Placement of piezoelectric ceramic sensors in ultrasonic wire-bonding transducers

Three identical lead zirconate titanate (PZT) sensor rings have been inserted in different locations of an ultrasonic wire bonding transducer driver in order to measure the bonding parameters. Voltage outputs from these three sensors were compared in order to find an optimal position for the sensor placement. One side of the PZT sensor ring is partially coated with a specially designed electrode pattern and the rings are only poled in this small region. With this specific electrode pattern, sensor resonance can be eliminated and the sensor has a flat frequency response. From the sensor signals, different bonding qualities can be distinguished. Hence, the transducer can provide a feedback for real-time bond quality control and thus can be called a 'smart transducer'. The optimum position is found to be near the back of the transducer if second harmonic detection is used. By placing the sensor at this optimized position, an in situ automatic wire bonding process control system can be implemented.