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PCB Selective Soldering Technology in Detail

Looking back at the development process of the electronics industry in recent years, we can notice that a very obvious trend is reflow soldering technology. In principle, conventional 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, minimizing production costs. However, temperature sensitive components limit the application of reflow soldering, whether it is an interposer or an SMD. Then people turned their attention to the choice of welding. In most applications, selective soldering can be used after PCB reflow soldering. This will be a cost-effective and efficient way to complete the remaining inserts and is fully compatible with future lead-free soldering.


Process characteristics of selective welding

The process characteristics of selective soldering can be understood by comparison with wave soldering. The most obvious difference between the two is that the lower portion of the PCB in the wave soldering is completely immersed in the liquid solder, while in selective soldering, only a certain portion of the area is in contact with the solder wave. Since the PCB itself is a poor heat transfer medium, it does not heat the solder joints that melt adjacent components and PCB areas during soldering. The flux must also be pre-coated before soldering. Compared to wave soldering, the flux is only applied to the part of the PCB to be soldered, not the entire PCB. In addition, selective soldering is only suitable for soldering of the interposing components. Selective soldering is a completely new approach to thoroughly understand that selective soldering processes and equipment are necessary for successful soldering.


Selective welding process

Typical selective soldering processes include: flux coating, PCB preheating, dip soldering, and drag soldering.


Flux coating process

In the selective soldering, the flux coating process plays an important role. At the end of solder heating and soldering, the flux should be sufficiently active to prevent bridging and prevent oxidation of the PCB. The flux is sprayed by the X/Y robot carrying the PCB through the flux nozzle and the flux is sprayed onto the PCB to be soldered. Fluxes are available in single-nozzle spray, micro-hole spray, and simultaneous multi-point/pattern spray. The most important thing is the accurate spraying of the flux during the microwave peak welding after the reflow process. The micro-hole spray type will never contaminate the area outside the solder joint. The minimum flux point pattern diameter of micro-spraying is greater than 2mm, so the positional accuracy of the solder deposited on the PCB is ±0.5mm, so that the flux can always be covered on the part to be welded. The tolerance of the sprayed soldering dose is provided by the supplier. The technical specification should be The flux usage is specified and a 100% safety tolerance range is usually recommended.


Preheating process

The main purpose of preheating in the selective soldering process is not to reduce the thermal stress, but to remove the solvent pre-drying flux, so that the solder has the correct viscosity before entering the solder wave. During soldering, the effect of preheating the heat on the soldering quality is not a critical factor. PCB thickness, device package size, 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 the flux is sprayed; another point of view is that soldering is not required without preheating. The user can arrange the selective soldering process according to the specific situation.


Welding process

The selective soldering process has two different processes: a drag welding process and a dip soldering process.

The selective drag welding process is performed on a single small tip solder wave. The drag welding process is suitable for welding on very tight spaces on the PCB. For example: individual solder joints or pins, single row pins can be dragged. The PCB moves at different speeds and angles on the solder wave of the tip to achieve the best solder quality. In order to ensure the stability of the welding process, the inner diameter of the tip is less than 6mm. After the flow direction of the solder solution is determined, the tip is installed and optimized in different directions for different soldering needs. The robot can approach the solder wave from different angles, that is, from 0° to 12°, so the user can solder various components on the electronic components. For most devices, the tilt angle is recommended to be 10°.


Compared with the dip soldering process, the soldering solution of the 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 joint is transmitted by the solder wave, but the solder wave quality of the single tip is small, and only the temperature of the solder wave is relatively high, so that the requirements of the drag welding process can be achieved.


For example: solder temperature is 275 ° C ~ 300 ° C, drag speed 10mm / s ~ 25mm / s is generally acceptable. Nitrogen is supplied to the soldering area to prevent solder wave oxidation, and the solder wave eliminates oxidation, so that the drag welding process avoids the occurrence of bridging defects, which increases the stability and reliability of the drag welding process.


The machine is characterized by high precision and flexibility. The modular design system can be customized according to the customer’s special production requirements and can be upgraded to meet the needs of future production development. The robot’s radius of motion covers the flux nozzles, preheating and soldering tips, so the same equipment can be used for different welding processes. The machine-specific synchronization process can greatly shorten the board process cycle. The ability of the robot to make this selective weld has 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 that the parameters of each board are highly repeatable. Secondly, the 5-dimensional movement of the robot enables the PCB to contact the tin surface at any optimized angle and orientation. Good welding quality. The tin wave height stylus mounted on the manipulator splint device is made of titanium alloy. The height of the tin wave can be measured periodically under program control. The height of the tin wave can be controlled by adjusting the speed of the tin pump to ensure process stability.


Despite these advantages, the single-nozzle solder wave dragging process is also inadequate: the soldering time is the longest in the three processes of flux spraying, preheating and soldering. And because the solder joints are one by one, the soldering time will increase greatly as the number of solder joints increases, and the soldering efficiency cannot be compared with the conventional wave soldering process. However, the situation is changing, and the multi-tip design maximizes throughput. For example, double-welded nozzles can double the throughput and flux can be designed as dual nozzles.


The immersion selective welding system has multiple soldering 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. Depending on the size of the PCB, single or multiple boards can be transported in parallel, and all solder joints will be soldered, preheated and soldered in parallel at the same time. However, due to the different distribution of solder joints on different PCBs, special solder nozzles are required for different PCBs. The size of the tip is as large as possible to ensure the stability of the soldering process and does not affect the surrounding adjacent devices on the PCB. This is important and difficult for the design engineer because the stability of the process may depend on it.


The immersion soldering process can be used to solder solder joints of 0.7mm to 10mm. The soldering process of short pins and small pads is more stable and the possibility of bridging is small. The distance between adjacent solder joints, devices and soldering tips should be More than 5mm.