SMT process quality inspection

First, check the content

(1) Is the component missing?

(2) Whether the component is mislabeled

(3) Is there a short circuit?

(4) Is there welding?

The first three conditions are well checked and the reasons are clear, but the cause of the virtual weld is more complicated.

Second, the judgment of virtual welding

1. Use an in-circuit tester-specific device (commonly referred to as a needle bed) for inspection.

2. Visual inspection (including the use of a magnifying glass, microscope). When visually finding that the solder joint penetration is too small, or there is a crack in the middle of the solder joint, or the surface of the solder joint is raised, or the solder is not in contact with the SMD, it should be noted that even if a slight phenomenon causes a hidden danger, it should be immediately judged whether there is a problem in the batch soldering. . The method of judging is to check if there are more solder joints in the same position on the PCB. For example, it is only a problem on a single PCB, which may be caused by scraping of the solder paste, deformation of the leads, etc., such as the same position on many PCBs. There is a problem, which may be caused by a defective component or a pad problem.

Third, the reasons and solutions for virtual welding

1. The pad design is defective. There should be no through holes in the mat. Through holes can cause solder loss and solder shortage. The pad pitch and area also require a standard match. Otherwise, the design should be corrected as soon as possible.

2. The PCB board is oxidized, that is, the pad is not lit. If there is oxidation, use an eraser to remove the oxide layer and make it brighter. The PCB is wet and can be dried in a dry box if suspected. There are oil stains, sweat stains and other pollution on the PCB board. At this time, it is cleaned with absolute ethanol.

3. Scratch and rub the solder paste on the PCB on which the solder paste is printed, thereby reducing the amount of solder paste on the associated pads, thereby making the solder insufficient. It should be replenished in time. Supplemental methods can be done with a dispenser or with a bamboo stick.

4. SMD (Surface Mounted Components) is of poor quality, expired, oxidized, deformed, resulting in virtual soldering. This is why it is more common.

(1) The oxidizing component is not bright. The melting point of the oxide increases. At this time, it is possible to use more than three Baidu electric ferrochrome and rosin-type flux for welding, but use more than two Baidu SMT reflow soldering and use less corrosive unclean. Solder paste is difficult to melt. Therefore, the oxidized SMD should not be welded by the reflow oven. When purchasing components, be sure to check for oxidation and use it at the time of purchase. Similarly, oxidized solder paste cannot be used.

(2) The surface mounting members of the multi-legs have small leg portions and are easily deformed by external force. Once deformed, the phenomenon of soldering or lack of soldering occurs. Therefore, it must be carefully inspected and repaired in time after welding.