A Method for the Rheological Characterization of Thick-Film Pastes

A complete rheological profile of a thick-film ink can provide an understanding of the effects of composition on properties and enables one to predict and control the screening behavior. A complete profile includes the thixotropic and pseudoplastic properties as well as the viscosity behavior during post-shear recovery when leveling takes place. This paper presents a set of experimental techniques with which to measure shear thinning and recovery as experienced during the screening process. The basic problem in the measurement of the viscosity profile of a pseudoplastic paste during recovery is that the shearing involved in making the measurement itself causes a change in the paste viscosity. We have developed a technique which measures viscosity at essentially zero shear. The method involves the relaxation of a spring-driven cone against the viscous drag of the paste repeated at short intervals during the recovery. The moment of inertia of the cone, the compliance of the viscometer spring, the cone geometry, and the shearing action are critical factors to consider in obtaining accurate and reproducible values of the viscosity. Experimental data describing the measurement of viscosity of Newtonian standards and of the theological characteristics of thick-film pastes are presented and discussed.