Microstructure and impression creep behavior of lead-free Sn–5Sb solder alloy containing Bi and Ag

Creep behavior of the Sn–5Sb–1.5Bi, Sn–5Sb–1.5Ag, and Sn–5Sb–1Ag–1Bi alloys was studied by impression testing and compared to that of the Sn–5Sb base alloy. The tests were carried out under constant punching stress in the range 40–180 MPa and at temperatures in the range 298–373 K. Results showed that Sn–5Sb–1.5Bi had the lowest creep rate, and thus the highest creep resistance among all materials tested. This is mainly due to the strong solid solution hardening effect of Bi in Sn, which decreases the minimum creep rate. The creep resistance of the Ag-containing alloys was higher than that of the binary base alloy, due to the formation of spherical and rod-shape Ag3Sn particles along the grain boundaries and inside the grains. The average n-values of 5.3, 6.0, 5.6 and 5.1 were obtained at low stresses and 11.5, 12, 11.7 and 10.5 at high stresses for Sn–5Sb, Sn–5Sb–1.5Bi, Sn–5Sb–1.5Ag and Sn–5Sb–1Ag–1Bi, respectively. The low-stress regime activation energies in the range of 40.6–53.8 kJ mol−1, which are close to the value of 46 kJ mol−1 for dislocation climb, assisted by vacancy diffusion through dislocation cores in the Sn, and stress exponents in the range 5–6, suggest that the operative creep mechanism is dislocation viscous glide controlled by dislocation pipe diffusion. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 72 kJ mol−1 are indicative of a dislocation climb mechanism with back stress similar to those noted in dispersion strengthening.

► Addition of Bi to Sn–5Sb resulted in strong solid solution hardening.
► Addition of Ag to Sn–5Sb resulted in the formation of Ag3Sn compound.
► Addition of both Bi and Ag to the Sn–5Sb alloy improved creep resistance.
► The low stress creep mechanism was pipe-diffusion-controlled viscous glide.
► The high stress creep mechanism was dislocation climb with back stress.