Difficulties in PCB selective soldering process
In the PCB electronics industry welding process, more and more 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 temperature is overcome. The problem of sensitive components and the choice of soldering are also compatible with future lead-free soldering, all of which make the range of applications for soldering wider and wider.
Process characteristics of selective soldering 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 certain areas are 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, and a thorough understanding of selective soldering processes and equipment is necessary for successful soldering.
Process for Selective Soldering Typical selective soldering processes include: flux coating, PCB preheating, dip soldering, and drag soldering.
Flux Coating Process The flux coating process plays an important role in selective soldering. 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 microwave peak after the reflow soldering process, the most important thing is the accurate spraying of the flux. The micro-hole spray type will never contaminate the area outside the solder joint. The minimum solder dot pattern diameter of micro-spraying is greater than 2mm, so the positional accuracy of the solder deposited on the PCB is ±0.5mm, so as to ensure that the flux always covers the soldered part. The tolerance of the spray soldering dose is provided by the supplier. 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 heat on solder quality is not a critical factor. PCB material thickness, device package size, and flux type determine the preheat 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 welding process has two different processes: the drag welding process and the 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 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 accurate 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. But 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.
Process characteristics of selective soldering 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 certain areas are 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, and a thorough understanding of selective soldering processes and equipment is necessary for successful soldering.
Process for Selective Soldering Typical selective soldering processes include: flux coating, PCB preheating, dip soldering, and drag soldering.
Flux Coating Process The flux coating process plays an important role in selective soldering. 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 microwave peak after the reflow soldering process, the most important thing is the accurate spraying of the flux. The micro-hole spray type will never contaminate the area outside the solder joint. The minimum solder dot pattern diameter of micro-spraying is greater than 2mm, so the positional accuracy of the solder deposited on the PCB is ±0.5mm, so as to ensure that the flux always covers the soldered part. The tolerance of the spray soldering dose is provided by the supplier. 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 heat on solder quality is not a critical factor. PCB material thickness, device package size, and flux type determine the preheat 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 welding process has two different processes: the drag welding process and the 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 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 accurate 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. But 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.