In the production process, the quality of SMT mainly depends on the quality of the solder joints.
At present, in the electronics industry, although the research on lead-free solder has made great progress, it has been promoted and applied worldwide, and environmental protection issues have received widespread attention. The soldering technology using Sn-Pb solder alloy is still the main technology of electronic circuits.
A good solder joint should be :
(1) a complete, smooth, shiny surface;
(2) Appropriate amount of solder and solder completely cover the solder joints of the pads and leads, and the component height 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 300 or less, and the maximum should not exceed 600.
SMT processing appearance inspection content:
(1) Whether the components are missing;
(2) Whether the components are mislabeled;
(3) Whether there is a short circuit;
(4) Whether there is virtual welding; the cause of virtual welding is relatively complicated.
First, the judgment of the virtual welding
1. Use the special equipment of online tester for inspection.
2. Visual or AOI test. When it is found that the solder joint solder has too little solder infiltration, or there is a broken joint in the middle of the solder joint, or the solder surface is convex or spherical, or the solder does not blend with the SMD, it is necessary to pay attention, even a slight phenomenon may cause hidden dangers. It should be immediately judged whether there is a problem of batch soldering. The method of judging is to see if there are more solder joints in the same position on the PCB. For example, it is only a problem on the individual PCB, which may be caused by scraping of the solder paste, deformation of the pins, etc., such as the same position on many PCBs. There are problems, it is likely to be caused by poor components or problems with the pads.
Second, the cause and solution of the virtual welding
1. The pad design is defective. The presence of vias in the pads is a major defect in PCB design. It is not necessary to use them. Do not use them. The vias will cause solder loss and solder shortage. The pad pitch and area also need standard matching. Otherwise, the design should be corrected as soon as possible.
2. The PCB has an oxidation phenomenon, that is, the pad is not bright. If there is oxidation, use an eraser to remove the oxide layer to make it bright. The PCB board is damp and can be dried in a dry box if suspected. The PCB board is contaminated with oil stains, sweat stains, etc., at this time, it should be cleaned with absolute ethanol.
3. The PCB on which the solder paste is printed, the solder paste is scraped and rubbed, so that the amount of solder paste on the relevant pads is reduced, so that the solder is insufficient. It should be replenished in time. The method of supplementing can be done with a dispenser or with a bamboo stick.
4. SMD (surface mount components) is of poor quality, expired, oxidized, deformed, resulting in virtual soldering. This is the reason why it is more common.
(1) The oxidized component is not bright. The melting point of the oxide increases,
At this time, more than three hundred degrees of electric ferrochrome and rosin-type flux can be used for welding, but it is difficult to melt with more than two hundred degrees of SMT reflow and a less corrosive no-clean solder paste. Therefore, the oxidized SMD should not be welded by the reflow oven. When buying components, be sure to see if there is oxidation, and use it when you buy it back. Similarly, oxidized solder paste cannot be used.
(2) The surface mount components of multiple legs have small legs and are easily deformed under the action of external force. Once deformed, the phenomenon of virtual welding or lack of welding will occur. Therefore, it is necessary to carefully check and repair in time after welding.