研究了230 ℃, 5×10³ A/cm²条件下液-固电迁移对Ni/Sn-9Zn/Ni线性焊点界面反应的影响. 在液-固电迁移过程中, Ni/Sn-9Zn/Ni焊点表现出明显的反极性效应, 即阴极界面金属间化合物(IMC)持续生长变厚, 并且一直厚于阳极界面IMC. 由于排除背应力的影响, Sn-9Zn液态钎料中Zn原子的反常迁移行为归因于其有效电荷数在高温下为正值, 即在电子风力作用下Zn原子向阴极界面定向迁移, 从而导致焊点在液-固电迁移过程中发生反极性效应. 回流焊后, Ni/Sn-9Zn/Ni焊点两侧界面上均生成了较薄的Ni₅Zn₂₁层. 液-固界面反应过程中(无电流)焊点两侧界面IMC均随时间延长而生长变厚, 从而消耗钎料中的Zn原子并使界面处的相平衡发生变化, 导致界面IMC由Ni₅Zn₂₁转变为[Ni₅Zn₂₁+(Ni, Zn)₃Sn₄]. 与之相较, 液-固电迁移过程中阴阳两极界面IMC的类型一直为Ni₅Zn₂₁, 并未发生IMC类型的转变. 这是由于, 在电子风力作用下, 阴极界面附近钎料中Zn原子的含量充足, Zn与Ni反应生成Ni₅Zn₂₁型IMC; 同时, 电子风力也阻碍了Zn原子向阳极界面的扩散, 从而抑制了阳极界面IMC的生长, 导致界面IMC较薄, 因此阳极界面也未发生IMC类型的转变. 此外, 运用反证法进一步验证了Zn的有效电荷数在高温下是正值.

The effect of liquid-solid electromigration (EM) on the interfacial reaction in Ni/Sn-9Zn/Ni interconnects was investigated under a current density of 5×10³ A/cm² at 230 ℃. A reverse polarity effect was revealed, i.e., the interfacial intermetallic compounds (IMC) at the cathode grew continuously and was remarkably thicker than those at the anode. This results from the directional migration of Zn atoms from the anode toward the cathode, which is induced by the positive effective charge number (Z ^*) of Zn atoms but not the back-stress. A thin Ni₅Zn₂₁ layer formed at each interface after soldering. The initial Ni₅Zn₂₁ interfacial IMC gradually transformed into [Ni₅Zn₂₁+(Ni, Zn)₃Sn₄] after liquid-solid interfacial reaction for 8 h, due to the local equilibrium at the interface changed with decreasing of Zn atoms content. The interfacial IMCs at both anode and cathode were identified as Ni₅Zn₂₁, and no IMC transformation occurred undergoing liquid-solid EM, because the Zn atoms content at the cathode was enough under electron current stressing, and the diffusion of Zn atoms toward anode was inhibited. The reverse proving was proposed to explain the positive value Z ^* of Zn atoms. The abnormal directional migration of Zn atoms toward the cathode prevented the dissolution of cathode substrate, which is beneficial to improving the EM reliability of micro-bump solder interconnects.