Nanomechanical characterization of SAC305 solder joints - effects of aging

In this work, aging phenomena in solder joints have been explored by nano-mechanical testing. Using nanoindentation techniques, the elastic, plastic, and creep behavior of SAC305 solder joint materials have been explored for various aging conditions. Single crystal solder joints were first extracted from PBGA assemblies (14 × 14 mm, 0.8 mm ball pitch, 0.46 mm ball diameter), and then subjected to aging at T = 125 C for various aging times. After the aging exposures, the joints were loaded in the nano-indentor, 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, the creep response of the aged and non-aged solder joint materials have also been measured for various stress levels. With this approach, aging effects have been quantified in joints, and their magnitudes correlated to those observed in testing of miniature bulk specimens. The measured results have demonstrated that the mechanical behavior of actual solder joints degrade significantly with aging. The results show that the aging induced degradations of the mechanical properties (modulus, hardness) in the SAC joints were of similar order (30-40%) as those seen previously in the testing of larger “bulk” uniaxial solder specimens. The creep rate of the tested SAC305 joints were found to increase by up to 75X with one year of aging. These degradations, while significant, were much less than those observed in larger bulk solder uniaxial tensile specimens with several hundred grains, where the increase was over 900X. Additional testing has been performed on very small tensile specimens with approximately 10 grains, and the aging-induced creep rate degradations found in these specimens were on the same order of magnitude as those observed in the single grain joints. Thus, the lack of the grain boundary sliding creep mechanism in the single grain joints is an important factor in avoiding the extremely large creep rate degradations ocurring in larger bulk SAC samples. The test results also show that the elastic, plastic, and creep properties of the solder joints and their sensitivities to aging are highly dependent on their crystal orientations. Polarized light microscopy and Electron Back Scattered Diffraction (EBSD) were used to identify the grain structure and crystal orientations in the tested joints. Microstructural analysis of the joints has shown that Ag3Sn precipitates coarsen during isothermal aging, which results in fewer but larger particles. The Sn dendrite cell size also was observed to grow significantly, but the boundaries are no longer well defined due to coarsening.