A new failure mechanism of electromigration by surface diffusion of Sn on Ni and Cu metallization in microbumps

Yuan Wei Chang*, Chia Chia Hu, Hsin Ying Peng, Yu Chun Liang, Chih Chen*, Tao Chih Chang, Chau Jie Zhan, Jing Ye Juang

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

21 Scopus citations

Abstract

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 Ni3Sn4 and Cu3Sn 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.

Original languageEnglish
Article number5935
JournalScientific reports
Volume8
Issue number1
DOIs
StatePublished - 1 Dec 2018

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