Abstract: The thermosonic flip chip bonding has been realized under the pressure constrain pattern of flip chip.Fragment of pad, circle band and ridge have been observed on the bonding interface. The ridge indicates high bonding strength, and fragment of pad indicates remove of interface surface, and the circle band indicates distribution of bonding strength.With the FEA tool, the value and distribution of stress were calculated under different bonding phases, and the formation theory of the circle band interface has been revealed on the term of stress. The edge of bonding interface has better stress conditions for surface remove and atom diffusion, which the primary factor for the formation of circle band interface.And the thermo-sonic vibration changed the distribution of stress and stress field, which caused the edge of bonding interface in better stress conditions, and promoted the formation of circle band interface.