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采用同步辐射实时成像技术对比研究了Cu/Sn-52In/Cu微焊点在120和180 ℃,2.0×10⁴ A/cm²条件下液-固电迁移过程中In、Sn和Cu原子的扩散迁移行为及其对界面反应的影响。由于没有背应力,液-固电迁移条件下Sn-52In焊点中In原子的有效电荷数Z^*为负值是其定向扩散迁移至阳极的物理本质,这与Sn-52In焊点固-固电迁移条件下背应力驱使In原子迁移至阴极的机理不同。基于液态金属焓随温度的变化关系,修正了计算液态金属Z^*的理论模型,计算获得In原子在120和180 ℃下的Z^*分别为-2.30和-1.14,为电迁移方向提供了判断依据。液-固电迁移过程中In和Cu原子同时由阴极扩散至阳极并参与界面反应使得界面金属间化合物(intermetallic compounds,IMC)生长表现为“极性效应”,即阳极界面IMC持续生长变厚,并且厚于阴极界面IMC,温度越高,界面IMC的“极性效应”越显著。液-固电迁移过程中阴极Cu基体的溶解与时间呈抛物线关系,温度越高,阴极Cu的溶解速率越快。

Electromigration (EM), which describes the mass transport due to the momentum exchange between conducting electrons and diffusing metal atoms under an applied electric field, has become a serious reliability issue in high-density packaging. With the increasing demands for miniaturization, liquid-solid (L-S) EM will pose a critical challenge to the reliability of solder interconnects. In this work, The interfacial reactions and diffusion behaviors of In, Sn and Cu atoms in Cu/Sn-52In/Cu interconnects during L-S EM under a current density of 2.0×10⁴ A/cm² at 120 and 180 ℃ have been in situ studied by using synchrotron radiation real-time imaging technology. During L-S EM, since there was no back-stress, the In atoms directionally migrated toward the anode due to the negative effective charge number (Z^*) of In, which is different from the In atoms directionally migrated toward the cathode due to the back-stress induced by the preferential migration of the Sn atoms over the In atoms toward the anode during the solid-solid (S-S) EM. Furthermore, a modified expression for calculating the effective charge number Z^* of liquid metals was proposed based on the enthalpy changes of melting process. The Z^* of In atoms was calculated to be -2.30 and -1.14 at 120 and 180 ℃, respectively, which was consistent with the migration behavior of In atoms. The model provides a theoretical basis for determining the direction of the EM. The polarity effect, evidenced by the IMC layer at the anode growing continuously while that at the cathode was restrained, was resulted from the directional migration of In and Cu atoms toward the anode during L-S EM, which was more significant at high temperature. The consumption of cathode Cu during L-S EM followed a parabolic relationship with the EM time, and the consumption rate was magnitude higher at high temperature. The migrations of In atoms was discussed in terms of diffusion flux.