lower part of the PCB to be soldered, not the entire PCB. In addition, selective soldering is only suitable for soldering of plug-in components. Selective soldering is an entirely new approach, and a thorough understanding of the selective soldering process and equipment is necessary for successful soldering.

The process of selective soldering
Typical selective soldering processes include: flux spraying, PCB preheating, dip soldering and drag soldering.

Flux coating process
In selective soldering, the flux coating process plays an important role. At the end of soldering heat and soldering, the flux should be active enough to prevent bridging and prevent oxidation of the PCB board. Flux spraying is carried by the x/y manipulator to pass the PCB board through the flux nozzle, and the flux is sprayed onto the position to be soldered on the PCB board. Flux can be sprayed with single nozzle, micro-hole spray, synchronous multi-point/pattern spray. In the microwave peak selection after the reflow process, it is important to spray the flux accurately. Microbore jet will not stain areas other than solder joints. The diameter of the micro-sprayed flux point pattern is greater than 2mm, so the positional accuracy of the flux deposited on the PCB board is ±0.5mm to ensure that the flux always covers the welded part. The tolerance of the sprayed flux is provided by the supplier, and the technical specification should The amount of flux to be used is specified, and a 100% safety tolerance range is usually recommended.

Preheating process
The main purpose of preheating in a selective soldering process is not to reduce thermal stress, but to pre-dry the flux to remove the solvent, so that the flux has the correct viscosity before entering the solder wave. During soldering, the influence of the heat brought by preheating on the soldering quality is not a key factor. The thickness of the PCB board material, device packaging specifications and the type of flux determine the setting of the preheating temperature. In selective soldering, there are different theoretical explanations for preheating: some process engineers believe that the PCB board should be preheated before flux spraying; another point of view is that preheating is not required and soldering is performed directly. The user can arrange the process flow of selective welding according to the specific situation.

Welding process
There are two different processes for selective soldering: drag soldering and dip soldering. The selective drag soldering process is done on a single small tip solder wave. The drag soldering process is suitable for soldering in very tight spaces on the PCB board. For example: individual solder joints or pins, single row pins can be drag soldered. The soldering quality achieved by the PCB board moving on the solder wave of the soldering tip at different speeds and angles. In order to ensure the stability of the welding process, the inner diameter of the welding tip is less than 6mm. After the flow direction of the solder solution is determined, the soldering tips are installed and optimized in different directions for different soldering needs. The manipulator can approach the solder wave from different directions, that is, different angles between 0° and 12°, so the user can solder various devices on the electronic components. For most devices, the recommended tilt angle is 10°. Compared with the dip soldering process, the solder solution of the drag soldering process and the movement of the PCB board make the heat conversion efficiency during welding better than that of the dip soldering process. However, the heat required to form the solder joint is transferred by the solder wave, but the quality of the solder wave of a single solder tip is small, and only the temperature of the solder wave is relatively high, the requirements of the drag soldering process can be met. Example: Soldering temperature of 275 degree Celsius～300 degree Celsius and drag speed of 10mm/s～25mm/s are usually acceptable. Nitrogen is supplied in the welding area to prevent the oxidation of the solder wave. The solder wave eliminates the oxidation, so that the drag soldering process avoids the generation of bridging defects. This advantage increases the stability and reliability of the drag soldering process. The machine has the characteristics of high precision and high flexibility. The system of modular structure design can be completely 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 robot can cover the flux nozzle, preheat and solder nozzle, so the same equipment can complete different welding processes. The machine-specific synchronous process can greatly shorten the single-board process cycle. The capabilities of the manipulator give this selective welding the characteristics of high-precision and high-quality welding. The first is the highly stable positioning capability of the manipulator (±0.05mm), which ensures that the parameters produced by each board are highly repeatable and consistent; the second is the 5-dimensional movement of the manipulator, which enables the PCB board to contact the tin surface at any optimized angle and orientation to obtain welding. quality. The tin wave height stylus installed on the manipulator splint device 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 to ensure the process stability. Despite the above advantages, the single-nozzle solder wave drag soldering process also has shortcomings: the soldering time is long in the three processes of flux spraying, preheating and soldering. And because the solder joints are drag soldered one by one, as the number of solder joints increases, the soldering time will increase significantly, and the soldering efficiency cannot be compared with the traditional wave soldering process. But things are changing, and multiple nozzle designs can maximize throughput, for example, double nozzles can be used to double throughput, and flux can also be designed with dual nozzles on PCB board.