The characteristic of BGA soldering is that the thermal conductivity of the PCB motherboard is much worse than that of the chip due to its size, thickness, material, etc. Compared with the chip, the temperature rise speed is obviously much slower, and it takes much longer to heat through. When the motherboard is at a low temperature, the heat conducted downward from the direction of the chip will be quickly absorbed by the motherboard, which has little effect on heating the solder ball.Therefore, we should pay attention to the following two points:
1. Preheat the chip in advance. After all, it is a multi-layer structure, and the substrate is PCB material. Therefore, no matter how good its thermal conductivity is, there will be a temperature difference between the upper and lower layers.A sudden large temperature difference will easily damage the chip, so we put forward the second point of view: 2. Heating on the hot air type must be divided into heating sections. According to experience, the chip has a strong ability to withstand high temperature, but the ability to withstand high temperature for a long time is not strong. Therefore, the heating time, especially in the high temperature area, should be shortened as far as possible. Once the tin ball is found to be melted through,Stop heating immediately and proceed to the next step.The soldering paste must be added during soldering.When the solder ball melts, give it a downward pressure from the top of the chip to make full contact between the solder ball and the solder joint.3. Several problems of BGA manual welding 1. Why should we preheat in advance?Our plan is to preheat in advance for welding, desoldering and welding, and it is better to preheat in advance for leaded/lead-free. The curve we provide is also formulated on the basis of preheating in advance.Why do you want to warm up in advance?Because the temperature rise of the motherboard is much slower than that of the chip, if the temperature of the motherboard is very low, the heat conducted by the chip will be quickly dissipated by the motherboard, which has limited effect on the temperature rise of the solder ball and is basically ineffective heating, while too long high temperature heating time will be harmful to the chip.2. What problems should be paid attention to in BGA chip resoldering?The operation should be completed at one go. After the chip is removed, the tin on the chip should be scraped off immediately by taking advantage of the heat. The motherboard should not be removed from the shelf. After the chip is processed, the solder on the board should be removed with a tin suction line.Wipe the solder paste with alcohol or washing water after the motherboard and chip are scraped, and then apply a new solder paste on the pad. The amount should be appropriate, preferably a very thin layer, but every point should be coated.Do not add too much solder paste, because too many chips will float on the solder paste.3. Why is the method of desoldering and soldering lead-free chips not applicable when soldering lead-free chips?In practice, lead-free chips may be soldered, such as replacing new chips.Users who have experience in operation may find that the same operation is very smooth in unsoldering lead-free chips, but when soldering lead-free chips, the solder balls do not melt and can not be soldered.So what is the reason for this?The reason is very simple, the contact between the solder ball and the pad is a small plane when desoldering and soldering, while the contact between the solder ball and one side of the motherboard is a point when soldering, and their thermal conductivity is different.The solution is to preheat to 10 to 20 degrees, or postpone the heating time, and preheat for a while.4. How to check whether the welding is qualified?After welding, first check whether the chip is welded flat. From the four sides of the chip, each side of the chip should be parallel to the motherboard. You can see that the solder ball is not elongated or flattened.If not, then it may be a problem of operation, or it may be that the motherboard is deformed.Check the residual solder paste. If you use high-quality solder paste, the solder paste should be kept bright, and the amount of solder paste under the chip should be relatively large.In this way, on the one hand, the solder ball is wrapped by the soldering paste, which is not easy to oxidize, and on the other hand, the soldering paste is still active when the soldering is completed.On the contrary, if the flux has been basically volatilized and the residual solder paste on the board turns black, it will have an impact on the success rate and firmness 0755-36842859.5. What problems should be paid attention to in continuous operation?If the user needs to continue the welding operation, please note that the next operation will not be carried out until the temperature of the preheating table drops below 70 degrees.6. Is the additional welding not as firm as the replanting ball?In general, such a situation does not exist, it can only be that the welding is not good, and the welding saves time, effort and cost, which is much more cost-effective than re-planting the ball.
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