Date of Award

Spring 1992

Project Type

Dissertation

Program or Major

Engineering

Degree Name

Doctor of Philosophy

First Advisor

James E Krzanowski

Abstract

Ultrasonic wire bonding is widely used in the electronic industry to connect semiconductor chips to packages. Even though the popularity of the technique has increased in recent times, questions remain about the bonding mechanism, and factors affecting bondability and reliability. In this thesis, answers were provided to many of these questions using TEM to examine bonded cross section and plan view specimens.

A detailed investigation of the Al wire and substrate showed dynamically annealed well recovered grains while microstructural observations of other substrates revealed wide varieties of response mechanisms. For example, Ni formed a dislocation cell structure, Cu formed a partially recovered structure, while Cu alloys and stainless steel formed planar dislocation arrays. These observed transformations were correlated with basic material properties and literature reported cyclic deformation studies to determine factors contributing to substrate plastic deformation during bonding. It appeared that the plastic deformation of the substrate is not a requirement for good bonding, but since extensive plastic deformation can occur during bonding, it could have important implications in bond strength and reliability.

A model developed to estimate microstructural transformations was effective when applied to different metal substrates but somewhat less effective with Cu alloys.

The extent and type of intermetallic phases that formed at the wire-substrate interface after thermal aging, thermal cycling and in the as-bonded conditions were characterized for different Au and NiB plated substrates using EDS. Specimens were also examined for the extent of Kirkendall porosity and the conditions of the unreacted portions of the wire and substrate. It was found that the extent of interfacial reactions depended strongly on substrate metallurgy. For example, in the NiP/immersion Au specimen the original Au layer was still present after bonding, and transformed completely to Al-Au intermetallics after only 1.5 h at 90$\sp\circ$C. The same treatment resulted in no intermetallic phase formation for Ni-B specimens with the interface remaining chemically and structurally sharp.

Finally, mechanisms of bonding and microstructural development were proposed.

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