1. Insufficient flux activity.
2. The wettability of the flux is insufficient.
3. The amount of flux coating is too small.
4. Uneven flux coating.
5. The circuit board can not be coated with flux in regions.
6. The circuit board is not tinted regionally.
7. Some pads or solder feet are seriously oxidized.
8. Unreasonable circuit board wiring (unreasonable distribution of components).
9. The direction of the board is wrong.
10. The tin content is insufficient, or the copper exceeds the standard; [the melting point (liquid line) of the tin liquid rises due to the excessive impurities]
11. The foaming tube is blocked and the foaming is not uniform, which causes the uneven coating of the flux on the circuit board.
12. The setting of the air knife is unreasonable (the flux is not evenly blown).
13. The board speed and preheating are not well matched.
14. Improper operation method when hand dipping tin.
15. The inclination of the chain is unreasonable.
16. The wave crest is uneven.
2. Improvement measures:
1. Design according to PCB design specifications. The long axis of the two end chips is perpendicular to the welding direction, and the long axis of the SOT and SOP should be parallel to the welding direction. Widen the pad of the last pin of SOP (design a thief pad)
2. The pins of the plug-in components should be shaped according to the hole distance and assembly requirements of the printed board. If the short soldering process is used, the soldering surface component pins should be exposed to the surface of the printed board by 0.8~3mm. correct
3. Set the preheating temperature according to the PCB size, whether it is a multilayer board, how many components, and where there are mounted components, etc.
4. The tin wave temperature is 250±5℃, and the soldering time is 3~5s. When the temperature is slightly lower, the conveyor belt speed should be slower
5. Replace the flux
