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In the process of SMT patch processing, quality control is particularly critical to prevent large quantities of defective products from appearing, resulting in huge repair costs. For SMT chip processing manufacturers, it is necessary to in-depth management and practice their solutions for the following quality content.
1. Check content
(1) Whether the components are missing. (2) Whether the components are pasted incorrectly. (3) Whether there is a short circuit. (4) Whether there is false welding.
The first three cases are easy to check, and the reasons are clear and easy to solve, but the reasons for the false welding are more complicated.
2. Judgment of Welding
1. Use special equipment for online tester (commonly known as needle bed) for inspection.
2. Visual inspection (including magnifying glass and microscope). When there is too little solder in the solder joints, poor solder infiltration, or cracks in the middle of the solder joints, or the surface of the solder is convex spherical, or the solder is not compatible with SMD, etc., you should pay attention to it. Even a slight phenomenon will cause If there is a hidden danger, it should be immediately judged whether there is a lot of false welding problem. The method of judgment is: to see if there are many problems with the solder joints at the same position on the PCB. If it is only a problem on an individual PCB, it may be caused by the solder paste being scratched, pin deformation, etc., such as the same position on many PCBs. There are problems. At this time, it is likely to be caused by a bad component or a problem with the pad.
3. Causes and solutions of false welding
1. The pad design is defective. There should be no through-holes on the pads, as the through-holes will cause solder loss and insufficient solder; the pad spacing and area also need standard matching, otherwise the design should be corrected as soon as possible.
2. The PCB board is oxidized, that is, the pad is black and does not shine. If there is oxidation, an eraser can be used to remove the oxide layer to reproduce the bright light. If the PCB board is damp, it can be dried in a drying box if it is suspected. The PCB board is polluted by oil stains, sweat stains, etc. At this time, it should be cleaned with absolute ethanol.
3. For PCBs that have been printed with solder paste, the solder paste is scratched and rubbed, which reduces the amount of solder paste on the relevant pads, resulting in insufficient solder. It should be made up in time. The method of making up can be made up with a dispenser or pick a little with bamboo sticks.
4. SMD (Surface Mounted Components) is of poor quality, expired, oxidized, and deformed, resulting in virtual soldering. This is a more common reason.
(1) The oxidized components are dark and not shiny. The melting point of the oxide increases. At this time, it can be soldered with more than 300 degrees of electric ferrochrome and rosin-type flux, but with more than 200 degrees of SMT reflow soldering and the use of less corrosive no-clean The solder paste is difficult to melt. Therefore, the oxidized SMD is not suitable for welding with reflow soldering furnace. Be sure to check whether there is oxidation when buying components, and use them in time after buying them. In the same way, oxidized solder paste cannot be used.
(2) The surface mount components with multiple legs have small legs and are easily deformed under the action of external force. Once deformed, the phenomenon of weak welding or lack of welding will definitely occur. Therefore, it is necessary to carefully check and repair in time before welding.
