1. Is the opening of steel mesh reasonable?
2. PCB plate oxidation, not easy to tin?
3. Is the design of PCB board reasonable?
4. Is the furnace temperature appropriate? Is it stable enough?
5. Is nitrogen added?
6. components oxidation, not easy to tin?
7. Check the parameter settings of SPI and historical SPI data. Is there any abnormal situation?
8. Check whether it is tin-sprayed board or not. If it is, when PCB is manufactured, the ink contaminates the pad so that it is resistant to be soldered.
9. Is solder paste in good condition?