A comparison of the creep behavior of joint-scale SAC105 and SAC305 solder alloys

In this paper, the creep behavior of Sn98.5Ag1.0 Cu0.5 (SAC105) solder alloy is experimentally characterized and compared with that of Sn96.5Ag3.0Cu0.5 (SAC305). Both solder alloys were characterized at joint-scale ({\sim}{\rm 180}~\mu{\rm m} thickness) under conditions of shear. In terms of motivation, SAC105 is more cost effective than SAC305 due to its lower silver content, however, it is known to exhibit inferior reliability under stimuli which induce thermal fatigue. The objective of this paper is to quantify the behavior of SAC105 when subjected to creep loading at different temperatures. To this end, the parameters of an Anand viscoplastic constitutive model are derived for thermally preconditioned SAC105 from a series of creep tests under varying constant shear stresses (5 to 15 MPa), constant shear strain rates [1E-6 (1/s) to 1E-2 (1/s)] and temperatures (20 to 100°C). The predictions of the Anand model are compared graphically with the experimental data to illustrate goodness-of-fit, and the data is found to be tightly bound to the Anand predictions. An extensive comparison is made of the creep behaviors of the two SAC alloys. The microstructure of both alloys is investigated and found to consist of β-Sn dendrites with precipitates of Ag₃ Sn and Cu₆ Sn₅ in the interdendritic region. SAC305 is noted to have more Ag₃ Sn precipitates and smaller tin dendrites than SAC105, which are the primary reasons for its superior creep resistance. From a practical perspective, SAC105 is found to be significantly less creep resistant than SAC305. SAC105 is, however, preferable to SAC305 for applications which may experience high strain rate stimuli (e.g., portable electronic devices) as it undergoes plastic flow at lower stress levels. SAC305 is superior for applications which feature thermomechanical fatigue (e.g., desktop computers and servers) as it accumulates less strain than SAC105. The Anand model parameters presented here could, however, enable practitioners to determine the suitability of the lower cost SAC105 alloy for applications which induce thermal fatigue.