PCB Technical - Causes and solutions of false welding

With the advancement of technology, some electronic products such as mobile phones and tablet computers are trending towards being light, small and portable. The electronic components used in SMT processing are also becoming smaller. 0201 size is given instead. How to ensure the quality of solder joints has become an important issue for high-precision placement. The solder joint is used as a bridge for soldering, and its 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.

At present, in the electronics industry, although the research of lead-free solder has made great progress, it has begun to be promoted and applied worldwide, and environmental protection issues have also received widespread attention. The soldering technology using Sn-Pb solder alloy is still It is the main connection technology of electronic circuits.

A good solder joint should not fail in its mechanical and electrical properties during the life cycle of the equipment. Its appearance is as follows:

(1) Complete, smooth and shiny surface;

(2) Appropriate amount of solder and solder completely cover the soldering part of the pad and lead, and the height of the component is moderate;

(3) 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.

SMT processing appearance inspection 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 a false welding; the reason for the false welding is relatively complicated.

1. Judgment of Welding

1. Use special equipment for online tester for inspection.

2. Visual or AOI inspection. When it is found that there is too little solder in the solder joint, the solder infiltration is poor, or there is a crack in the middle of the solder joint, or the surface of the solder is convex, or the solder is not compatible with the SMD, etc., you must pay attention to it. Even a slight phenomenon can cause hidden dangers. 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 a single 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.

2. Causes and solutions of false welding

1. The pad design is defective. The existence of through-holes on the pads is a major defect in PCB design. Do not use them if they are less than absolutely necessary. The through-holes will cause solder loss and insufficient solder; the spacing and area of the pads also need to be standardized, 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 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 and makes the solder insufficient. 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 Mount Components) is of poor quality, expired, oxidized, 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 rises,

At this time, it can be soldered with more than 300 degree electric ferrochrome and rosin-type flux, but it is difficult to melt with more than 200 degree SMT reflow soldering and the use of less corrosive no-clean solder paste. Therefore, the oxidized SMD is not suitable for welding with reflow soldering furnace. When buying components, you must see if there is oxidation, and use them in time after you buy them. In the same way, oxidized solder paste cannot be used.

(2) Surface mount components with multiple legs have small legs and are easily deformed under the action of external force. Once deformed, the phenomenon of missing or missing welding will definitely occur. Therefore, it is necessary to carefully check and repair in time before welding.