Browsing by Author "Liang, Yu-Chun"

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A new failure mechanism of electromigration by surface diffusion of Sn on Ni and Cu metallization in microbumps
(2018-04-12)
Chang, Yuan-Wei
;
Hu, Chia-chia
;
Peng, Hsin-Ying
;
Liang, Yu-Chun
;
Chen, Chih
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 mu m diameter microbump, the cross-section area and the volume are only 1/25 and 1/125 of a 100 mu m 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 mu m microbumps.
Generic Rules to Achieve Bump Electromigration Immortality for 3D IC Integration
(2013)
Chen, Hsiao-Yun
;
Shih, Da-Yuan
;
Wei, Cheng-Chang
;
Tung, Chih-Hang
;
Hsiao, Yi-Li
An immortal solder micro-bump (mu bump) electromigration (EM) lifetime has been demonstrated for 3D IC integration. This ultimate goal was achieved under strictly controlled conditions, including the optimal design of bump metallurgy, geometry, optimized processes, along with well-defined stressing conditions and manufacturing window. The current carrying capability and EM lifetime of mu bump have been investigated as functions of stressing conditions which are correlated with the degradation mechanisms. When stressed under the appropriate g conditions, all mu bump test samples survived prolonged stressing, some over 13,000 hours, without a failure. Cross-sectional analyses revealed that the entire solder joint almost all transformed into intermetallic compounds (IMCs) with very minor or no voids. The resistance plots showed an initial fast rise in resistance due to IMC formation, then gradually leveled off and eventually reached a steady state. The observed degradation mechanism is dominated by IMC formation, which is the same as that of the user conditions. On the other hand, void formation that eventually led to open failure was the dominant degradation mechanism when samples were aggressively stressed. In other words, when all other conditions were the same, the stressing conditions make a huge difference in determining between an almost immortal EM lifetime vs. a short lifetime using the same high quality mu bumps. The boundary that separates the stressing conditions is roughly defined and will be discussed. In addition, since full IMC mu bump will become inevitable in the future miniaturized solder interconnect structure, the EM behavior of IMC dominated mu bump has also been evaluated in this study. Under highly accelerated stressing conditions of 174 degrees C, at 1.6x10(5) A/cm(2) current density, the IMC dominated mu bumps were able to survive more than 600 hours and are still going strong. In comparison, solder mu bump failed quickly after just 107 hours when stressed under the same condition. This comparison study clearly demonstrated that IMC dominated joint has significantly higher current carrying capability than that of the solder joint. After reviewing all the data, we have concluded that the failure criteria for solder mu bump should be raised significantly higher than the 20% criteria traditionally used for the much larger C4 bumps.