Mechanical Characterization and Aging Behavior of iSAC Lead Free Solder
Authors: Mazumder Golam Rakib , Chakraborty Souvik , Maruf Mahbub Alam , Suhling Jeffrey C. , Lall Pradeep 
Published: November 2024

This study is focused on the evolution of mechanical properties of a new SAC+X lead-free solder material, commonly named iSAC. Energy Dispersive Spectroscopy (EDS) analysis reveals that this solder material has Bi, Ni, Zn, and Ge as additional elemental ingredients within the traditional Sn-Ag-Cu (SAC) matrix. The presence of these additional element helps restrict the coarsening of intermetallic compounds (IMCs) at copper pad interfaces, and thereby hinders crack propagation during harsh operating conditions that involve fluctuating temperatures. The additional elements can form solid solutions, and hard precipitates which can contribute to solid solution strengthening, and precipitate hardening — thus increasing the strength and hardness to the solder material, compared to other conventional SAC alloys.

To effectively define the thermomechanical performance of iSAC solder alloy, it is important to understand the evolutions of its micromechanical properties concerning the extent of its exposure to high-temperature isothermal aging conditions. Miniature tensile samples of the iSAC material with controlled microstructures were then fabricated by a reflow process in glass tubes. Uniaxial tensile tests were then performed on reflowed rectangular cross-section solder specimens over a range of temperatures and strain rates. Using the stress-strain data, the mechanical properties of the iSAC material were determined and compared to other lead-free alloys. Nano-indentation is often termed as an effective way to evaluate the micromechanical properties and hardness of lead-free solder alloys since it allows the evaluation of micromechanical properties within a fairly small test domain area within the alloy matrix. Also, in this study, bulk iSAC solder samples were first cut into small pieces and melted to form small uniaxial specimens. Those rectangular bar samples were then reflowed, following the supplier-provided reflow profile to incorporate joint-like microstructures within the samples. Selected samples were then subjected to various isothermal aging conditions (0, 1, 2, 5, 10, 20, and 30 days of aging) at 125 °C in a box oven. Finally, nanoindentation was performed on the samples to evaluate the effective elastic modulus and hardness on the iSAC material after various durations of aging. In addition, nanoindentation creep tests were performed to evaluate the evolution of the secondary creep rate with aging. The evolutions of the mechanical properties were plotted vs. aging time, and the results were compared to other lead-free alloys including SAC305, QSAC10, QSAC20, and SACQ.

The analysis of the mechanical properties and aging behavior of the iSAC alloy has revealed its superior performance compared to conventional pure SAC solder alloys like SAC305. In particular, the iSAC alloy exhibits a slower degradation rate of its mechanical properties over time. However, when compared to solder alloys containing an optimized level of Bi (2–3%), such as SACQ, the degradation rate of the mechanical properties on the iSAC alloy with isothermal aging fell within an intermediate range. These findings offer valuable insights for selecting solder alloys, particularly in harsh environmental conditions, where iSAC presents itself as a viable option, providing a balance between the performance of pure SAC and Bi-doped SAC solder materials.