PCB manufacturing selective soldering technics difficulties

• In the PCB manufacturing electronics industry welding technics, more and more pcb manufacturers are turning their attention to selective welding. Selecting welding can complete all the solder joints at the same time, so that the production cost is minimized, and the reflow soldering is overcome the problem of temperature-sensitive components affected, and the selective soldering is also compatible with future lead-free soldering. These advantages make the range of applications for soldering wider and wider.

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  The following describes the technics characteristics of selective soldering:

  The technics 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 and melt the solder joints on adjacent components and PCB fabrication areas. 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 plug-in components. Selective soldering is a completely new approach to thoroughly understand that selective soldering technics and equipment are necessary for successful soldering.

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  Selective welding process:

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

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  Flux coating technics:

  In the selective soldering, the flux coating technics plays an important role. At the end of solder heating and soldering, the flux should be sufficiently active to prevent bridging connection and 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 specify flux usage and a 100% safety tolerance range is usually recommended.

  Preheating technics:

  The main purpose of preheating in the selective soldering technics of pcb is not to reduce the thermal stress, but to remove the solvent pre-drying flux, so that the flux has the correct viscosity before entering the solder wave. During soldering, the effect of preheating 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 technics engineers believe that the PCB should be preheated before the flux is sprayed; another point of view is that soldering can proceed directly without preheating. The user can arrange the process flow for selective soldering according to the specific situation.

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  Welding technics:

  The selective soldering technics has two different technics: a drag weldingtechnics and a dip soldering technics.

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  The pcb manufacturing selective drag welding technics is performed on a single small tip solder wave. The drag welding technics 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 tip solder wave to achieve the best solder quality. In order to ensure the stability of the welding technics, 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°.

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  Compared with the dip soldering technics, the soldering solution of the drag soldering technics and the movement of the PCB board make the heat conversion efficiency during soldering better than the dip soldering technics. However, the heat required to form the weld seam 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 can it meets the requirements of the drag welding technics. For example: solder temperature is 275 ° C ~ 300 ° C, drag speed 10mm / s ~ 25mm / s is usually acceptable. Nitrogen is supplied to the soldering area to prevent solder wave oxidation, and the solder wave eliminates oxidation, so that the drag welding technics avoids the occurrence of bridging defects, which increases the stability and reliability of the PCB soldering technics.

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• 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 nozzles, so the same equipment can be used for different welding technics. The machine-specific synchronization process can greatly shorten the board process cycle. The ability of the robot to make this selective welding 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. And obtain 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 technics stability.

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• Despite these advantages, the single-nozzle solder wave dragging process also have disadvantages: the soldering time is the longest in the three processes of flux spraying, preheating and soldering. And because the solder joints are dragging 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 technics. But the situation is changing, and the multi-tip design maximizes pcb manufacturing yield. For example, double-welded nozzles can double the throughput and flux can be designed as dual nozzles.

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