PCB Blog - Reasons and solutions of PCB board welding defects

Looking back at the development of the electronic PCB board industry process in recent years, we can notice a clear trend that is reflow soldering technology. In principle, traditional inserts 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, reducing production costs to a minimum. However, temperature-sensitive components limit the application of reflow soldering, whether it is plug-in or SMD. Then people turned their attention to selective welding. Selective soldering can be used after reflow soldering in most applications. This will be an economical and efficient way to solder the remaining inserts and is fully compatible with future lead-free soldering.

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 itself is a poor heat transfer medium, it does not heat and melt the solder joints in adjacent components and PCB areas when soldering. Flux must also be pre-applied before soldering. In contrast to 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 the soldering heat and soldering, the flux should be active enough to prevent bridging and prevent oxidation of the PCB. Flux spraying is carried by the X/Y manipulator to pass the PCB over the flux nozzle, and the flux is sprayed onto the PCB to be soldered. Flux has a single nozzle spray, 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 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 specify For flux usage, 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 material, device package specifications and flux type determine the preheating temperature setting. In selective soldering, there are different theoretical explanations for preheating: some process engineers believe that the PCB 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. For example: individual solder joints or pins, single row pins can be dragged. The quality of soldering achieved by the PCB moving on the solder wave of the solder 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 nozzles 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: The solder temperature is 275℃～300℃, and the drag speed is 10mm/s～25mm/s is 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 welding process avoids the generation of bridging defects. This advantage increases the stability and reliability of the drag welding 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 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 stability of the process. Despite the above-mentioned 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 since 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 multi-tip designs can maximize throughput. For example, using dual solder nozzles can double the output, and the flux can also be designed with dual nozzles.

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 lower than that of the robot type. Depending on the size of the PCB, single-board or multi-board parallel transfer is possible, and all the points to be soldered will be fluxed, preheated and soldered at the same time in parallel. However, due to the different distribution of solder joints on different PCB, 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 peripheral adjacent devices on the PCB. 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, solder joints of 0.7mm to 10mm can be soldered. The soldering process of short leads and small-sized pads is more stable, and the possibility of bridging is small. The distance between adjacent solder joint edges, devices and solder tips should be greater than 5mm on PCB board.