In the course of the development of PCB, an obvious trend is reflow soldering technology. Basically, traditional plug-in parts can also use reflow soldering process, which is commonly known as through-hole reflow soldering. The advantage is that all the solder joints are completed at the same time, keeping the production cost to a minimum. However, temperature sensitive elements limit the application of reflow soldering, whether it is plug-in or SMD. People turn their attention to welding. In many applications, selective welding is used after reflow soldering. This is a very effective way.
Process characteristics of selective welding
The process characteristics of selective welding can be understood by comparing with wave soldering. The most obvious difference between the two is that the lower part of PCB in wave soldering is completely immersed in liquid solder, while in selective soldering, only some specific areas are in contact with solder waves. Because PCB itself is a poor heat conduction medium, it does not heat and melt solder joints in adjacent components and PCB areas during welding. Flux must also be applied beforehand before welding. Compared with wave soldering, the flux is only applied on the lower part of the PCB to be welded, not the whole PCB. In addition, selective welding is only suitable for the welding of plug-in components. Selective welding is a new method, and a thorough understanding of selective welding process and equipment is necessary for successful welding.
Selective welding process
Typical selective welding processes include flux spraying, PCB preheating, immersion welding and drag welding.
Introduction of process characteristics and process flow of selective welding of PCB circuit board
Preheating process
Flux coating process
Flux coating process plays an important role in selective welding. At the end of welding heating and welding, the flux should be active enough to prevent bridging and oxidation of PCB. Flux spraying is carried by the Xamp Y manipulator with PCB through the top of the flux nozzle, and the flux is sprayed to the PCB position to be welded. Flux has many ways of single-nozzle spray, micro-hole spray and synchronous multi-point / pattern spray. The most important thing for microwave peak selection after reflow soldering is accurate spraying of flux. The microporous spray will never contaminate the area outside the solder joint. The minimum flux pattern diameter of micro-spot spraying is larger than that of 2mm, so the position accuracy of flux deposited on PCB is ±0.5mm in order to ensure that the flux is always covered on the welded part, the tolerance of spray welding dose is provided by the supplier, and the flux usage should be specified in the technical specification.
Welding process
There are two different selective welding processes: drag welding and immersion welding.
The selective towing process is completed on the solder wave of a single small nozzle. The towing process is suitable for welding in a very tight space on the PCB. For example: individual solder joints or pins, single-row pins can be dragged soldering process. PCB moves on the solder wave of the nozzle at different speeds and angles to achieve the best welding quality. In order to ensure the stability of the welding process, the inner diameter of the welding nozzle is less than 6mm. After the flow direction of the solder solution is determined, the welding nozzles are installed and optimized in different directions for different welding needs. The manipulator can approach the solder wave from different directions, that is, from 0 °to 12 °, so the user can weld various devices on the electronic assembly. for most devices, the tilt angle is recommended to be 10 °.
Compared with the immersion welding process, the solder solution of the drag welding process and the movement of the PCB plate make the heat transfer efficiency better than that of the immersion welding process. However, the heat needed to form the weld joint is transferred by the solder wave, but the solder wave quality of a single nozzle is small, and only the temperature of the solder wave is relatively high, can it meet the requirements of the towing process. Example: the solder temperature is 275 ℃ ~ 300 ℃, and the pulling speed 10mm/s~25mm/s is usually acceptable. Nitrogen is supplied in the welding area to prevent solder wave oxidation, which eliminates oxidation and makes the towing process avoid bridging defects, which increases the stability and reliability of the towing process.
The machine has the characteristics of high precision and high flexibility, the system designed by the module structure can be completely customized according to the customer's special production requirements, and can be upgraded to meet the needs of future production development. The motion radius of the manipulator can cover flux nozzles, preheating and solder nozzles, so the same equipment can complete different welding processes. The unique synchronous process of the machine can greatly shorten the veneer process cycle. The ability of the manipulator makes this selective welding have the characteristics of high precision and high quality welding. The first is the highly stable and accurate positioning ability of the manipulator (±0.05 mm), which ensures that the production parameters of each plate are highly repeated and consistent; 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 welding quality. The tin wave height measuring needle installed on the manipulator splint device is made of titanium alloy. The tin wave height can be measured regularly under program control, 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 advantages, the single-nozzle solder wave towing welding process also has shortcomings: the welding time is the longest in the three processes of flux spraying, preheating and welding. And because the solder joint is a drag welding, with the increase of the number of solder joints, the welding time will increase greatly, so the welding efficiency can not be compared with the traditional wave soldering process. However, the situation is changing, and the design of multi-welding nozzles can maximize the output, for example, the use of double welding nozzles can double the output, and the flux can also be designed as double nozzles.
There are several solder nozzles in the immersion selective welding system, and the solder joints to be soldered with PCB are designed one to one. Although the flexibility is not as flexible as the robot hand type, the output is equivalent to the traditional wave soldering equipment, and the cost of the equipment is lower than that of the robot hand type. According to the size of the PCB, single-board or multi-board parallel transmission can be carried out, and all solder joints will be completed flux spraying, preheating and welding in parallel at the same time. However, due to the different distribution of solder joints on different PCB, it is necessary to make special solder nozzles for different PCB. The size of the welding nozzle is as large as possible to ensure the stability of the welding process without affecting the surrounding adjacent devices on the PCB, which is important and difficult for the design engineer, because the stability of the process may depend on it.
By using the immersion selective welding process, the solder joint of 0.7mm~10mm, the welding process of short pin and small size pad is more stable, the possibility of bridging is less, and the distance between the edge of adjacent solder joint, device and nozzle should be greater than 5mm. The main purpose of preheating in selective welding process is not to reduce thermal stress, but to remove solvent pre-drying flux, so that the flux has the correct viscosity before entering the solder wave. During welding, the influence of preheating heat on welding quality is not a key factor. The preheating temperature is determined by the thickness of PCB material, the specification of device packaging and the type of flux. In selective welding, there are different theoretical explanations for preheating: some process engineers think that PCB should be preheated before flux spraying; another view is that welding directly without preheating. The user can arrange the selective welding process according to the specific situation.
