Generation of PCBA welding Pores _ How to improve the problem of PCBA welding pores
2022-11-09
PCBA welding type
1.Reflow welding
First of all, the first welding process of PCBA is reflow welding. After SMT mounting, PCB board will be reflow welding to complete the patch welding.
2.Wave crest welding
Reflow welding is the welding of patch components, and wave soldering is needed for plug-in components. Generally, PCB board is inserted into the components, and then through the wave furnace to complete the plug-in components and PCB board welding.
3.dip tin welding
For some large components, or other factors, can not be wave soldering, it is often used to weld tin furnace, tin furnace welding is simple and convenient.
4. Manual welding
Manual welding refers to the use of electric iron welding staff, generally in PCBA processing plants need manual welding personnel.
The generation of porosity in PCBA welding
The pores generated by PCBA plate welding, which is also called bubbles, are generally generated by reflux welding and wave soldering in the process of PCBA processing. PCBA is composed of multiple processes. Only through different PCBA welding types can a complete PCBA board be produced.
How to improve the porosity problem of PCBA welding
1.Bake
Bake PCB and components exposed to air for a long time to prevent moisture.
2.Control of solder paste
Tin paste containing moisture is also easy to produce pores, tin beads. First of all, good quality solder paste is selected. The return temperature and stirring of the solder paste are strictly carried out according to the operation. The exposure time of the solder paste in the air is as short as possible.
When lead-free solder paste is used, the bubbles inside the solder joint are a heat retention place when the components work. The heat generated during the work of the components will accumulate in the bubbles, resulting in the solder joint temperature can not be exported smoothly through the solder pad. The longer the working time, the more heat stored, the greater the impact on the reliability of the solder joint. In order to achieve the desired wetting and final interbonding with SAC alloys, the flux in SAC solder paste must operate at a higher temperature than the flux in lead solder paste. The flux operates at a higher temperature and the surface tension of SAC alloy is greater than that of tin lead alloy, increasing the likelihood of volatiles trapped in the molten solder. These volatiles cannot be easily discharged from molten solder. It is difficult to avoid the formation of air bubbles, but we can remove them by means! Because ordinary air reflow equipment can not produce vacuum environment, the oxygen inside the furnace and the air bubbles inside the solder joint can not be effectively excluded, in order to prevent the nitrogen oxide of the solder joint to protect the return furnace because the pressure of nitrogen is higher than atmospheric pressure, but the air bubbles inside the solder joint produce more, so how to solve the problem of bubbles after the use of lead-free solder paste?
First of all, after welding, gradient vacuumization is carried out at this stage before cooling, that is, the vacuum degree is gradually improved, because the solder is still in the liquid state after welding. At this time, the bubbles are distributed with each position of the solder joint. Gradient vacuumization can first remove the bubbles from the surface, and the bubbles at the bottom will move upward, and the bubbles will overflow evenly with the decrease of pressure. If the air is evacuated instantaneously, it will leave explosive openings in the solder joint. Secondly, pre-vacuum, lead-free solder paste in heating before the oxygen should be evacuated in the working area, to avoid the formation of solder oxide film in the heating process, vacuum environment can also increase the wetting area. Of course, in addition to the above problems will affect the blistering of lead-free solder paste after use, there are also many small details in our usual work. As long as we pay attention to these in our work, we believe that these things can be avoided.
3.workshop humidity control
Planned to monitor the workshop humidity, control between 40-60%.
4.set a reasonable furnace temperature curve
The furnace temperature is tested twice a day to optimize the furnace temperature curve, and the heating rate cannot be too fast.
5.Flux spraying
In the over wave soldering, the flux spraying amount can not be too much, spraying reasonable.
6.optimize the furnace temperature curve
The temperature of the preheating zone should meet the requirements, not too low, so that the flux can be fully volatilized, and the speed of the furnace can not be too fast.
There may be many factors affecting the PCBA welding bubble, which can be analyzed from PCB design, PCB humidity, furnace temperature, flux (spray size), chain speed, tin wave height, solder composition and other aspects. It is necessary to obtain a better process after several times of debugging.
