The reason for the tin bead problem
From a design point of view, the PCB pad design is unreasonable, and the large grounding pad of the special packaged device extends beyond the device pin too long.
The material package does not match the pad size, and the component body is pressed on the pad to cause the solder paste to overflow
After the solder paste is printed, the pressure of the chip is too large, and some of the solder paste is squeezed out of the pad to the bottom of the component body or outside of the pad. During reflow soldering, the part of the solder paste that has been squeezed out cannot be properly retracted onto the pad. , The formed tin beads.
During the reflow soldering process, the slope of the temperature curve is too fast, and the flux solvent in the solder paste is violently vaporized to produce an explosive spray, which leads to the splashing of tin powder and the formation of tin beads around the pad.
In the process of solder paste printing, since the bottom of the steel mesh is not cleaned, there is residual tin powder around the PCB pad. During the reflow soldering process, tin beads can also be generated.
If the stencil hole design is to directly open the hole according to the pad size 1:1 in the gerber file, no review and optimization will be done. In this way, the solder paste is printed with full openings, and the solder paste is extruded from the pad after component placement and reflow soldering. After the formation of tin beads.