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<p>Traditional Sn–Ag–Cu (SAC) solders face significant challenges in advanced electronics applications, particularly in 5G technology and electric vehicles.&#xa0;Their inadequate thermal fatigue resistance and susceptibility to mechanical degradation at elevated temperatures limit reliability. Strategic incorporation of alloying elements offers a promising solution for enhancing SAC properties. Bismuth (Bi) and nickel (Ni) are particularly crucial, mitigating intermetallic compound (IMC) growth, refining microstructure, and enhancing mechanical strength. This study investigates the effects of adding 0.1 wt.% Ni and 3.0 wt.% Bi to Sn-3.5Ag-0.5Cu (SAC355) solder. We specifically analyze their impact on thermal parameters, microstructure, phase transformations, and mechanical properties to address these performance gaps. A comprehensive analysis was conducted on directionally solidified SAC355 using advanced characterization techniques, including optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and tensile testing. The base SAC355 alloy exhibited a dendritic microstructure with large Ag₃Sn and Cu₆Sn₅ platelets, along with a eutectic mixture of β-Sn. Ni addition (0.1 wt.%) effectively suppressed β-Sn dendrite coarsening and facilitated the formation of thermally stable (Cu,Ni)₆Sn₅ IMCs. Incorporating 3.0 wt.% Bi generated nanoscale Bi-rich precipitates within the β-Sn matrix. These precipitates inhibited IMC overgrowth and promoted effective grain boundary pinning. The combined Ni and Bi additions produced a unique bimodal IMC architecture: coarse (Cu,Ni)₆Sn₅ and fine Ag₃Sn. This synergy stabilized Cu₆Sn₅ against polymorphic phase transformations. DSC analysis confirmed critical thermal enhancements. The melting temperature decreased significantly from 214.1°C (base) to 209.8°C, while the pasty range narrowed from 10.7°C to 6.7°C. This reduction is critical for minimizing thermal stress in temperature-sensitive devices. Tensile testing demonstrated superior mechanical performance for the SAC355-Ni-Bi alloy. This enhancement, attributed to Bi’s solid-solution strengthening and Ni’s IMC stabilization,&#xa0;improved ductility (elongation increased by 167%) while mitigating brittle fracture.</p>

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Microstructural and mechanical enhancements in Sn–Ag–Cu solder alloys via nickel and bismuth additions

  • 期刊论文

  • 发表日期：

    2025年9月23日

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  • A. M. El-Taher,
  • M. M. Fadel,
  • Sally A. Eladly,
  • Ashraf S. Abd Elrahman,
  • H. Algarni,
  • H. Elhosiny Ali,
  • M. Ragab

摘要

Traditional Sn–Ag–Cu (SAC) solders face significant challenges in advanced electronics applications, particularly in 5G technology and electric vehicles. Their inadequate thermal fatigue resistance and susceptibility to mechanical degradation at elevated temperatures limit reliability. Strategic incorporation of alloying elements offers a promising solution for enhancing SAC properties. Bismuth (Bi) and nickel (Ni) are particularly crucial, mitigating intermetallic compound (IMC) growth, refining microstructure, and enhancing mechanical strength. This study investigates the effects of adding 0.1 wt.% Ni and 3.0 wt.% Bi to Sn-3.5Ag-0.5Cu (SAC355) solder. We specifically analyze their impact on thermal parameters, microstructure, phase transformations, and mechanical properties to address these performance gaps. A comprehensive analysis was conducted on directionally solidified SAC355 using advanced characterization techniques, including optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and tensile testing. The base SAC355 alloy exhibited a dendritic microstructure with large Ag₃Sn and Cu₆Sn₅ platelets, along with a eutectic mixture of β-Sn. Ni addition (0.1 wt.%) effectively suppressed β-Sn dendrite coarsening and facilitated the formation of thermally stable (Cu,Ni)₆Sn₅ IMCs. Incorporating 3.0 wt.% Bi generated nanoscale Bi-rich precipitates within the β-Sn matrix. These precipitates inhibited IMC overgrowth and promoted effective grain boundary pinning. The combined Ni and Bi additions produced a unique bimodal IMC architecture: coarse (Cu,Ni)₆Sn₅ and fine Ag₃Sn. This synergy stabilized Cu₆Sn₅ against polymorphic phase transformations. DSC analysis confirmed critical thermal enhancements. The melting temperature decreased significantly from 214.1°C (base) to 209.8°C, while the pasty range narrowed from 10.7°C to 6.7°C. This reduction is critical for minimizing thermal stress in temperature-sensitive devices. Tensile testing demonstrated superior mechanical performance for the SAC355-Ni-Bi alloy. This enhancement, attributed to Bi’s solid-solution strengthening and Ni’s IMC stabilization, improved ductility (elongation increased by 167%) while mitigating brittle fracture.