Ultrasonic wire-bond quality monitoring using piezoelectric sensor

This paper describes an ultrasonic wire-bond quality monitoring technique which involves the detection of changes in the mechanical impedance of the bond zone during bond formation by monitoring the changes in the ultrasonic vibrations of an ultrasonic wire-bonding transducer system. A lead zirconate titanate (PZT) piezoelectric ceramic having properties similar to the commercial PZT-5A has been selected as the sensor material. A PZT piezoelectric sensor is installed on an appropriate location on the transducer in order to measure the ultrasonic amplitude and bonding time during the bonding process. This sensor has dimensions 1 mm X 1 mm X 0.245 mm to avoid excessive loading on the transducer and to obtain a flat frequency response. A bond quality monitoring system has been developed to process and record the non-linear signal detected by the sensor. More significant changes are observed at the higher-frequency harmonics (second harmonic) of the ultrasonic signal. Good correlation is found between the change in the second harmonic and the bond quality. Bonding process drift towards unacceptable bond quality can be identified. This technique will be used to develop a multiparameter-based automatic process-control system for wire bonding. (C) 1998 Elsevier Science S.A.