SMT processing solder joint quality inspection

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. How to ensure the quality of solder joints has become an important issue for high-precision PCBA products. The solder joint is used as a bridge for components and PCB, 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, 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 also soldering 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, solder completely cover the soldering part of the pad and lead, and the height of the soldering is moderate;

(3) Good solderability, 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 30degree, and the maximum should not exceed 60degree.

SMT processing appearance inspection content:

• Whether the components are missing;

• Whether the components are mounted correctly;

• Whether there is a short circuit;

• Whether there is a pseudo welding; the reason for the pseudo welding is relatively complicated.

I. How to judge the quality of welding?

1. Use special equipment (e.g. 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 angle, or the solder is not compatible with the SMD. It should be immediately judged whether there is a lot of pseudo 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., If there are problems at the same position on many PCBs, it is likely to be caused by a bad component or a problem with the pad.

II. Causes and solutions of pseudo 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 suspected damp, it can be dried in a drying box. If the PCB board is polluted by oil stains, sweat stains, etc., it should be cleaned with absolute ethanol.

3. For PCBs that have been printed with solder paste, if the solder paste is scratched or 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 done with a dispenser or pick a little with small sticks.

4. SMD (Surface Mount Components) is of poor quality, expired, oxidized, deformed, resulting in pseudo soldering. This is a more common reason.

The oxidized components are dark and not shiny. The melting point of the oxide rises. it can be soldered with more than 300 degree electric solder iron and rosin-type flux, because it is difficult to solder with 240 degree SMT reflow soldering and the use of less corrosive non-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.