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.
Highlights
Microbumps in three-dimensional integrated circuit (3D-IC) have emerged as a critical technology for the demand of better performance and higher packaging density[1,2,3,4]
Sn atoms diffused along the surface of the circumferences of the Ni and the Cu UBMs during EM testing, and formed Ni3Sn4 and Cu3Sn, respectively
Unlike the slight resistance changes induced by UBM dissolution and void propagation, the void induced by the surface diffusion of Sn caused several hundred milli-ohm of resistance increase
Summary
Microbumps in three-dimensional integrated circuit (3D-IC) have emerged as a critical technology for the demand of better performance and higher packaging density[1,2,3,4]. There were two main EM failure mechanisms reported[9,10]: the void propagation along the IMC/solder interface[11,12,13,14] and the dissolution of under-bump-metallization (UBM)[15,16,17,18,19,20] Both of them were correlated with the current crowding effect in the FC bumps. The small amount of solder in microbumps did reduce the current crowding effect[21,22], it could be fully transformed into the IMC in a short time These characteristics significantly elongated the EM failure and caused the microbumps immortal in some specific test conditions[23,24,25]. A new EM failure mechanism, the surface diffusion of Sn, was found in microbumps with small volume of solder
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