Advanced process characterization for 125 kHz wire bonder ultrasonic transducers

A high frequency ultrasonic transducer for wire bonding is conceived, designed, prototyped, tested and industrially produced. The influence of each feature of the ultrasonic transducer design, such as constituent material, amplifier geometry, mounting flange, capillary fixing, on the wire bonding process results is clearly identified and carefully analyzed through thorough process tests. The assembly and aging characteristics of the transducers are measured and the scattering of the vibration properties in the mass production is statistically quantified by laser interferometer measurements and compared with that of conventional horns. The transducer is mounted on the wire bonder with a flange whose special geometry is calculated by means of FEM simulations and patented. This flange allows the mechanical stiffness of the coupling transducer-wire bonder to be dramatically increased and the parasitic mechanical dynamical vibrations at the horn tip to be significantly reduced. Process tests show that the reduction in mechanical vibrations obtained in this way allows a wire bonder scarcely capable normally to attain the 80 μm fine pitch process, to perform easily the 60 μm fine pitch process. A beneficial impact of the mechanical coupling horn-wire bonder on the process performance is ascertained. The use of titanium as horn material, characterized by a low thermal expansion coefficient, is proven by process tests to be effective in improving the placement accuracy of the wire bonder. The high vibration frequency of the transducer (125 kHz) is proven by process tests to be effective in improving the ball roundness and the fine pitch wire bonding capabilities of the wire bonder and in decreasing the minimum wire bonding temperature and the applied bond force. The new approach to characterize the process performance of new ultrasonic transducer designs is of general importance for wire bonding technology.