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Solder pastes and isotropic conductive adhesives (ICAs) are widely used as an interconnect in the electronic industry. Paste printing process accounts for majority of the assembly defects in the electronic manufacturing industry. This study investigates the effect of shear rates on the viscosities of the pastes (solder pastes and ICAs) used for flip chip assembly. Empirical models such as the power law and the Cross model were used to quantify the viscosity over a range of shear rates for solder pastes and fit to the experimental data. The shear rate dependence viscosity of solder pastes could be used to study the flow behavior experienced by the pastes during the stencil printing process. From the results, viscosities of all three types of pastes were said to be dependence on shear rate. In a stencil printing process, as if the viscosity of the solder paste was too high, the paste might not wet the surface of the substrate and more energy was needed to force the paste penetrate the aperture. This study has revealed that the fitting of the Cross model is generally of better quality than the power law model because the qualitative behavior of the Cross model throughout the whole range of shear rates (0.001 s−1 to 100 s−1) is essentially the same as the experimental data.

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