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In the production and use of solder paste, particular factors must be considered that determine its viscosity because this property is of crucial importance for printing solder paste onto PCBs as well as for the homogeneous mixing of solder powder with flux. Theoretically, the motion of bodies can be described with Hooke's law (1676), in the case of elastic behavior, and with the Newtonian laws (1685), in the case of pure viscosity. However, fluids occurring in practice are partially subject to both and must therefore be considered under both aspects. This is the task of rheology.
The term derives from the Greek rhei (flow) and logos (study ) and dates back to 1928. Contrary to what one might expect, this field has a long history. In Japan, the first rheological research facility was founded in 1973, 45 years later than abroad. Lately, polymer rheology as a new subject has gained in importance in connection with the research of plastics. The behavior of solder paste is another important topic within the field.