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62 SMT Magazine • April 2015 speed of the mass of fluid during the break- off process will follow a more gradual passage through the three phases of filament break-off for a dense suspension controlled by inertia, viscosity and capillary forces, as described by Bonnoit [2] , and thus decrease the probability of islands of suspension that will develop into sat- ellites. In the observations above, quantities have been obtained as statistical quantities of a large number of droplets. This is appropriate on one level, since the positioning error of any droplet must be ensured to some level of confidence. At the same time, it is essential to understand that the rheological qualities of jetting fluids are dependent not only on the local shear on the fluid, but also on a relaxation time scale of the fluid. A plot of the viscosity of the solder paste as a function of the local shear on the fluid is presented in Figure 7. The relaxation time of the paste will pri- marily affect the first of a series of droplets as can be seen in Figure 8. The plot in Figure 8 shows the effect of various piezo actuation pro- files on the evolution of deposit diameter in a number of strips consisting of 20 deposits. For this set of piezo actuation profiles, it is appar- ent that the behaviour of the diameter of the first deposit is insensitive to the choice of actu- ation profile. If an adjustment of the actuation profile is inadequate to maintain a constant deposit diameter throughout the entire strip, it is possible that a preferential actuation pattern for the first deposit could be used to attend to this strip property. An example of jetting results for a 0.4 mm pitch uBGA can be seen in Figure 9. Although the photograph in Figure 9 only provides an aesthetic test of the jetting quality, a number of the quantified quantities described above can figure 7: viscosity of three solder pastes and an adhesive as a function of the local shear stress on the fluid. FeAture JETTiNG STraTEGiES FOr MBGaS: a QuESTiON OF GivE aND TaKE continues