Fracture reliability concern of (Au, Ni)Sn4 phase in 3D integrated circuit microbumps using Ni/Au surface finishing

Au surface finishing on Ni layer in solder joints can lead to the replacement of β-Sn by (Au, Ni)Sn4 phase in microbumps. A thorough crack in (Au, Ni)Sn4 is found after the sample being annealed at 150 °C for 1000 h. Phase transformation of (Au, Ni)Sn4 to Ni3Sn4 and AuNi2Sn4 is observed. This transformation will lead to − 10.5% volume shrinkage and may result in tensile stress, which becomes the reason for a thorough crack formation. Kirkendall voids have been found at the interface between Ni3Sn4 and electroplated Ni layer. Marker movement indicates that there was a dominant Ni diffusing flux.

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