How to judge the quality of PCB SMT processing?
2023-12-08
With the development trend of electronic products to be light, small and portable, the electronic components used in SMT processing are also becoming smaller, and the resistance and capacity parts of the previous 0402 are also replaced by a large number of 0201 sizes. How to ensure the quality of solder joints has become an important topic for high-precision patches. As a bridge for welding, the quality and reliability of solder joints determine the quality of electronic products. That is to say, in the process of PCBA processing, the quality of SMT is ultimately manifested as the quality of solder joints.
A good solder joint should be one that does not fail its mechanical or electrical properties over the life of the equipment. Its appearance is as follows:
1. Complete, smooth and shiny surface;
2. The appropriate amount of solder and solder completely cover the soldering parts of the pads and leads, and the component height is moderate; Good wettability: the edge of the solder joint should be thin, and the wetting angle between the solder and the surface of the pad should be less than 300, and the maximum should not exceed 600.
The quality of PCB SMT processing can be judged by visual inspection:
1. Whether there is any omission of components;
2. Whether the components are mislabeled;
3. Whether there is a short circuit;
4. Whether there is false welding;
The first three good inspections, the reasons for virtual welding are relatively complex, and it is more troublesome to inspect, so let's introduce them to you.
First, the judgment of virtual welding
1. Use special equipment for online tester for inspection.
2. Visual or AOI inspection. When it is found that the solder joint is too little and the solder is poorly wetted, or there is a broken seam in the middle of the solder joint, or the surface of the solder is convex and spherical, or the solder and SMD are not compatible, etc., it is necessary to pay attention, even if the slight phenomenon will cause hidden dangers, and it should be immediately judged whether there is a batch of virtual soldering problems.
3. The way to judge is to see if there are problems with the solder joints at the same position on many PCBs, if it is only a problem on individual PCBs, it may be due to the solder paste being scratched, the pins are deformed, etc., such as there are problems in the same position on many PCBs, at this time, it is likely to be caused by bad components or problems with the pads.
Second, the causes and solutions of virtual welding
1. The pad design is defective. The presence of through holes in the pads is a major defect in PCB design, do not use the through holes, the through holes will cause the loss of solder and cause insufficient solder; the spacing and area of the pads also need to be matched according to the standard, otherwise the design should be corrected as soon as possible.
2. The PCB board has oxidation, that is, the pad is black and not shiny. If there is oxidation, you can use an eraser to remove the oxide layer to make it bright again. If the PCB board is wet, if it is suspected, it can be placed in a drying oven to dry. If the PCB board is contaminated with oil stains, sweat stains, etc., it should be cleaned with absolute ethanol.
3. The PCB that has been printed with solder paste is scraped and rubbed, which reduces the amount of solder paste on the relevant pads and makes the solder insufficient. It should be made up in a timely manner. The method of making up can be made up with a dispenser or a small amount with a bamboo skewer.
4. SMD (surface mount components) is of poor quality, expired, oxidized, and deformed, resulting in false soldering. This is the most common reason. When buying components, be sure to see if there is oxidation, and use them in time after buying. Similarly, oxidized solder paste cannot be used.
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