To meet the demand of cryogenic reliability on the electronic packaging for deep space satellite, the three-dimensional interfacial morphology and property of eutectic SnPb solder joint under an extreme thermal shocking of 77-423 K were investigated. After thermal shocking, Cu-Sn IMC layers coarsened in the cross-sectional observation and micro-cracks formed in Cu 6 Sn 5 layer. The rough Cu 6 Sn 5 layer in the top view gradually smoothed, with broken Cu 6 Sn 5 particles and exposed Cu 3 Sn. In the bottom-view observation, the Cu 6 Sn 5 layer of as-soldered joint was overlapped by Cu 3 Sn layer. But no defects were generated in Cu 3 Sn layer. The shear force of solder joint was greatly reduced due to broken Cu 6 Sn 5 with the changing fracture characteristics from the solder-controlled mode to the solder/IMC mixed mode.