Chang Yuan-Wei, Hu Chia-Chia, Peng Hsin-Ying, Liang Yu-Chun, Chen Chih, Chang Tao-Chih, Zhan Chau-Jie, Juang Jing-Ye
Department of Materials Science and Engineering, National Chiao Tung University, Hsin-chu, 30010, Taiwan, Republic of China.
Electronic and Optoelectronic System Research Laboratories, Industrial Technology Research Institute, Hsin-chu, Taiwan, 31040, Republic of China.
Sci Rep. 2018 Apr 12;8(1):5935. doi: 10.1038/s41598-018-23809-1.
Microbumps in three-dimensional integrated circuit now becomes essential technology to reach higher packaging density. However, the small volume of microbumps dramatically changes the characteristics from the flip-chip (FC) solder joints. For a 20 µm diameter microbump, the cross-section area and the volume are only 1/25 and 1/125 of a 100 µm diameter FC joint. The small area significantly enlarges the current density although the current crowding effect was reduced at the same time. The small volume of solder can be fully transformed into the intermetallic compounds (IMCs) very easily, and the IMCs are usually stronger under electromigration (EM). These result in the thoroughly change of the EM failure mechanism in microbumps. In this study, microbumps with two different diameter and flip-chip joints were EM tested. A new failure mechanism was found obviously in microbumps, which is the surface diffusion of Sn. Under EM testing, Sn atoms tend to migrate along the surface to the circumference of Ni and Cu metallization to form NiSn and CuSn IMCs respectively. When the Sn diffuses away, necking or serious void formation occurs in the solder, which weakens the electrical and mechanical properties of the microbumps. Theoretic calculation indicates that this failure mode will become even significantly for the microbumps with smaller dimensions than the 18 µm microbumps.
三维集成电路中的微凸点如今已成为实现更高封装密度的关键技术。然而，微凸点的小体积极大地改变了倒装芯片（FC）焊点的特性。对于直径为20μm的微凸点，其横截面积和体积仅分别为直径100μm的FC焊点的1/25和1/125。尽管同时电流拥挤效应有所降低，但小面积显著增大了电流密度。小体积的焊料很容易完全转变为金属间化合物（IMC），并且IMC在电迁移（EM）作用下通常更强。这些导致了微凸点中EM失效机制的彻底改变。在本研究中，对两种不同直径的微凸点和倒装芯片焊点进行了EM测试。在微凸点中明显发现了一种新的失效机制，即Sn的表面扩散。在EM测试下，Sn原子倾向于沿表面迁移至Ni和Cu金属化层的周边，分别形成NiSn和CuSn IMC。当Sn扩散离开时，焊点中会出现颈缩或严重的空洞形成，这会削弱微凸点的电气和机械性能。理论计算表明，对于尺寸小于18μm微凸点的微凸点，这种失效模式将变得更加显著。
