Mechanism of improved electromigration reliability using Fe-Ni UBM in wafer level package
Mechanism of improved electromigration reliability using Fe-Ni UBM in wafer level package
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DOI：
10.1016/j.jmst.2017.11.046
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发表时间：
2017-12
影响因子：
10.9
通讯作者：
Li-Yin Gao;Hao Zhang;Cai-Fu Li;Jingdong Guo;Zhi-Quan Liu
中科院分区：
文献类型：
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作者：
Li-Yin Gao;Hao Zhang;Cai-Fu Li;Jingdong Guo;Zhi-Quan Liu
采用Ni含量分别为73wt%和45wt%的Fe-Ni薄膜作为圆片级芯片封装的凸点下金属化层（UBM），通过电迁移（EM）测试评价了其可靠性，并与商用Cu UBM进行了比较。对于Sn3.8Ag0.7Cu（SAC）/Cu焊点，300 h后Cu阴极出现空洞，1000 h后出现Cu阴极耗尽和裂纹等典型失效。而SAC/Fe-Ni焊点在1000 h EM后仍保持完好状态，未出现任何失效。通过Weibull分析计算出Fe-73 Ni UBM和Fe-45 Ni UBM的特征寿命分别为2121 h和2340 h，是Cu UBM的特征寿命（698 h）的3倍以上。Fe-Ni焊点的失效模式随金属间化合物（IMC）生长行为的不同而不同，均为钎料与IMC层之间的阴极界面裂纹。分析了与阴极溶解和裂纹萌生有关的原子通量。当添加Fe-Ni UBM时，由于IMC的低扩散性和与Fe原子的电子流相反的转移方向，阴极溶解被抑制。焊料中较小的电磁通量导致Fe-Ni焊点中较小的空位通量，这可以解释焊料空洞和裂纹的延迟以及在电磁下更长的寿命。
Fe-Ni films with compositions of 73 wt% of Ni and 45 wt% of Ni were used as under bump metallization (UBM) in wafer level chip scale package, and their reliability was evaluated through electromigration (EM) test compared with commercial Cu UBM. For Sn3.8Ag0.7Cu(SAC)/Cu solder joints, voids had initiated at Cu cathode after 300 h and typical failures of depletion of Cu cathode and cracks were detected after 1000 h EM. While the SAC/Fe-Ni solder joints kept at a perfect condition without any failures after 1000 h EM. Moreover, the characteristic lifetime calculated by Weibull analysis for Fe-73Ni UBM (2121 h), Fe-45Ni UBM (2340 h) were both over three folds to Cu UBM’s (698 h). The failure modes for Fe-Ni solder joints varied with the different growth behavior of intermetallic compounds (IMCs), which can all be classified as the crack at the cathodic interface between solder and outer IMC layer. The atomic fluxes concerned cathode dissolution and crack initiation were analyzed. When Fe-Ni UBM was added, cathode dissolution was suppressed due to the low diffusivity of IMCs and opposite transferring direction to electron flow of Fe atoms. The smaller EM flux within solder material led a smaller vacancy flux in Fe-Ni solder joints, which can explain the delay of solder voids and cracks as well as the much longer lifetime under EM.