Anomalous creep in Sn-rich solder joints

This paper discusses the creep behavior of example Sn-rich solders that have become candidates for use in Pb-free solder joints. The specific solders discussed are Sn-3.5Ag, Sn-3Ag-0.5Cu, Sn-0.7Cu and Sn-10In-3.1Ag, used in thin joints between Cu and Ni/Au metallized pads. The creep behavior of these joints was measured over the range 60-130degreesC. The four solders show the same general behavior. At all temperatures their steady-state creep rates are separated into two regimes with different stress exponents (n). The low-stress exponents range from similar to3-6, while the high-stress exponents are anomalously high (7-12). Strikingly, the high-stress exponent has a strong temperature dependence near room temperature, increasing significantly as the temperature drops from 95 to 60degreesC. The anomalous behavior of the solders appears to be due to the dominant Sn constituent. Joints of pure So have stress exponents, n, that change with stress and temperature almost exactly like those of the Sn-rich solder joints. Surprisingly, however, very similar behavior is found in Sn-10In-3.1Ag, whose primary constituent is gamma-InSn. Research on creep in bulk samples of pure Sn suggests that the anomalous temperature dependence of the stress exponent is due to a change in the dominant mechanism of creep. Whatever its source, it has the consequence that conventional constitutive relations for steady-state creep must be used with caution in treating Sn-rich solder joints, and qualification tests that are intended to verify performance should be carefully designed.