PCBA Tech - About SMT chip processing solder joint inspection skills

The electronic components used in SMT processing are getting smaller and smaller, and the 0402 resistance and capacitance components have been replaced by the 0201 size. How to ensure the quality of solder joints has become an important issue for high-precision surface mount. The solder joints are used as a bridge for soldering, and their quality and reliability determine the quality of electronic products. In other words, in the production process, the quality of SMT ultimately manifests as the quality of solder joints.

SMT patch processing solder joint inspection method

At present, in the electronics industry, although great progress has been made in the research of lead-free solders, they have been promoted and applied all over the world, and environmental issues have also received widespread attention. The soldering technology using tin-lead solder alloy is still the main connection technology for electronic circuits.

good solder joints should be within the service life of the equipment, and its mechanical and electrical properties will not fail. Its appearance is as follows:

(1) Complete, smooth and shiny surface;

(2) The proper amount of solder and solder completely cover the soldering part of the pad and the lead, and the height of the components is moderate;

(3) Good wettability; the edge of the solder joint should be thin, and the wetting angle between the solder and the pad surface should be less than 300°, and the maximum should not exceed 600°.

Surface mount processing visual inspection content:

(1) Are there any missing parts;

(2) Whether the component is connected incorrectly;

(3) Whether there is a short circuit;

(4) Whether there is a false welding; the reason for the false welding is more complicated.

1. Judgment of false welding.

1. Use the special equipment of the online tester to check.

2. Visual inspection or AOI inspection. When it is found that there is too little solder in the solder joint, or there is a crack in the middle of the solder joint, or the surface of the solder is convex spherical, or the solder is incompatible with SMD, etc., It should be noted that even minor phenomena will cause hidden dangers, and it is necessary to immediately determine whether there is a lot of false welding problem. The method of judgment is to see whether there are problems with the same position solder joints on many printed circuit boards. For example, the problems on individual printed circuit boards may be caused by solder paste scratches and pin deformation. If there is a problem in the same position on many PCB printed circuit boards, it may be caused by defective components or pads.

2. The reason and solution of virtual welding.

1. The PCB pad design is defective. The existence of through holes in the pads is a major flaw in the design of printed circuit boards. If it is less than 10,000, do not use it. Through holes will cause solder loss and lead to solder shortage; the pad spacing and area also need to be matched with standards, otherwise the design should be revised as soon as possible.

2.2. The PCB board is oxidized, that is, the solder board is dark. If there is oxidation, you can use an eraser to remove the oxide layer to reproduce the bright light. The PCB board is wet, if in doubt, it can be dried in a drying oven. The PCB board is polluted by oil stains, sweat stains, etc., at this time, use absolute ethanol to clean it.

3. On the PCB printed with solder paste, the solder paste is scratched, which reduces the amount of solder paste on the relevant pads, resulting in insufficient solder. Make up in time. Make-up method can use a dispenser or bamboo stick to pick a little makeup.

4. SMD (Surface Mounted Components) is of poor quality, expired, oxidized and deformed, resulting in virtual soldering. This is a common cause.

(1) The oxidation component is dark and not bright. The oxide melting point rises, and it can be soldered with electric ferrochrome above 300 degrees plus rosin type flux, but it is difficult to melt with SMT reflow soldering above 200 degrees and weaker corrosive no-clean solder paste. Therefore, oxidized SMD should not be soldered in a reflow oven. When buying components, you must check for oxidation, and use them in time after you buy them. Likewise, oxidized solder paste cannot be used.

(2) Multi-leg surface mount components have small legs and are easily deformed under external force. Once it is deformed, there will definitely be a phenomenon of PCB soldering or lack of soldering. Therefore, it must be carefully checked before and after soldering and repaired in time.