Abstract: The vibration velocity of tool tip and chip on thermosonic flip chip (TSFC) bonding was monitored with a laser Doppler vibrometer, and the "stall" phenomena was observed from the virtual value curve of vibration velocity, i.e., after the TSFC bonding started a fever milliseconds, the chip velocity decreases suddenly, while the tool tip velocity still increases.Stall indicated that the bump pad interface has formed initial bonding strength.And the frequency characters of the vibration velocity signals were also obtained.It is found that the 3rd harmonics of chip vibration velocity signal indicates the stall phenomena, i.e., when the 3rd harmonics appeared, the stall happens.Experiment results show that little bonding force is good for produce stall.