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<p>This study investigates electromigration (EM) failure in Sn-3.0Ag-0.5Cu (SAC305) flip chip solder joints featuring asymmetric Cu under bump metallization (UBM) thicknesses (25&#xa0;μm on the chip vs. 45&#xa0;μm on the printed circuit board (PCB)). Under three EM conditions (150&#xa0;°C/1.0 × 10<sup>4</sup> A/cm<sup>2</sup>, 150&#xa0;°C/0.7 × 10<sup>4</sup> A/cm<sup>2</sup>, and 135&#xa0;°C/1.0 × 10<sup>4</sup> A/cm<sup>2</sup>), the failure rate under the most severe condition (56.25%) was 1.29 and 2.25 times the rates observed under the other two conditions, respectively. Temperature influenced EM failure more significantly than current density. Solder joints in which the thinner Cu UBM served as the cathode exhibited rapid failure, characterized by near-complete dissolution of the Cu UBM and subsequent crack propagation, ultimately resulting in open-circuit failure. In contrast, only two joints failed when the thicker Cu UBM served as the cathode. Finite element simulations revealed that current crowding at the electron flow entrance (corresponding to the thinner cathode side) reached approximately 10 times the nominal current density, accompanied by localized Joule heating that elevated the temperature to 154.15&#xa0;°C. The severe current crowding, combined with localized Joule heating, significantly enhanced the Cu atomic flux, thereby accelerating open-circuit failure in the solder joints with the thinner Cu UBM serving as the cathode. These findings offer fundamental insights into the EM behavior of flip chip solder joints with asymmetric Cu UBM configurations.</p>

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Dominant role of thinner Cu UBM in electromigration failure of Sn-3.0Ag-0.5Cu flip chip solder joints

  • 期刊论文

  • 发表日期：

    2026年7月2日

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  • S. Meng,
  • H. L. Liu,
  • M. L. Huang

摘要

This study investigates electromigration (EM) failure in Sn-3.0Ag-0.5Cu (SAC305) flip chip solder joints featuring asymmetric Cu under bump metallization (UBM) thicknesses (25 μm on the chip vs. 45 μm on the printed circuit board (PCB)). Under three EM conditions (150 °C/1.0 × 104 A/cm2, 150 °C/0.7 × 104 A/cm2, and 135 °C/1.0 × 104 A/cm2), the failure rate under the most severe condition (56.25%) was 1.29 and 2.25 times the rates observed under the other two conditions, respectively. Temperature influenced EM failure more significantly than current density. Solder joints in which the thinner Cu UBM served as the cathode exhibited rapid failure, characterized by near-complete dissolution of the Cu UBM and subsequent crack propagation, ultimately resulting in open-circuit failure. In contrast, only two joints failed when the thicker Cu UBM served as the cathode. Finite element simulations revealed that current crowding at the electron flow entrance (corresponding to the thinner cathode side) reached approximately 10 times the nominal current density, accompanied by localized Joule heating that elevated the temperature to 154.15 °C. The severe current crowding, combined with localized Joule heating, significantly enhanced the Cu atomic flux, thereby accelerating open-circuit failure in the solder joints with the thinner Cu UBM serving as the cathode. These findings offer fundamental insights into the EM behavior of flip chip solder joints with asymmetric Cu UBM configurations.