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26 SMT Magazine • April 2017 PREDICTING SOLDER PASTE TRANSFER EFFICIENCY AND PRINT VOLUME RHEOLOGY MEASUREMENT Cone and Plate Rheometer A Malvern Kinexus rheometer fitted with a cone and plate probe was used to measure rhe- ology. The device among many other things, is capable of creating a continuum of shear rates, while measuring the viscosity of the solder paste at each shear rate. Figure 3 is a photo of the device. Figure 4 is a close-up of the cone and plate mechanism. Because of the heat that is de- veloped from the friction of the solder paste trapped between the cone and plate, a cool- ing unit is connected to the base plate. A wa- ter bath maintained at 25°C is adjacent to the device, and a continuous stream of 25°C water flows under the bottom plate, maintaining an isothermal condition for the viscosity measure- ment. Shear viscosity was measured while shear rate was continuously logarithmically ramped from 0.1 to 100 s-1 for a total measurement time of 2 minutes. 10 measurements were made per decade of shear viscosity. Results and Discussion Table 1 details the metal loading and ini- tial viscosities of the three pastes at room tem- perature (25°C). As mentioned above, the met- al loading needs to be decreased as powder size distribution decreases in order to obtain the same 10 RPM Malcolm measured viscosity. It is interesting to note that the divergence in viscosities tends to be greater at lower shear rates. Conversely, the viscosities converged at the higher shear rates. The print volume reproducability (CpK) of the pastes made with Type 3, 4 and 5 powders when the area ratio was 0.79 was 2.89 ±0.14 (Figure 5). This converts to an expected defect rate of less than 0.6 parts per trillion. The performance of the three pastes di- verged at a 0.54 aspect ratio (Figure 6). In other words, a 210 μ (.008") square aperture in a 100μ (.004") thick stencil. The paste made with Type 3 powder showed a CpK of 1.23. This translates in an estimated defect rate of greater than 100 per million opportunities. The paste using Type 4 powder showed a CpK of 1.82, translating to an expected defect rate of approximately 50 per billion opportunities. Finally, the paste that used the type 5 pow- der showed a CpK of 3.48, translating to an in- Figure 4: Close-up of the cone and plate mechanism. Table 1: Metal Loading and Viscosity of 3 Pastes Used. Figure 3: Cone and plate rheometer.