1.Reflow welding
First of all, the first welding process of PCBA is reflow welding. After SMT mounting, PCB board will be reflow welding to complete the patch welding.
2.Wave crest welding
Reflow welding is the welding of patch components, and wave soldering is needed for plug-in components. Generally, PCB board is inserted into the components, and then through the wave furnace to complete the plug-in components and PCB board welding.
3.dip tin welding
For some large components, or other factors, can not be wave soldering, it is often used to weld tin furnace, tin furnace welding is simple and convenient.
4. Manual welding
Manual welding refers to the use of electric iron welding staff, generally in PCBA processing plants need manual welding personnel.
The generation of porosity in PCBA welding
The pores generated by PCBA plate welding, which is also called bubbles, are generally generated by reflux welding and wave soldering in the process of PCBA processing. PCBA is composed of multiple processes. Only through different PCBA welding types can a complete PCBA board be produced.
How to improve the porosity problem of PCBA welding
1.Bake
Bake PCB and components exposed to air for a long time to prevent moisture.
2.Control of solder paste
Tin paste containing moisture is also easy to produce pores, tin beads. First of all, good quality solder paste is selected. The return temperature and stirring of the solder paste are strictly carried out according to the operation. The exposure time of the solder paste in the air is as short as possible.
When lead-free solder paste is used, the bubbles inside the solder joint are a heat retention place when the components work. The heat generated during the work of the components will accumulate in the bubbles, resulting in the solder joint temperature can not be exported smoothly through the solder pad. The longer the working time, the more heat stored, the greater the impact on the reliability of the solder joint. In order to achieve the desired wetting and final interbonding with SAC alloys, the flux in SAC solder paste must operate at a higher temperature than the flux in lead solder paste. The flux operates at a higher temperature and the surface tension of SAC alloy is greater than that of tin lead alloy, increasing the likelihood of volatiles trapped in the molten solder. These volatiles cannot be easily discharged from molten solder. It is difficult to avoid the formation of air bubbles, but we can remove them by means! Because ordinary air reflow equipment can not produce vacuum environment, the oxygen inside the furnace and the air bubbles inside the solder joint can not be effectively excluded, in order to prevent the nitrogen oxide of the solder joint to protect the return furnace because the pressure of nitrogen is higher than atmospheric pressure, but the air bubbles inside the solder joint produce more, so how to solve the problem of bubbles after the use of lead-free solder paste?
First of all, after welding, gradient vacuumization is carried out at this stage before cooling, that is, the vacuum degree is gradually improved, because the solder is still in the liquid state after welding. At this time, the bubbles are distributed with each position of the solder joint. Gradient vacuumization can first remove the bubbles from the surface, and the bubbles at the bottom will move upward, and the bubbles will overflow evenly with the decrease of pressure. If the air is evacuated instantaneously, it will leave explosive openings in the solder joint. Secondly, pre-vacuum, lead-free solder paste in heating before the oxygen should be evacuated in the working area, to avoid the formation of solder oxide film in the heating process, vacuum environment can also increase the wetting area. Of course, in addition to the above problems will affect the blistering of lead-free solder paste after use, there are also many small details in our usual work. As long as we pay attention to these in our work, we believe that these things can be avoided.
3.workshop humidity control
Planned to monitor the workshop humidity, control between 40-60%.
4.set a reasonable furnace temperature curve
The furnace temperature is tested twice a day to optimize the furnace temperature curve, and the heating rate cannot be too fast.
5.Flux spraying
In the over wave soldering, the flux spraying amount can not be too much, spraying reasonable.
6.optimize the furnace temperature curve
The temperature of the preheating zone should meet the requirements, not too low, so that the flux can be fully volatilized, and the speed of the furnace can not be too fast.
There may be many factors affecting the PCBA welding bubble, which can be analyzed from PCB design, PCB humidity, furnace temperature, flux (spray size), chain speed, tin wave height, solder composition and other aspects. It is necessary to obtain a better process after several times of debugging.
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