Nanomechanical characterization of SAC solder joints - reduction of aging effects using microalloy additions

In this work, we have examined the ability of microalloy additions (dopants) to reduce aging effects in solder joints by nanoindentation testing of several sets of doped/non-doped alloys. The investigated solder joint alloys included SAC105, SAC105+Ni, and SAC105+Mn. For the doped alloys, the base SAC105 solder in the PBGA component solder balls was modified by microalloying an additional small amount (< 0.05%) of the dopant material (Ni, Mn, etc.). The tested joints were extracted from 14 × 14 mm PBGA assemblies (0.8 mm ball pitch, 0.46 mm ball diameter) that were part of the iNEMI Characterization of Pb-Free Alloy Alternatives Project. After extraction, the joints were then subjected to various aging conditions at high temperature (0–6 months aging at T = 125 °C). After aging, the joints were loaded in the nanoindentation system, and the load-deformation behavior during indentation was used to characterize the mechanical properties of the solder joints for various aging conditions including modulus, hardness, and yield stress. Using constant force at max indentation, we have also measured the creep response of the aged and non-aged solder joint materials for various stress levels. With this approach, we have been able to quantify aging effects in joints and compare the behavior of the standard and doped alloys. Our results have shown that addition of the microalloy elements significantly reduced the aging degradations of the mechanical properties and creep resistance in the joints. For example, the average reduction of the effective elastic modulus of SAC105 joints was 32.9% with 180 days of aging, while the analogous average reductions were 7.0% and 8.1% for the SAC105+Ni and SAC105+Mn joints. Similarly, the average hardness (yield stress) degradation for the SAC105 joints was 45.9%, while those for the SAC105+Ni and SAC105+Mn were 10.5% and 10.7%, respectively. Finally, the creep rate increased 69.1X for the aged SAC105 joints, while the creep rate degradations for the SAC105+Ni (6.5X) and SAC105+Mn (9.0X) were an order of magnitude smaller. Our findings have also suggested that the addition of Ni was slightly more effective than Mn for the SAC105+X dopants.