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24 SMT Magazine • April 2017 Commercially available solder pastes use an upper and lower limit for viscosity. Generally, this viscosity is measured at one shear rate, typi- cally the shear associated with a spiral viscometer rotating at 10 revolutions per minute (RPM). If the powder contained in solder paste is dissolved by the acid activator in the solder paste flux, the viscosity of the solder paste will quickly increase. If the viscosity increases more than 20 to 30% above the upper limit of the specification, poor printing results will be highly likely. The solder paste will not roll evenly over the stencil and ap- ertures in the stencil will remain unfilled, causing skips in the paste printing pattern. The purpose of the study was to determine if there is a correlation between print yields and rheology. The ultimate goal would be to use rheological measurements to better predict the print volume and print volume repeatability of any sample of solder paste. Another benefit of this test method is the reduction in wasted solder paste. Using a spiral viscometer requires an entire 500-g jar of paste. Because of the lengthy exposure to relatively high shear, this 500-g sample is no longer suit- able for use in an assembly process. Only a very few grams of solder paste are used in the cone and plate rheology measurement. This study was limited to one no-clean paste flux, SAC 305 lead-free powder and three parti- cle size distributions. It should be noted that wa- ter soluble solder paste also can increase in vis- cosity and become unprintable due to the same mechanism. In fact, water soluble pastes may tend to do this more quickly when exposed to elevated temperatures. They also can increase in viscosity in the presence of very low humidity. Evaluating water soluble paste behavior is be- yond the scope of this study. Experimental Conditions Three solder pastes were manufactured for the experiment using commercial scale equip- ment. There have been multiple examples of lab size solder paste batches showing differing rheological properties versus commercial scale batches. In this experiment the same batch of flux was used to make all three pastes. Note that using a given lot of paste flux, varying the particle size distribution at a fixed metal loading will lead to different viscosity results. A Type 3 powder at 89% metal load- ing will have a lower viscosity than a Type 4 powder at the same metal loading. These dif- ferences in viscosity are more pronounced at lower shear rates. Figure 1 shows one lot of flux with two PSDs of SAC 305 powder at one met- al loading. The three pastes were also subjected to a standard print volume and print volume re- peatability evaluation. A standard test vehicle was used. (Figure 2), using an uncoated .004 in. (100μ) thick, laser cut stainless steel sten- cil. Print volumes were measured using a Koh Young Aspire solder paste inspection (SPI) de- vice. These measurements were then exported to Minitab statistical analysis software program and process capability was reported. PREDICTING SOLDER PASTE TRANSFER EFFICIENCY AND PRINT VOLUME Figure 1: Viscosity (Y-axis) vs. viscometer rotational velocity (RPM X-axis). Figure 2: Print volume test vehicle.