Thixotropy flow behaviour of solder and conductive adhesive pastes

This paper presents results on the thixotropic behavior of two suspensions; solder paste and isotropic conductive adhesives (ICAs). These materials are widely used as bonding medium in the electronics industry. Solder paste are metal alloys suspended in a flux medium while the ICAs consist of silver flakes dispersed in an epoxy resin. The thixotropy behavior was investigated through two rheological test; (i) hysteresis-loop test and (ii) steady shear rate test. In the hysteresis-loop test, the shear rate was increased from 0.01 to 10 s(-1) and then decreased from 10 to 0.01 s(-1). Meanwhile, in the steady shear rate test, the materials were subjected to a constant shear rate of 0.1, 1 and 10 s(-1) for a period of 1800s. The solder paste exhibited a higher degree of structural breakdown compared to the isotropic conductive adhesives (ICAs). Both the suspensions showed a high degree of shear thinning behavior with time. Existing thixotropy model such as Weltman and Hahn were applied to understand the rate of structural change for solder paste and ICAs. The Weltman model (rgreater than or equal to0.95) showed a strong correlation with the experimental data compared to Hahn model (rless than or equal to0.95). The rate of structural breakdown increases with the value of B-1, from Weltman model. The change in the microstructure of solder paste increases with the B-1 but these trends were only observed for isotropic conductive adhesives at lower shear rates. (C) 2004 Kluwer Academic Publishers.