Rheological techniques for measuring normal stress differences of solder paste [PCBs]

Like most solids in liquid suspensions at high solid volume fraction, solder paste exhibits very complicated rheological behaviour. Many rheological characteristics, such as shear thinning, pseudoplasticity, and thixotropy are observed in this material. These characteristics are known to have different impacts on the solder paste's primary application: stencil printing. In fact, at various stages of the printing process, different and sometimes conflicting rheological characteristics ate required of the solder paste. These characteristics have been receiving increasing attention in the solder paste literature. However, one rheological property that has been overlooked is the normal stresses generated in the solder paste. These two stresses are the normal components to the shear stress. These stresses are only found in non-Newtonian materials and are part of its non-linear viscoelastic properties. This paper describes a methodology to measure the normal stress differences of solder paste under steady shear. In the method used to measure the normal stresses, a special type of rheometer, known as a rheogoniometer is used. For comparative normal stress measurements, only certain types of geometries can be used for solder paste. A typical shear rate experiment is described using cone and plate, and parallel plate geometries. Potential errors in the measurements of normal stress differences are also discussed.