Effect of particle size distributions on the rheology of Sn/Ag/Cu lead-free solder pastes

The dependence of viscosities on the particle size distribution of lead-free solder powders (Sn/Ag/Cu) is investigated using a parallel-plate rheometer. Experimental results show that for a fixed weight fraction of particles, as the increase of size ratios, the viscosities of the solder paste decrease especially at low shear rates. In addition, for solder paste with more small particles introduced, shear-thickening appears before the onset of shear thinning. The higher the relative fraction, the more dramatically viscosities increase, which is consistent with modeling results using Cross equation. A possible mechanism is proposed to reveal the relationship between microstructure and corresponding rheological behaviors.