Rupture time analyses of the Sn–3.5Ag solder alloys containing Cu or Bi Original Research Article

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 ε̇min were lowest for the 0.75 Cu specimens, and highest for the 10 Bi specimens. The stress exponents (n) of ε̇min were usually around 4 ± 1, with the exception of the 0.5 Cu and 0.75 Cu alloys with n=6.0∼7.6, and the 10 Bi alloy which showed n ~ 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 β-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.