Mechanical Characterization of SAC Solder Joints at High Temperature Using Nanoindentation

In this work, we have used nanoindentation methods to explore the creep behavior, and aging effects of SAC305 solder joints at several elevated testing temperatures from 25 to 125 oC. A special high temperature stage and test protocol was used within the nanoindentation system to carefully control the testing temperature, and make the measurements insensitive to thermal drift problems. Solder joints were extracted from 14 × 14 mm PBGA assemblies (0.8 mm ball pitch, 0.46 mm ball diameter) that were built as part of the iNEMI Characterization of Pb-Free Alloy Alternatives Project. Since the properties of SAC solder joints are highly dependent on crystal orientation, polarized light microscopy was utilized to determine the orientation of the tested joints. For all the experiments, only single grain solder joints were used to avoid introducing any unintentional variation from changes in the crystal orientation across the joint cross-section. After extraction, the single grain solder joints were subjected to various aging conditions. Nanoindentation testing was then performed on the aged specimens at five different testing temperatures (T = 25, 50, 75, 100, and 125 oC). In order to understand creep response of the solder joints at different temperatures, a constant force at max indentation was applied for 900 sec while the creep displacements were monitored. With this approach, we were able to measure the creep strain rate as a function of both temperature and prior aging conditions. As expected, our results have shown that indent/testing temperature has a significant impact on the mechanical properties and creep strain rate of solder joints. The measured data have also shown that the effects of aging on solder joints properties become much more significant as the testing temperature increases. In particular, the aging induced degradation rates at high temperatures (100-125 oC) were more than 100X those seen at room temperature. Nanoindentation pile-up effects, although insignificant at room temperature, were observed during high-temperature testing and corrections were made to limit their influence on the test results.