Rheological techniques for measuring normal stress differences ofsolder paste PCBs

摘要

Like most solids in liquid suspensions at high solid volumefraction, solder paste exhibits very complicated rheological behaviour.Many rheological characteristics, such as shear thinning,pseudoplasticity, and thixotropy are observed in this material. Thesecharacteristics are known to have different impacts on the solderpaste's primary application: stencil printing. In fact, at variousstages of the printing process, different and sometimes conflictingrheological characteristics ate required of the solder paste. Thesecharacteristics have been receiving increasing attention in the solderpaste literature. However, one rheological property that has beenoverlooked is the normal stresses generated in the solder paste. Thesetwo stresses are the normal components to the shear stress. Thesestresses are only found in non-Newtonian materials and are part of itsnon-linear viscoelastic properties. This paper describes a methodologyto measure the normal stress differences of solder paste under steadyshear. In the method used to measure the normal stresses, a special typeof rheometer, known as a rheogoniometer is used. For comparative normalstress measurements, only certain types of geometries can be used forsolder paste. A typical shear rate experiment is described using coneand plate, and parallel plate geometries. Potential errors in themeasurements of normal stress differences are also discussed