How does the phenomenon of tin-free solder joints occur during SMT welding?

The phenomenon of solder joints without tin is usually called the missing printing phenomenon. It is mainly manifested in the phenomenon that the solder paste is not printed on the PCB board normally during the solder paste printing stage. The main reasons for the missing printing phenomenon are usually as follows:

1. If there is no tin on the pins before reflow soldering, it may be that the the holes on stencil is blocking and the viscosity of the solder paste is too high. At this time, it is necessary to clean the stencil, replace the new solder paste, and correct the parameters of the screen printing machine. Or the solder ability of components or pads is too low, or the PCB, components or pads are contaminated, thus this batch of incoming materials needs to be removed.

2. If there is no problem with the printing and placement of the patch, but after reflow soldering, it is found that some pins of some chips have insufficient soldering, it is likely that the amount of flux is insufficient, thus we need to increase its amount; or If it is excessive preheating, the preheating temperature should be lowered appropriately; or if the peak temperature of reflow soldering is too low, it should be adjusted to a moderate position.