Process characteristics of selective welding

The process characteristics of selective welding can be understood by comparing it 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 the liquid solder, while in selective soldering, only a specific area is in contact with the solder wave. Due to the fact that PCB itself is a poor heat conduction medium, it will not heat and melt the solder joints in adjacent components and PCB areas during soldering. Flux must also be applied in advance before welding. Compared to wave soldering, flux is only applied to the lower part of the PCB to be soldered, rather than the entire PCB. In addition, selective welding is only applicable to the welding of plug-in components. Selective welding is a new method, and a thorough understanding of the selective welding process and equipment is necessary for successful welding.

The process of selective welding

The typical selective soldering process includes: flux spraying, PCB preheating, immersion soldering, and drag soldering.

Flux coating process

In selective soldering, the flux coating process plays an important role. At the end of welding heating and welding, the flux should have sufficient activity to prevent bridging and oxidation of the PCB. Flux spraying is carried by X/Y robotic arms to the PCB through the flux nozzle, and the flux is sprayed onto the position to be soldered on the PCB. Flux has a variety of ways, including single nozzle spray, micropore spray, and synchronous multi-point/pattern spray. The most important thing in microwave peak selection welding after reflow soldering is accurate spraying of solder flux. The microporous spray type will never contaminate the area outside the solder joint. The minimum diameter of the solder dot pattern for micro dot spraying is greater than 2mm, so the position accuracy of the solder deposited on the PCB by spraying is ± 0.5mm to ensure that the solder always covers the welded area. The tolerance of the amount of solder sprayed is provided by the supplier, and the technical manual should specify the amount of solder used, usually recommending a safe tolerance range of 100%.

Preheating process

The main purpose of preheating in selective welding processes 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 soldering process. During welding, the heat generated by preheating is not a key factor affecting the welding quality. The thickness of PCB material, device packaging specifications, and type of flux determine the setting of preheating temperature. In selective soldering, there are different theoretical explanations for preheating: some process engineers believe that PCBs should be preheated before flux spraying; Another viewpoint is that welding can be carried out directly without preheating. Users can arrange the selective welding process according to specific circumstances.

Welding process

There are two different types of selective welding processes: drag welding process and immersion welding process.

The selective drag welding process is completed on a single small soldering nozzle solder wave. The drag soldering process is suitable for soldering in very tight spaces on PCBs. For example, individual solder joints or pins can be subjected to drag soldering process for a single row of pins. The PCB moves on the solder wave of the solder nozzle at different speeds and angles to achieve optimal soldering quality. 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 nozzle is installed and optimized in different directions for different welding needs. The robotic arm can approach the solder wave from different directions, i.e. from 0 ° to 12 °, at different angles. Therefore, users can solder various devices on electronic components. For most devices, a tilt angle of 10 ° is recommended.

Compared with the immersion soldering process, the solder solution and PCB board movement of the drag soldering process result in better thermal conversion efficiency during soldering. However, the heat required to form a weld connection is transferred by the solder wave, but the quality of the solder wave for a single nozzle is small. Only when the temperature of the solder wave is relatively high can the requirements of the drag welding process be met. For example, a soldering temperature of 275 ℃ to 300 ℃ and a dragging speed of 10mm/s to 25mm/s are usually acceptable. Nitrogen is supplied in the welding area to prevent solder wave oxidation, which eliminates oxidation and prevents bridging defects in the drag welding process. This advantage increases the stability and reliability of the drag welding process.

The machine has the characteristics of high precision and high flexibility, and 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 movement radius of the robotic arm can cover the flux nozzle, preheating and soldering nozzle, so the same equipment can complete different soldering processes. The unique synchronous process of the machine can greatly shorten the single board process cycle. The capabilities possessed by the robotic arm enable this type of selective welding to have the characteristics of high precision and high-quality welding. Firstly, the precise positioning capability of the robotic arm with high stability (± 0.05mm) ensures that the parameters produced for each board are highly consistent and repetitive; Secondly, the 5-dimensional motion of the robotic arm enables the PCB to contact the solder surface at any optimized angle and orientation, achieving optimal soldering quality. The tin wave height measuring needle installed on the robotic arm clamp device is made of titanium alloy and can be periodically measured under program control. The tin wave height is controlled by adjusting the speed of the tin pump to ensure process stability.

Despite the many advantages mentioned above, the single nozzle solder wave drag welding process also has shortcomings: the welding time is the longest among the three processes of flux spraying, preheating, and welding. And due to the fact that the solder joints are drag welded one by one, as the number of solder joints increases, the soldering time will significantly increase, and the soldering efficiency cannot be compared with traditional wave soldering processes. But the situation is changing, and a multi nozzle design can maximize production. For example, using dual welding nozzles can double production, and dual nozzles can also be designed for flux.

The immersion selective soldering system has multiple soldering nozzles and is designed one-to-one with the PCB solder joints. Although it is not as flexible as a robotic arm, its output is equivalent to traditional wave soldering equipment, and the equipment cost is relatively low compared to a robotic arm. According to the size of the PCB, single board or multi board parallel transfer can be carried out, and all solder joints will be sprayed, preheated, and soldered in parallel at the same time. However, due to the different distribution of solder joints on different PCBs, specialized soldering nozzles need to be made for each PCB. The size of the soldering nozzle should be as large as possible to ensure the stability of the soldering process and not affect the adjacent components on the PCB. This is important and difficult for design engineers because the stability of the process may depend on it.

By using immersion selective welding technology, welding points of 0.7mm to 10mm can be welded. The welding process for short pins and small-sized pads is more stable, and the possibility of bridging is also small. The distance between adjacent welding point edges, devices, and solder tips should be greater than 5mm.