PCB Technical - Process characteristics of PCB circuit class

PCB circuit board industry process development process, an obvious trend in reflow soldering technology. In principle, traditional plug-in parts can also be reflow soldered, which is commonly referred to as through-hole reflow soldering. The advantage is that it is possible to complete all solder joints at the same time, minimizing production costs. However, temperature-sensitive components limit the application of reflow soldering, whether it is an interposer or SMD. Then people turned their attention to selective soldering. In most applications, selective soldering can be used after reflow soldering. This will become an economical and effective way to complete the remaining plug-in parts, and it is fully compatible with future lead-free soldering.

Process characteristics of selective soldering

The process characteristics of selective soldering can be understood by comparing with wave soldering. The most obvious difference between the two is that in wave soldering, the lower part of the PCB is completely immersed in liquid solder, while in selective soldering, only some specific areas are in contact with the solder wave. Since the PCB itself is a poor heat conduction medium, it will not heat and melt the solder joints of adjacent components and the PCB 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, rather than the entire PCB. In addition, selective soldering is only applicable to the soldering of plug-in components. Selective welding is a brand new method. A thorough understanding of selective welding processes and equipment is necessary for successful welding.

Selective soldering process

The typical selective soldering process includes: 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 heating and soldering, the flux should have sufficient activity to prevent bridging and prevent PCB oxidation. Flux spraying is carried by the X/Y manipulator to carry the PCB through the flux nozzle, and the flux is sprayed onto the PCB to be soldered. The flux has multiple methods such as single nozzle spray, micro-hole spray, and synchronous multi-point/pattern spray. The most important thing for microwave peak selective soldering after the reflow soldering process is the accurate spraying of the flux. The micro-hole jet will never contaminate the area outside the solder joints. The minimum flux point pattern diameter of micro-point spraying is greater than 2mm, so the position accuracy of the flux deposited on the PCB is ±0.5mm to ensure that the flux is always covered on the welded part. The spray flux tolerance is provided by the supplier, and the technical specification should To specify the amount of flux used, a 100% safety tolerance range is usually recommended.

Preheating process

The main purpose of preheating in the selective soldering process is not to reduce thermal stress, but to remove the solvent and pre-dry the flux, so that the flux has the correct viscosity before entering the solder wave. During soldering, the influence of the heat from preheating on soldering quality is not a key factor. PCB material thickness, device packaging specifications and flux type determine the setting of preheating temperature. In selective soldering, there are different theoretical explanations for preheating: some process engineers believe that the PCB should be preheated before the flux is sprayed; another view is that the preheating is not required and the soldering is directly performed. The user can arrange the selective welding process 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 completed on a single small soldering tip solder wave. The drag soldering process is suitable for soldering in very tight spaces on the PCB. For example: individual solder joints or pins, single-row pins can be drag soldered. The PCB moves on the solder wave of the soldering tip at different speeds and angles to achieve the best soldering quality. 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, at different angles between 0° and 12°, so users can solder various devices on 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 soldering better than that of the dip soldering process. However, the heat required to form the weld connection is transferred by the solder wave, but the solder wave quality of a single solder tip is small, and only the relatively high temperature of the solder wave can meet the requirements of the drag soldering process. Example: The solder temperature is 275 degree Celsius～300 degree Celsius, and the pulling speed is 10mm/s～25mm/s usually acceptable. Nitrogen is supplied in the welding area to prevent the solder wave from oxidizing. The solder wave eliminates the oxidation, so that the drag soldering process avoids bridging defects. This advantage increases the stability and reliability of the drag soldering process.