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28 SMT Magazine • April 2017 finitesimally small expected defect rate of 1 in 2.56 x 10-17 opportunities. The results are three rheology profiles that are different at low and medium shear rates, and three different print performance levels at an area ratio of 0.54. The ques- tion is whether or not there is a cause and effect between these two observations. A second type of rheology test that may be more relevant to the print process is the shear stress sweep. In this measure- ment, the shear stress is cycled from low to high and back to low. Intuitively, this is similar to a set of squeegee blades pushing a bead of paste back and forth. Figure 7 shows the results of the three test pastes. Again, there is a consistent pattern between the Type 3, 4 and 5 pastes. This is another indication that a rhe- ological footprint may be a pre- dictor of print quality. There is a distinct hyster- esis between the high to low stress sweep and the low to high sweep. At high stress, the sol- der paste behaves mostly like a viscous liquid and shows shear thinning, but as the stress de- creases and viscosity increases, a discontinuity occurs at about 100, 75 and 65 Pa for types 3, 4 and 5 powder respectively. The discontinuity viscosity is lower for the type 5 powder than the types 3 and 4. After the discon- tinuity, the viscosity decreases somewhat with decreasing shear stress. During the low stress to high stress sweep, the viscosity in- creases with shear stress until another discontinuity occurs at about 4 Pa for all three pastes, af- ter which the viscosity decreases slowly showing a quasi-solid be- havior until the stress is about 100 Pa when shear thinning starts. Figure 7c shows the normal force measured during the shear sweep measurements. The nor- mal force is perpendicular to the direction of the shear stress. The data is rather noisy, but PREDICTING SOLDER PASTE TRANSFER EFFICIENCY AND PRINT VOLUME Figure 5: Histogram of deposits @ AR=0.79. Figure 6: Histogram of print deposits @ AR=0.54.