High Temperature Mechanical Behavior of SAC and SAC+X Lead Free Solders

In this work, we have investigated the mechanical behavior of several SAC and SAC+X lead free solder alloys at extreme high temperatures up to 200 oC. The studied alloys included SAC305 (96.5Sn-3.0Ag-0.5Cu), SAC_Q (92.8Sn-3.4Ag-0.5Cu-3.3Bi), and Innolot (90.95Sn-3.8Ag-0.7Cu-3.0Bi-0.15Ni-1.4Sb). The solder uniaxial test specimens were formed in high precision rectangular cross-section glass tubes using a vacuum suction process. The samples were initially cooled in a water bath and were later reflowed. The reflow profile was chosen to closely mimic profiles used for BGA assemblies, so that the obtained microstructures were similar to those found in typical solder joints. For each of 4 elevated temperatures (T = 125, 150, 175, and 200 oC), tensile stress-strain tests were performed at three strain rates (SR = 0.001, 0.0001, and 0.00001 sec-1). For each alloy and testing temperature, the stress-strain curve shape and high temperature tensile properties (initial modulus, yield stress, and ultimate tensile strength) of the solders were measured and compared. As expected, the results have shown significant degradations of the mechanical properties of lead-free solders at higher temperatures. However, it was found that the addition of dopants (e.g. Bi, Ni, and Sb) in the SAC+X alloys improved their high temperature properties significantly. The measured data have been used to determine the Anand model parameters for each alloy. Good correlations were obtained between the Anand model predictions and the experimental data over a wide range of strain levels, temperatures, and strain rates.