Electromigration behavior of the Cu/Au/SnAgCu/Cu solder combination
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The electromigration behavior of the Cu/Au/SnAgCu/Cu combination was investigated under 103 A/cm2 of current stressing at ambient temperature. The Au layer, when it acts as a cathode, was consumed continuously, and no significant compound was found at the interface. Meanwhile, Cu6Sn5 was formed at the anodic Cu layer, and the thickness of the compound increased with increasing time. The Au atoms were found to be trapped in Cu6Sn5 within the solder matrix. The AuSn4 compound precipitated while attaching to Cu6Sn5 at the Cu6Sn5/solder interface. The thermomigration effect was found to be insignificant in this work as no obvious reaction occurred at the cathode/anode sides or in the solder matrix without current stressing.
I. INTRODUCTION
Recently, with the miniaturization, high performance, and intensive input/output (I/O) numbers of electronic devices, the pitch feature size and the current density of each contacted pad have been reduced and increased significantly, respectively, for very large-scale integration (VLSI) circuit designs in the semiconductor industry. Generally speaking, flip-chip technology is usually applied to connect Si chips with printed circuit boards (PCBs). Electromigration (EM), which occurs within solder joints and results in failure of the solder joint, is becoming a critical issue for joint reliability as solder joints become ever smaller. According to previous studies,1–5 under high current stressing, atomic mass transfer of the solder joint has frequently been found. Due to the momentum transfer between electrons and metal atoms, when the applied current density exceeds 102 A/cm2,6 atomic migration—also called “electromigration”—takes place. Not only does electromigration lead to atomic migration, but it also plays an important role in compound formation in the solder joints, including eutectic Sn– Pb,6–10 composite Sn–Pb,11–14 and lead-free solder systems.15–18 In addition to electromigration, thermomigration also had a large effect on atomic migration in flip-chip joints under current stressing. Due to the effect of Joule heating, the temperature inside the solder is increased. Ye et al.19 reported the existence of a thermal gradient in the eutectic Sn–Pb solder joint with high current stressing. With 1.3 × 104 A/cm2 of current stressing, three-
dimensional coupled thermoelectrical finite element simulation was utilized to predict the temperature gradient inside the solder. In addition, infrared microscopy was also applied to detect the real temperature at the lead-free Sn–Ag solder joint under 105–106 A/cm2 of current stressing.20 It was found that the conductive metal line (the Al line) was a heat generator,20 and a thermal gradient of ≈1700 °C/cm exists inside the solder in the horizontal direction. Huang et al. estimated the energy of atomic migration under electromigration or thermomigration effect21 by assuming that the atomic migration energy caused by 104 A/cm2 current stressing was approximately the same as the energy caused by a thermal gradient of 1000 °C/cm. For this reas
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