SMT solder joint quality control

SMT solder joint quality control

The electronic components used in SMT processing are getting smaller and smaller, and the resistor-capacitance parts of 0402 before are also largely replaced by 0201 size. How to ensure the quality of solder joint becomes an important topic of high precision patch. As a welding bridge, the quality and reliability of solder joint determine the quality of electronic products. That is to say, in the production process, the quality of SMT is ultimately expressed as the quality of solder joints.

At present, in the electronic industry, although the research of lead-free solder has made great progress, has begun to promote the application in the world, and the environmental protection problem is also widely concerned by people, the use of Sn-Pb solder alloy soft brazing technology is still the main connection technology of electronic circuits.

Good solder joints should not fail in mechanical or electrical properties during the service life of the equipment. Its appearance is shown as:

(1) a complete and smooth surface;

(2) Appropriate amount of solder and solder completely cover the welding parts of the pad and lead, and the height of the component is appropriate;

(3) good wettability; The edge of the welding point should be thin, and the wetting Angle between the solder and the pad surface should be less than 300, and the maximum is not more than 600.

SMT processing appearance inspection content:

(1) Whether the components are omitted;

(2) there is no wrong attachment of components;

(3) Whether there is short circuit;

(4) there is no weld; The reason of the soldering is relatively complicated.

First, the judgment of virtual welding

1. Adopt special equipment of online tester for inspection.

2. Visual or AOI inspection. When it is found that too little solder solder infiltration is bad, or there are cracks in the middle of the solder joint, or the solder surface is convex ball, or the solder and SMD is not melt, it is necessary to pay attention to, even a slight phenomenon will cause hidden trouble, should immediately determine whether there is a batch of soldering problem. The way to judge is to see if there are problems with solder joints at the same position on more PCB. If the problem is only on a single PCB, it may be caused by the scratch of solder paste, pin deformation and other reasons. If there are problems at the same position on many PCB, it is likely to be caused by bad components or problems with solder pads.

Causes and solutions of virtual welding

1. Defective pad design. The existence of through holes in the solder pad is a major defect of PCB design. It is not necessary to use them unless it is ten thousand. The through holes will cause solder loss and solder shortage. Pad spacing and area also need to match the standard, otherwise the design should be corrected as soon as possible.

2. There is oxidation phenomenon on PCB board, that is, the welding plate is black and not bright. If there is a phenomenon of oxidation, the eraser can be used to remove the oxide layer, so that its light reproduce. PCB board damp, such as doubt can be placed in the drying box drying. PCB board has oil stains, sweat stains and other pollution, at this time to clean with anhydrous ethanol.

3. For PCB with solder paste printed, the solder paste is scratched and rubbed, which reduces the amount of solder paste on related pads and leads to insufficient solder. Should be made up in time. The method can be used to fill the dispensing machine or pick up a little with a bamboo stick.

4.SMD (surface paste components) is of poor quality, expired, oxidized and deformed, resulting in virtual welding. This is the more common reason.

(1) oxidized elements are black and not bright. The melting point of the oxide rises,

At this time with more than 300 degrees of electric ferrochrome and rosin type flux can be welded, but with more than 200 degrees of SMT reflow welding coupled with the use of less corrosive cleaning solder paste is difficult to melt. Therefore, the oxidized SMD should not be welded by reflow furnace. Buy components must see whether there is oxidation, and buy back to use in time. Similarly, oxidized solder paste should not be used.

(2) the surface mount component with multiple legs, its legs are small and easy to deform under the action of external force. Once deformed, the phenomenon of welding or lack of welding will certainly occur, so it is necessary to check carefully before and after welding and repair in time

SMT electronic patch information