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.
Temperature-dependent electromigration failure was investigated in solder joints with Cu metallization at 126 C, 136 C, 158 C, 172 C, and 185 C. At 126 C and 136 C, voids formed at the interface of Cu 6 Sn 5 intermetallic compounds and the solder layer. However, at temperature 158 C and above, extensive Cu dissolution and thickening of Cu 6 Sn 5 occurred, and few voids were observed. We proposed a model considering the flux divergency at the interface. At temperatures below 131 C, the electromigration flux leaving the interface is larger than the incoming flux. Yet, the incoming Cu electromigration flux surpasses the outgoing flux at temperatures above 131 C. This model successfully explains the experimental results. V
In this study, the temperature map distribution in the Sn3.0Ag0.5Cu solder bump with Cu column under current stressing is directly examined using infrared microscopy. It is the radiance changes between the different materials of the surface that cause the unreasonable temperature map distribution. By coating a thin layer of black optical paint which is in order to eliminate the radiance changes, we got the corrected temperature map distribution. Under a current stress of 1.15 × 104 A/cm2 at 100℃C, the hot-spot temperature is 132.2℃ which surpasses the average Cu column temperature of 129.7℃C and the average solder bump temperature of 127.4 ℃. Thermomigration in solder may still occur under a large current stressing.
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