Investigation of the Mechanical Behavior of SAC305 Solder Joints at Extreme High Temperatures Using Nanoindentation

Solder joints provide mechanical support, and electrical and thermal interconnection between packaging levels in microelectronics assemblies. Proper functioning of these interconnections and the reliability of the electronic packages depend largely on the mechanical properties of the solder joints. Lead free solders typically provide excellent thermo-mechanical properties and are commonly used as interconnections in electronic packages. However, the exposure of lead free solder joints to complicated thermal histories can degrade their mechanical behavior. For example, aging of lead free solders at high temperature leads to microstructural evolution resulting in reduced mechanical properties and increased creep deformations. Previous investigations on mechanical characterization of lead free solders have mainly emphasized mechanical testing at temperatures up to 125 °C. However, many electronic products are deployed in harsh environment applications including well drilling, geothermal energy, automotive power electronics, and aerospace engines, where solders are exposed to very high temperatures from 125-200 °C. Mechanical properties of lead free solders at elevated temperatures are limited.In this work, we have explored Mechanical behavior and aging effects in SAC305 (96.5Sn-3.0Ag-0.5Cu) solder joints at several extreme high testing temperatures (T = 125, 150, 175, and 200 °C) using the method of nanoindentation. A special high temperature stage and test protocols were used within the nanoindentation system to carefully control the testing temperature, and to make the measurements insensitive to thermal drift problems. Solder joints were extracted from 14 x 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.To study aging effects, solder joints were preconditioned for 0, 1, 5, 10, and 30 days at T = 125 °C in a box oven. Nanoindentation testing was then performed on the aged specimens at four different test temperatures (T = 125, 150, 175, and 200 °C) to extract the elastic modulus, hardness, and creep performance of the aged material. Throughout this study, a constant force of 10 mN was applied for 900 seconds to monitor the creep displacements and measure the creep strain rate as a function of both temperature and prior aging conditions.As expected, our results have shown that mechanical properties and creep strain rate of solder joints are highly dependent on the test temperature and degrade significantly as the temperature increases. In particular, the aging induced degradation rates for high temperature testing (175-200 °C) were about 10000X than those observed at 125 °C. In addition, the effects of aging on solder joints properties become much more significant as the aging time and test temperature increases.