The content and distributing of P in Electroless Nickel/Immersion Gold (ENIG) surface finishes effect the reliability of interface between solder joint and surface finishes. 289 I/Os lead-free Sn-3.0Ag-0.5Cu BGAs was assembled on the ENIG print circuit board by reflow soldering technology. The print circuit board was tested by random vibration. After random vibration test, the failure and not failure samples were studied by X-ray, SEM, and EDX et al. The failure mechanism was confirmed and the relation of failure and content and distributing of P in ENIG finishes was discussed. The failure and not failure samples after vibration test show similar microstructure at the interface of solder joint and surface finishes. The cross-sectioning of the affected area shows a “tooth decay” effect of corrosion of the nickel layer, and “mud cracks” in the topography on the surface. The enrichment of P on the surface accelerated the oxidation of Ni, and also reduced the mechanical intensity of interface. The oxidation of Ni and the enrichment of P on the surface result in the crack of solder/finish interface