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.