PCB Technical - Process characteristics of PCB selective soldering

In the soldering process of the PCB electronics industry, more and more circuit board soldering manufacturers have begun to focus on selective soldering. Selective soldering can complete all solder joints at the same time, minimizing production costs, and at the same time overcoming reflow. The problem of soldering affecting temperature-sensitive components, selective soldering can also be compatible with future lead-free soldering, these advantages have made the application of selective soldering wider and wider.

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 the lower part of the PCB is completely immersed in liquid solder in wave soldering, while in selective soldering, only part of the specific area is in contact with the solder wave. Because PCB itself is a kind of poor heat conduction medium, it will not heat and melt the solder joints of adjacent components and PCB area during welding. Flux must also be pre-applied before soldering. Compared with wave soldering, the flux is only coated on 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 process 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. When soldering heating and soldering ends, the flux should have sufficient activity to prevent bridging and prevent the PCB from oxidizing. The flux spraying is carried by the x/y manipulator and the PCB is passed over the flux nozzle, and the flux is sprayed onto the position of the PCB to be soldered. The flux has multiple methods such as single nozzle spray type, micro-hole spray type, 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 PCB 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 wavefront. During soldering, the influence of preheating heat 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 PCB should be preheated before flux spraying; another view is that preheating is not required and the soldering should be performed directly. The user can arrange the selective welding process according to the specific situation.

Selective soldering 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 a very tight space on the PCB board.

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 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. 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 the occurrence 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 modular structure design system can be fully customized according to the customer's special production requirements, and can be upgraded to meet the needs of future production development. The radius of motion of the manipulator can cover the flux nozzle, preheating and soldering nozzle, so the same equipment can complete different welding processes. The machine's unique synchronization process can greatly shorten the single board process cycle. The capabilities of the manipulator make this selective welding have the characteristics of high-precision and high-quality welding. The first is the highly stable and precise positioning capability of the manipulator (±0.05mm), which ensures that the parameters produced by each board are highly repeatable; secondly, the 5-dimensional movement of the manipulator enables the PCB to contact the tin surface at any optimized angle and orientation to obtain the best Good welding quality. The tin wave height stylus installed on the manipulator splint device is made of titanium alloy. The tin wave height can be measured regularly under program control. The tin wave height can be controlled by adjusting the tin pump speed to ensure process stability.

Despite all of the above advantages, the single-nozzle solder wave drag soldering process also has shortcomings: PCB soldering time is the longest among the three processes of flux spraying, preheating and soldering. And because the solder joints are dragged one by one, as the number of solder joints increases, the soldering time will increase significantly, and the welding efficiency cannot be compared with the traditional wave soldering process. However, the situation is changing. The design of multiple nozzles can maximize the output. For example, the use of dual welding nozzles can double the output, and the flux can also be designed as a double nozzle.