Anomalously high tensile creep rates from thin cast Sn3.9Ag0.6Cu lead-free solder

Abstract

The present paper compares the creep and microstructural changes during creep behavior of bulk and thin cast forms of Sn3.9Ag0.6Cu. The processing parameters of the thin cast material were selected to result in a very fine microstructure analogous to what occurs in very small size solder electronic interconnections. We found that the thin cast material is less creep resistant than the bulk material. A comparison of Ag element maps between as-crept bulk and thin cast material shows that the relevant climb process occurs in a very different environment in the bulk material as compared to the thin cast material. In the bulk material, the relevant climb process occurs within a finely dispersed intermetallic compound (IMC) eutectic, which covers broad areas within the material. In the thin cast material, the relevant climb process occurs primarily in the beta-Sn grains that continuously surround isolated, coarse IMC particles. This resulted in the activation energy of the bulk material being larger than that for the thin cast material. Finally, it is important to note that the strength deficiency of the thin cast material is persistent—once the material is cast in thin cast form, it will remain weak in comparison to the bulk material. Therefore, using data obtained from bulk material samples for the construction of thermomechanical models of very small scale solder interconnections is likely to result in significant, intrinsic errors.