Nanomechanical Characterization of Aging Effects in Solder Joints

Abstract

Over the past decade, it has been demonstrated the large changes that occur in the mechanical (constitutive) response and failure behavior of lead free solders after exposure to isothermal aging. For example, measured stress-strain data illustrate large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 50%) during the first 6 months after reflow solidification. Even more dramatic evolution was observed in the creep response of aged solders, where up to 10,000X increases were found in the steady state (secondary) creep strain rate (creep compliance) of lead free solders that were subjected to elevated temperature aging at 125 C. All of prior work was based on uniaxial testing of miniature bulk solder tensile specimens formed in glass tubes. However, there has been little work on aging effects on mechanical properties and creep behavior in individual solder joints. Such knowledge is crucial for optimizing the design, manufacturing, and reliability of microelectronic packages. Characterization of individual joints is quite challenging because of their extremely small size, and the difficulty in gripping them and applying controlled loadings. In this work, aging phenomena in actual solder joints have been explored by nano-mechanical testing of single lead free solder joints extracted from PBGA assemblies. The lead free solder joints in this study were extracted from Amkor CABGA, 14 x 14 mm, 192 balls, 0.8 mm ball pitch, 0.46 mm ball diameter. A variety of samples with 10 different solder ball alloys were studied. After extraction, the joints were subjected to various aging conditions (0 to 12 months of aging at T = 125 C). Using nanoindentation techniques, the stress-strain and creep behavior of the SAC solder materials were explored at the joint scale for various aging conditions. Mechanical properties characterized as a function of aging include the elastic modulus, hardness, and yield stress. Using a constant force at maximum indentation, the creep response of the aged and non-aged solder joint materials was also measured as a function of the applied stress level. With these approaches, aging effects in actual solder joints are being quantified and correlated to the magnitudes of those observed in testing of miniature bulk specimens. Empirical models were also developed to predict the observed behavior. In addition, an approach has been developed to predict tensile creep strain rates for low stress levels using nanoindentation creep data measured at very high compressive stress levels. In a second aspect of this study, the effects of silver content on SAC solder aging has been evaluated by testing joints from SACN05 (SAC105, SAC205, SAC305, and SAC405) test boards assembled with the same reflow profile. The observed aging effects in the SACN05 solder joints have been quantified and correlated with the magnitudes observed in tensile testing of miniature bulk specimens performed in prior studies. The ability of microalloy additions (dopants) to reduce aging effects in solder joints was also examined by nanoindentation testing of several sets of doped/non-doped alloys. The investigated solder joint alloys included: (1) SAC105, SAC205, SAC105+Ni, SAC105+Mn and SAC205+Ni and (2) low silver doped alloys, SAC0307, SAC0307+Bi (SACX) and SN100C. For the doped alloys, the base SAC solder in the PBGA component solder balls was modified by microalloying an additional small amount (< 0.05%) of the dopant material (Ni, Mn, Bi etc.). 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. With this approach, aging effects in joints have been quantified and compared to the behavior of the standard and doped alloys. Due to the variety of crystal orientations realized during solidification, it was important to identify the grain structure and crystal orientations in the tested joints. Polarized light microscopy and Electron Back Scattered Diffraction (EBSD) techniques have been utilized for this purpose. As another part of this work, the enhanced x-ray microdiffraction technique at the Advanced Light Source (Synchrotron) at the Lawrence Berkeley National Laboratory was employed to characterize several joints after various aging exposures (0, 1, and 7 days of aging at T = 125 C). For each joint, microdiffraction was used to examine grain growth, grain rotation, sub-grain formation, and residual strain and stress evolution as a function of the aging exposure.