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Fine-pitch Cu pillar bumps have been adopted for flip-chip bonding technology. Intermetallic compound (IMC) growth in Cu pillar bumps was investigated as a function of annealing or current stressing by in situ observation. The effect of IMC growth on the mechanical reliability of the Cu pillar bumps was also investigated. It is noteworthy that Sn exhaustion was observed after 240 h of annealing when current stressing was not applied, and IMC growth rates were changed remarkably. As the applied current densities increased, the time required for complete Sn consumption became shorter. In addition, Kirkendall voids, which would be detrimental to the mechanical reliability of Cu pillar bumps, were observed in both Cu 3 Sn/Cu pillars and Cu 3 Sn/Cu under-bump metallization interfaces. Die shear force was measured for Cu pillar samples prepared with various annealing times, and degradation of mechanical strength was observed.

The in situ intermetallic compound (IMC) growth in Cu pillar/Sn bumps was investigated by isothermal annealing at 120°C, 150°C, and 180°C using an in situ scanning electron microscope. Only the Cu 6 Sn 5 phase formed at the interface between the Cu pillar and Sn during the reflow process. The Cu 3 Sn phase formed and grew at the interfaces between the Cu pillar and Cu 6 Sn 5 with increased annealing time. Total (Cu 6 Sn 5 + Cu 3 Sn) IMC thickness increased linearly with the square root of annealing time. The growth slopes of total IMC decreased after 240 h at 150°C and 60 h at 180°C, due to the fact that the Cu 6 Sn 5 phase transforms to the Cu 3 Sn phase when all of the remaining Sn phase in the Cu pillar bump is completely exhausted. The complete consumption time of the Sn phase at 180°C was shorter than that at 150°C. The apparent activation energy for total IMC growth was determined to be 0.57 eV.

The reaction between Cu pillar and eutectic SnPb solder during isothermal annealing was studied systematically. Intermetallic compounds (IMCs), such as Cu6Sn5 and Cu3Sn, were formed in between Cu and SnThe parabolic rate law was observed on IMC formation, which indicated that the growth of IMCs was controlled by atomic diffusion (a diffusion-limited process). Annealing at 165°C for 160 h decreased the growth rate of Cu6Sn5, and at the same time increased the growth rate of Cu3Sn. This was when Sn in solder was exhausted completely. The activation energies for the growth of Cu3Sn and Cu6Sn5 were measured to be 1.77 eV and 0.72 eV, respectively. The Kirkendall void that formed at the interface between Cu pillar and solder obeyed the parabolic rate law. The growth rate of the Kirkendall void increased when the Sn in solder was consumed in its entirety.