Principles and preventive measures of tin beads in SMT assembly
Oct 19, 2021
two Principle of solder ball formation in flux
one Classification of solder balls
According to the number and size of tin beads, it can be divided into 4 situations.
In the case of a single solder powder, the diameter is 10-40 µm. If the size is 50 µm or more, it is considered that multiple solder powders are fused.
two Principle of solder ball formation in flux
・Solder paste collapses when heated
The solder paste collapses when heated, but it does not completely connect the two pads (see the 0.1mm position), but forms a thin tin bead in the green oil bridge (see the 0.2mm position).
・Flux outflow
As the flux flows out during melting, the solder powder that dissolves later flows out.
three Common causes of tin bead formation
①Improper setting of reflow soldering temperature curve;
②The flux failed to play its role;
③The opening of the template is too large or deformed seriously;
④The placement pressure is too large when patching;
⑤The solder paste contains water;
⑥ The printed board is not cleaned, leaving solder paste on the surface of the printed board and through holes;
⑦Using non-contact printing or excessive printing pressure;
⑧ Flux failure.
Four Common methods to prevent tin beads
The solder mask on the PCB circuit board is the most important factor affecting the formation of tin beads. In most cases, choosing an appropriate solder mask can avoid the formation of tin beads. The use of some specially designed fluxes can help avoid the formation of tin beads. In addition, make sure to use enough flux so that when the PCB circuit board leaves the wave crest, there will be some flux residue on the PCB circuit board, forming a very thin film to prevent the solder beads from adhering to the PCB circuit board superior. At the same time, the flux must be compatible with the solder mask, and the spraying of the flux must be strictly controlled by a flux spray system.
1. Reduce the solder temperature as much as possible;
2. Using more flux can reduce tin beads, but it will result in more flux residue;
3. Increase the preheating temperature as much as possible, but follow the flux preheating parameters, otherwise the activation period of the flux is too short;
4. Faster conveyor belt speed can also reduce tin beads.