Modeling the effects of temperature on the rheology of solder pastes and flux system

Solder paste is one of the most important process materials today in surface mount technology. Stencil printing of solder paste onto PCBs constitutes an important stage in the reflow soldering of surface mount devices. A high proportion of the solder-related defects can be attributed to the stencil printing process. This is likely to continue with the trend toward miniaturization and the implementation of die-size packages. To achieve repeatable solder deposits from board-to-board and pad-to-pad requires an understanding of the paste rheology. One of the key factors that influences solder paste rheology is temperature. A change in temperature will cause the viscosity of the solder paste to change. This change could be ambient or from the stencil printing process itself. This is likely to impact on the performance of the solder paste. In this paper, we present the effect of temperature on the rheological properties of solder paste and the flux vehicle system. Current models show a single variable dependence of viscosity with temperature. The model presented here incorporates shear rates and can be used for any solder paste or non-Newtonian material. The effects of temperature on solder paste flux medium, particle size and distribution, and metal alloy content are also presented.