Date of Award

3-2024

Document Type

Dissertation

Publisher

Santa Clara : Santa Clara University, 2024

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Panthea Sepehrband

Abstract

This thesis research uses a combination of computational and experimental approaches to provide atomistic insights into the mechanisms of ultrasonic bonding (UB), a family of solid-state metal-metal joining techniques, including ultrasonic wire bonding, ultrasonic flip-chip bonding, ultrasonic additive manufacturing, and ultrasonic spot welding. The work investigates the atomic-scale contact formation (i.e., the so-called Jump-to-Contact (JC) mechanism) between the UB counterparts, generation of a network of dislocations (i.e., one-dimensional crystallographic defects) at their interface, and the evolution of the network when the interface is under ultrasonic vibration. In particular, the thesis delivers invaluable insights on the mechanisms for contact formation, bond strengthening, and ultrasonic softening in UB.

The research first employs molecular dynamics (MD) simulations to study the atomic-scale contact formation between the two parts through the JC mechanism. The results show that the UB counterparts are more likely to fonn a contact at higher temperatures. As a consequence of the gap existing between parts' free surfaces prior to JC, the JC interface is under some level of tensile strain. The results suggest that the number of dislocations in the dislocation network formed at JC interface, may multiply immediately after formation. The tendency for dislocation multiplication (DM) increases at higher temperatures, lower misorientation angles between the parts, and higher JC strains (due to a higher gap size between the parts before JC).

Next, the work studies the effect of shear or vibration on the evolution of the dislocation network at JC interface. The results show that DM can occur during shear and vibration and that the characteristics of the profile of average atomic volume along the direction perpendicular to the interface can be used to predict the shear strength. One of the most important contributions of this work is indeed the explanation of the softening effect of the application of vibration (parallel to the interface) in UB. The MD results show that irrespective of the interface orientation, vibration always causes dissolution of the dislocation network through inducing DM. This points to the ultrasonic softening effect, i.e., freeing dislocations from their pinned equilibrium position. Additionally, the same effect of vibration is found to promote atomic mixing across the interface, indicative of enhanced diffusion, and possibly being the mechanism for bond strengthening in UB. Based on this mechanism, a simultaneous increase in vibration amplitude and decrease in vibration frequency is shown to improve bonding. It is also observed that the application of vibration increases the temperature in MD simulations, which agrees with the UB literature.

Lastly, the thesis research uses the collective understanding obtained from the MD simulation results to address a challenge in practical applications of UB, i.e., bondpad damage. It utilizes SEM/EDX imaging and 3D optical profilometry to analyze the fractured surfaces of wire and substrate after fracturing the wire bonds in ultrasonically wedge-bonded specimens. It establishes that a bulge on the wire and a cavity on the bondpad are always formed after breaking the bonds, pointing to the occurrence of fracture in the bondpad away from the interface. This observation is an example of bond strengthening in UB. Based on the observation of the occurrence of vibration-induced DM in MD simulations, it proposes a possible plastic deformation mechanism for bondpad damage, and it further suggests potential mitigation strategies for minimizing the cavity's depth for bonding wires on sensitive thin bondpads.

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