Analysis of mechanical properties in lead-free solders subjected to flash aging

Abstract

Lead-free solder alloys, particularly those based on tin, silver, and copper, have recently become very popular because of their better mechanical and electrical characteristics and environmentally friendly nature compared to lead-based solder alloys. Although previous studies have focused on how aging and doping affect the stress–strain behavior of SAC305 at the macroscale, this study investigates the effects of isothermal flash aging and zinc (Zn) doping on SAC305 at the nanoscale. Using molecular dynamics simulations, the stress–strain behavior of the SAC305 solder alloy with different percentages of Zn doping is analyzed after subjecting the alloys to varying isothermal flash aging conditions. The thermal aging process involves heating from 300 K to 420 K at 5 K ps⁻¹, holding at 420 K for durations ranging from no aging to 5 nanoseconds, and cooling back to 300 K at the same rate. The stress–strain behavior of these materials is examined under tensile loading at a strain rate of 1 × 10⁹ s⁻¹. The results show that mechanical parameters such as the ultimate tensile strength (UTS), Young's modulus (YM), and energy absorption capacity are greatly affected by isothermal aging. Longer aging durations lower the average atomic volume (AV) and produce denser materials, thereby improving the UTS and YM. However, the priority for long-term solder joint reliability is mechanical stability, which is best achieved through minimal changes in these properties over time. Radial distribution function (RDF) fluctuations are observed with aging, showing changes in the atomic structure. Among the Zn doping levels tested, 0.75% Zn proved to be the most effective in minimizing the effects of aging, as it exhibited the lowest RDF fluctuation with aging, along with the least variation in the UTS, YM, and energy absorption capacity, thus providing the best mechanical stability. This study highlights the potential of nanoscale Zn doping as an effective method for enhancing the mechanical reliability of lead-free solder alloys for high-performance and environmentally friendly electronic applications.