The reasons for the appearance of the "tin bead" of the SMT surface and the preventive control method
In the SMT soldering process, we consider that the potential reason to cause the “tin bead” of soldering point are highly possibility in the Reflow oven temperature setting, SMT Reflow soldering time, solder paste quality, solder paste printing thickness, and screen quality, as well as the mounter pressure. So finding out the root the problem of “ tin bead” and do the anticipate control to avoid the “ tin bead” in your PCB SMT process.
A. Solder paste quality is the possibly to cause the “tin bead”.The metal content in the solder paste. The quality of metal in solder paste is about 89-91%, and the volume ratio is about 50%.In general, the more metal content, the more closely the metal powder in the solder paste, the more chance that the powder of tin powder will be combined together and not be blown apart in the process of gasification, so it is not easy to form "tin bead".We concluded that If the metal content decreases, the probability of "tin bead" increasesd.
B． Solder paste in the oxide content. The oxide content of the solder paste also affects the soldering effect. The higher the oxide content, the greater the surface tension during the fusion of the metal powder with the solder pad, and in the "reflow soldering" the oxide content of the metal powder Content will increase, which is not conducive to the molten solder completely "wetting" resulting in small tin beads.
C.Solder paste in the metal powder size. Solder paste in the metal powder is a very small nearly spherical, commonly used solder powder diameter of about 25-45μm between the finer powder oxide content is low, which will make the "tin" phenomenon eased .
D.Solder paste thermal collapse effect. In the reflow oven preheating stage, if the solder paste is not very resistant to thermal collapse, the solder paste that has been printed before the soldering temperature (before the solder begins to melt) begins to collapse and some of the solder paste flows outside the solder pad , The solder begins to melt. Because of the internal stress, the solder paste shrinks into the solder joint and begins to infiltrate and climb to the solder tip. Sometimes there is not enough solder paste outside the solder paste due to lack of flux or other reasons. Shrinking back, when it is completely melted to form a "tin beads."
Thus, the quality and selection of solder paste also affect the production of solder beads, solder paste in the metal and oxide content, particle size of metal powder, solder paste thermal collapse effect in varying degrees affect the "tin "Formation.