Performance study of the ultrasonic wire bonder used in microelectronic packaging

Abstract

Studies of the dynamic behavior of an Uthe 70-PTL ultrasonic wire-bonding transducer (chapters 2 - 4) and investigations of using PZT piezoelectric sensor in monitoring the quality of ultrasonic wedge bonds during the bonding process (chapter 6) are presented and discussed in this thesis. Determination of the ultrasonic bondability of Al wire, the criteria for a "good" bond, and the requirements for establishing an optimum bonding condition for an ASM AB-500B semi-automatic ultrasonic wire wedge bonder are included (chapter 5). Conclusions and suggestions for future work are also presented (chapter 7). Original contributions reported in this research are as follows : (1) The resonance characteristics and the optimum operating frequency of the ultrasonic transducer mounted on the wire bonder were found by in-situ measurement of its electrical impedance spectrum (chapter 2). (2) Theoretical models and design procedures of the piezoelectric driver and the concentrator of this particular type of transducer were developed by modifying the existing one-dimensional theories. Using these modified models, it is expected that the general characteristics of these two parts can be predicted and described (chapter 2). (3) With the transducer being clamped as it would be in the wire bonder, the actual vibration displacement distributions along the concentrator and the bonding wedge were measured using a high frequency heterodyne interferometer (chapter 3). Standing wave patterns were obtained and compared to the concentrator model predictions. The model was found to be useful to describe the general characteristics of this device. (4) The resonant frequencies and vibration mode shapes of the transducer, with emphasis on the concentrator, were computed using FEM analysis (chapter 4). The computed results were in good agreement with those of the concentrator model and the interferometric measurements. (5) A PZT piezoelectric sensor was mounted on the transducer horn in order to detect the changes in the mechanical impedance of the bond zone during the bond formation by monitoring the changes in the ultrasonic vibration of the transducer (chapter 6). Studies were carried out from the selection of sensor material to the choice of an appropriate location for sensor placement, and also from the development of a bond quality monitor to the evaluations of the relationship between the sensor signal (fundamental and its higher order harmonics) and the bond quality under different bonding conditions. More significant changes (amplitude and frequency) were observed in the second harmonic of the ultrasonic signal. Good correlations were obtained between the change in amplitude of the second harmonic and the bond shear strength.