On-line non-destructive ultrasonic rheology measurement of solder pastes

Seman, Anton (2010) On-line non-destructive ultrasonic rheology measurement of solder pastes. PhD thesis, University of Greenwich.

Abstract

This thesis concerns the study of a non-destructive ultrasonic technique for characterising the rheological properties of solder pastes and, specifically, the use of through-mode microsecond ultrasonic pulses for evaluation of viscoelastic properties of solder paste materials. A wide range of flux systems and solder alloy particle distributions used in the industry are investigated to determine the correlation of the ultrasound attenuation and velocity to the viscosity of the solder paste and their correlation to paste printing performance. The work is part of a bigger study aimed at the development of an on-line quality control technique for paste manufacture based on both conventional rheological testes and ultrasound measurements.

A comparative study of standard fluids and both commercial and newly formulated solder pastes and flux vehicle systems demonstrated the utilisation of the ultrasound technique for on-line, non-destructive measurement of the viscosity of non-Newtonian materials such as solder pastes. The viscosity of the solder paste is governed by the intermolecular forces between the solder particles and the flux. The strength of these intermolecular forces depends on the probability of these particles rubbing up against one another while the paste is being sheared. The ultrasound viscosity results obtained were comparable to the rheometer viscosity results or to the viscosity provided by the solder paste manufacture. The ultrasound technique produced consistent results and was also proven to work at low temperatures. It may be used to help solder paste manufacturers to add the correct amount of flux or solder particles to their paste in order to reach a desired viscosity. Otherwise, it can be used as a quick go/no-go monitoring tool in the production line for predicting printing quality.

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