Application of Nanoindentation and Microdiffraction to Study Aging Effects in Lead Free Solder Interconnects

Abstract

In this work, we have performed a study on aging effects in actual solder joints extracted from PBGA assemblies using nanoindentation. The tested joints were extracted from 14 × 14 mm PBGA assemblies (0.8 mm ball pitch, 0.46 mm ball diameter) that are part of the iNEMI Characterization of Pb-Free Alloy Alternatives Project (16 different solder joint alloys available). Using nanoindentation techniques, the stress-strain and creep behavior of the aged SAC solder materials were explored at the joint scale for various aging conditions. After aging, the joints were loaded in the nano-indentor, and the load-deformation behavior during indentation has been used to characterize the mechanical properties of the solder joint 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 correlate their magnitudes to those observed in tensile testing of miniature bulk specimens. Our results have shown that the aging induced degradations of the mechanical properties (modulus, hardness) of single grain SAC305 joints were similar to those seen previously by testing of larger “bulk” solder specimens. However, the degradation of the creep response, while still significant, was less in the solder joints relative to larger uniaxial tensile specimens. This was due to the single grain nature of the joints considered, and the lack of the grain boundary sliding creep mechanism.