Date of Award

Summer 2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Engineering

First Advisor

Carol A. Handwerker

Committee Chair

Carol A. Handwerker

Committee Member 1

Eric P. Kvam

Committee Member 2

Fu Zhao

Committee Member 3

John E. Blendell

Abstract

Tin-based solder is ubiquitous in microelectronics manufacturing and plays a critical role in electronic packaging and attachment. While manufacturers of consumer electronics have made the transition to the use of lead-free solder, there are still a variety of reliability issues associated with these lead-free alternatives, particularly for high performance, high reliability applications. Because of these performance short-comings, researchers are still searching for a material, an alloy, or a unique alternative that can meet the thermal, mechanical, and electrical requirements for conventional reflow solder applications. In an effort to produce a more reliable alternative, Kim et al. proposed the low-temperature (200°C) sintering of copper-silver core-shell particles as a viable solderless interconnect technology. This technology is based on the silver atoms from the shell diffusing by surface diffusion to form sintered necks between copper particles, and therefore dewetting most of the copper surfaces. This study presents a 3-fold, in-depth evaluation of this Cu-Ag core-shell lead-free solderless interconnect technology focusing on solder paste development and prototyping, silver thin film stress relaxation and dewetting kinetics, and the environmental impacts associated with this new technology. ^ First, an evaluation of the starting particle consistency and sintered compact mechanical properties determined that a specific core-shell particle geometry (1µm average core diameter and 10nm shell thickness) outperformed other combinations, exhibiting the highest modulus and yield strengths in sintered compacts, of 620 MPa and 40-60 MPa respectively. In particular, yield strengths for sintered compacts are similar to those reported for Sn-3.5Ag-0.75Cu (a commonly used lead-free solder) for the same strain rate. Following particle evaluations, the development of a functioning flux formulation was a key factor in the creation of a viable drop-in replacement. The processing of the final flux/particle paste combination was optimized at a commercial test facility for printing on test boards containing a wide variety of pad shapes, sizes, and pitches and thus, validated the ability of the Cu-Ag core-shell paste to be a drop-in replacement for traditional solder paste using conventional manufacturing techniques. ^ The second study addresses the fundamental mechanisms behind interconnect formation. An assessment of the kinetics and microstructure evolution during silver thin film dewetting and defect formation provides essential materials science knowledge to understand and control the functionality of the Cu-Ag core-shell system. From an interrupted annealing study used to quantify dewetting kinetics, a range of surface diffusion coefficients were calculated from the experimental results, assuming that surface diffusion controlled dewetting. The two order of magnitude range in calculated diffusion coefficient demonstrates that the diffusion-limited kinetic models traditionally used to quantify hillock and hole growth kinetics during thin film relaxation and dewetting do not apply to the dewetting of Ag films. The presence of interface-limited kinetics was then validated through the non-uniform growth of individual hillocks over time. ^ Lastly, an environmental assessment compares the impacts associated with the manufacturing and materials for the Cu-Ag core-shell particle system and SAC 305, the most commonly used lead-free solder alloy that contains 96.5% tin, 3% silver, and 0.5% copper. By comparing the impacts on global warming, acidification, eutrophication, ozone depletion, ecotoxicity, smog, carcinogenics, non-carcinogenics, and respiratory effects associated with each technology, the environmental advantages and disadvantages of each system are clearly communicated. By utilizing this information and the versatility of the core-shell system, possible methods for reducing impacts of the Cu-Ag core-shell system are addressed in order to reduce its environmental footprint. ^ This multidimensional assessment provides a comprehensive validation in terms of technology, science, and environmental impacts of the Cu-Ag core-shell interconnect technology as a viable drop-in replacement for lead-based and lead-free solders for microelectronic manufacturing.

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