Rupture time analyses of the Sn-3.5Ag solder alloys containing Cu or Bi

Abstract

The creep rupture properties of Sn-3.5Ag based ternary alloys with varying amounts of Cu or Bi were investigated using rolled and heat-treated bulk specimens. Nominal compositions of the third element were 0% (Base), 0.5, 0.75, 1.0 and 1.5% Cu and 2.5, 4.8, 7.5 and 10% wt% Bi, respectively. The alloys generally showed the secondary and tertiary creep characteristics only, and the minimum strain rates (<(ε)over dot>(min)) were lowest for the 0.75 Cu specimens, and highest for the 10 Bi specimens. The stress exponents (n) Of <(ε)over dot>min were usually around 4 +/- 1, with the exception of the 0.5 Cu and 0.75 Cu alloys with n = 6.0similar to7.6, and the 10 Bi alloy which showed n similar to 2. Fractographic analyses revealed typical creep rupture by the nucleation and growth of cavities in the matrix for the Base and Cu-containing alloy except the 1.5 Cu specimens, which showed cavity nucleation by the cracking of brittle Cu6Sn5 particles. On the other hand, all the Bi-containing alloys showed cavity nucleation on grain boundaries due to the segregation of Bi to the beta-Sn grain boundaries. Subsequent rupture time analyses showed that creep rupture by necking could predict rupture times of the Base and the Cu-containing alloys reasonably, and that the model based on the continual nucleation of cavitating facets could explain the rupture times of the Bi-containing alloys reasonably well. The Kachanov equation made reasonable predictions of the rupture time for all cases. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.