The influence of wall slip in the measurement of solder paste viscosity

The wall slip phenomena is known to have a significant effect on the measurement of the viscosity of dense suspensions. In the measurement of the viscosity of solder pastes the effect of wall slip is such that the measured viscosity (also called the apparent viscosity) is much lower than the true viscosity of the paste. Therefore, correction needs to be applied to the measured viscosity in order to obtain the true viscosity of the solder paste. In this paper, we present work on the modeling of the influence of wall slip on viscosity measurement, and a model for predicting the true viscosity based on measurements using parallel plate viscometer. The apparent viscosity values measured at two different plate gaps, but at the same applied shear rate (also called the apparent shear rate), is used for predicting the true viscosity, the wall slip velocity and the thickness of the boundary slip layer. The model was validated using results from solder paste samples measured at three different plate gaps (H=0.5 mm, 1.0 mm and 1.5 mm). Our results show that the predicted values of the true viscosity using the data measured at any two gaps are in reasonably good agreement. The results also show that the influence of the wall slip is significant and that the ratio of the predicted viscosity to the apparent viscosity decreases with increasing apparent shear rate.