PCB Technical - PCB selective soldering technology details

Looking back on the development history of PCB technology in recent years, we can notice that an obvious trend is 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 the choice of welding. In most applications, selective soldering can be used after reflow soldering. This will become an economical and effective way to complete the soldering of the remaining plug-in parts, and it is fully compatible with future lead-free soldering.

Process characteristics of PCB 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 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. 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 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 robot (±0.05mm), which ensures the high repeatability of the parameters produced by each board; the second is the 5-dimensional movement of the robot so that the PCB can 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 the above advantages, the single-nozzle solder wave drag soldering process also has shortcomings: the welding time is the longest among the three processes of flux spraying, preheating and welding. 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 dual nozzle.

The immersion selective soldering system has multiple solder nozzles and is designed one-to-one with the PCB to be soldered. Although the flexibility is not as good as the robot type, the output is equivalent to the traditional wave soldering equipment, and the equipment cost is relatively low compared to the robot type. According to the size of the PCB, single board or multiple boards can be transferred in parallel, and all the points to be soldered will be sprayed, preheated and soldered in parallel at the same time. However, due to the different distribution of solder joints on different PCBs, special solder nozzles need to be made for different PCBs. The size of the soldering tip is as large as possible to ensure the stability of the soldering process without affecting the neighboring PCB components. This is important and difficult for the design engineer, because the stability of the process may depend on it.

Using the immersion selective soldering process, the solder joints of 0.7mm～10mm can be soldered. The soldering process of short pins and small-size pads is more stable, and the possibility of bridging is small. The distance between the edges of adjacent solder joints, devices and soldering tips should be More than 5mm.