Impedance Modeling of Ultrasonic Transducers Used in Heavy Aluminum Wire Bonding

Dynamic characteristics and ultrasonic properties of piezoelectric transducer determine the bonding quality in heavy aluminum wire bond packaging. In this paper, an impedance model for the ultrasonic transducers is developed by analytical calculations using the equivalent circuit method. This lumped model indicates the relationship between impedance and the other parameters such as material features, structure dimensions, loadings, driving electrical power, and material losses. This impedance model can be used not only for transducer design but also to predict the frequency and impedance at different loadings. The impedance model was numerically calculated using an analytical model and discussed with respect to three cases of loadings with and without a bonding tool and with a bonding tool in contact with the substrate. Four axial frequencies between 20 and 100 kHz were found for the ultrasonic transducer. The frequency and impedance of the transducer with the bonding tool were higher than those in the case without a bonding tool. When the bonding tool was in contact with the substrate, the resonance frequency decreased and the impedance increased. The experimental results obtained by frequency sweeping support the validity of the impedance model. The impedance model can be a benefit to transducer design and parameter match to electrical driver.