Study of composite transducers for ultrasonic wire bonding

Abstract

Lead zirconate titanate (PZT)/epoxy 1-3 composite rings were used as the driving elements in transducers for ultrasonic wirebonding applications. The composite transducers have cleaner axial vibration mode as damping in the epoxy led to reduced lateral mode coupling. These transducers can be used to produce narrow bond width which is essential for a fine-pitch wirebonding process. Work in this thesis included the following: 1. Hard PZT piezoceramic rings (PZT81and PZT82) and epoxy (Araldite LY5138-2/HY5138) were selected as the active and passive phases in the 1-3 structure, respectively. Their materials properties were measured and determined. 2. The composite rings were prepared by dice-and-fill technique. PZT81/epoxy composite rings were fabricated with PZT volume fraction o from 0.58 to 0.93 with epoxy width of ~77-81 um. The modified series and parallel model was applied to predict the materials properties and resonance characteristics of composite rings as a function of o and good agreement has been obtained. Low frequency lateral modes (radial and wall-thickness modes) were studied with finite element analysis (FEA) simulation results and they were found to become weaker as o decreases. 3. The nonlinear properties in PZT and composite rings caused by increased applied electric field and mechanical loading were studied. Complex materials parameters in the rings were evaluated as functions of the applied AC field and the clamped torque. It was found that under normal conditions, the applied field and torque were acceptable and did not cause serve nonlinearity in the materials. When subjected to the same increase in the applied field, the composite rings were found to have lower percentage change in the materials parameters. 4. Physical models for describing the piezoelectric driver and ultrasonic horn were described based on an one-dimensional elastic wave theory. By using an equivalent circuit including the loss factor, the electrical impedance spectra and the vibration amplitude distribution of the transducer can be simulated. By varying the input parameters and mechanical dimension, the relationships between materials and physical structures can be understood. FEA was also used to simulate the transducer prototype. By using composite rings, the non-axial resonances close to the working frequency in the transducer could be successfully eliminated. 5. A series of PZT82/epoxy composite rings with o ranging from 0.93 to 0.83 were assembled into 64 kHz ultrasonic wire bonding transducers. Their vibration and resonance characteristics were studied. The influence of using composites with varying o in the transducer behaviours were studied. The performances of these composite transducers were compared with conventional ceramic transducers. It was found that the non-axial and many other spurious resonances were suppressed in the composite transducers. The composite transducer can maintain their narrow lateral vibration profile under different input power thereby enabling fine-pitch bonding technology. A composite transducer with o =0.9 was installed in the wire bonder to perform preliminary bonding tests, and results indicated that the composite transducer can indeed produce bond-width ~11-16% narrower than that of a PZT transducer. 6. From the Above extensive set of performance study, a set of design guidelines was suggested and applied to the design of other higher frequency transducers. High frequency composite transducers were fabricated; their applications and performances were studied.