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Thermosonic flip chip bonding has been widely used in fine pitch IC packaging for its unique features. By using thermosonic flip chip bonding at an aluminum wire bonder, 1 mm*1 mm chip with 8 gold bumps is bonded successfully to Ag substrate, and the bonding strength reaches as high as about 30gf/bump. Dynamical characteristics of ultrasonic transducer are investigated by finite element method and experiments using impedance analysis and laser Doppler measurement. The FEM analysis shows that, ultrasonic transducer system vibrates in coupling of all excited modes, which results in complicated vibrations during bonding. The third axial vibration mode, which includes 1.5 wavelengths and 3 nodes, is the dominant working mode. However, the dominant vibration is disturbed significantly by undesirable non-axial modes such as flexural vibration. Moreover, there are some other un-wanted modes parasitizing closely to dominant mode. Velocities of transducer horn sampled by laser Doppler vibrometer show the system vibrates with several modes simultaneously. The impedance results using impedance analyzer prove there are some other undesirable frequencies close to the working mode. All non-working modes of ultrasonic transducer will disturb the bonding process and degrade bond quality, so they must be well controlled to obtain unique vibration mode and stable vibration for thermosonic flip chip bonding.