Effects of microstructure on the creep properties of the lead-free Sn-3.5Ag-Cu solders

The lead-free, Sn-3.5Ag and Sn-3.5Ag-0.7Cu alloys were prepared in two different forms; firstly, bulky as-cast alloys were cold rolled and thermally stabilized before the creep tests so that there would be very small amount of microstructural change during creep (TS), and secondary thin specimens were fast cooled from the melt (FC) to simulate microstructures of as-reflowed solders in flip chips. Cooling rates of the FC specimens ranged between 140 K/sec and 150 K/sec, and the resultant β-Sn globule size was about 5∼10 times smaller than that of the TS specimens. Subsequent creep tests showed ∼10² times lower ε_min and ∼10² times higher t_f for the FC specimens. Rupture time analyses based on the Kachanov equation proved to be accurate within a factor of three for all cases, however the model based on necking was good only for the TS specimens. Fractographic analyses raised a need to conduct creep tests with thicker specimens for the thin FC specimens.