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30 SMT Magazine • April 2017 distinct differences between the pastes made with the three types of powder are observed. All of the pastes show high normal force at high shear rates, which decrease with decreasing stress. However, the type 3 paste increases with decreas- ing stress below 100 Pa until and discon- tinuity is reached at about 20 Pa. In addi- tion, there is a strong hysteresis observed in the reverse sweep. The normal force remains almost constant with stress, un- til about 100 Pa, when it decreases slight- ly with stress until a minimum occurs at about 250 Pa and the normal force in- creases with stress. The paste made with type 5 powder shows similar increases of normal force between 100 and 1000 Pa, but almost constant normal force at stresses less than 50 Pa, the decreasing stress having high normal force than the increasing stress. The normal force be- havior for the type 4 paste is between that of the type 3 and type 5 pastes. Oscillation sweeps can also be used to look at the properties of solder pastes. During these measurements, the trun- cated cone rotates clockwise and coun- terclockwise sinusoidally. By making os- cillatory measurements, the solid-like (elastic, in phase with displacement) and liquid-like (viscous, out of phase with displacement) behavior of materials can be separated. Figure 8 shows frequency sweep measurement data. Here a step- wise sweep of frequencies between 10 and 0.1 Hz was made at a constant com- plex shear stress of about 10 Pa. Figure 8a shows complex modulus vs. frequen- cy, and in Figure 8b, the complex mod- ulus is divided into its in phase (elastic, solid-like) and out of phase (viscous, liq- uidlike) components. For all three pastes, the elastic modulus reaches a steady val- ue at about 10 Hz, but the viscous mod- ulus continues to increase with frequen- cy. Likewise, Figure 8c shows viscosity decreasing with frequency. The T5 paste has the highest viscosity at low frequen- cy and T3 the lowest viscosity, with T4 being somewhere in between. PREDICTING SOLDER PASTE TRANSFER EFFICIENCY AND PRINT VOLUME Figure 7: Shear stress sweep: (a) shear rate vs. shear stress; (b) shear viscosity vs. shear stress; (c) normal force vs. shear stress.

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