Dissolution of Stainless Steel Into Molten Sn-Bi Solder During Viscosity Measurement

Interface energy based on cluster model in molten Sn-Bi solder, which was related to Fermi energy and surface tension, was calculated to compare with the measurement results of viscosity . Viscosity measurements of the molten solder in carbon crucible had been carried out to detect the change of rotation torque of stainless steel 304 cone plate with dissolving into the solder. The operation temperature was 160 degree Celsius, and shear speed was 105.6 rpm. An increase of rotation torque was observed about 200 minutes after from dipping of the cone plate into molten solder.  After the viscosity test, a scanning electron microscope equipped with EDX observation of the surface of stainless steel cone plate contacted to the solder had been carried out. In the observed area of the plate, Sn and Bi elements could not be found out. Thus, this phenomenon was caused by elution of Fe elements of stainless steel into Sn, and it was identified with the increase of the liquids line of the tested solder. This phenomenon was considered to be related with the hall effect of Bi, due to calculated interfacial energy.