(Dr. Suresh Sitaraman, advisor)

"Thermo-Mechanical Modeling and Design of Micro-springs for Microelectronic Probing and Packaging"

Abstract

To continue feature size reduction, improved performance, and lower cost in microelectronics, an innovative, reliable interconnect technology is needed that can exceed National Technology Roadmap for Semiconductors (NTRS) requirements for a 50 µm pitch in 2012. Select opto-electronic applications may require even a 10 µm pitch.  Most of today’s interconnect and probing technologies are incapable of achieving these ultra-fine pitches, even with dramatic improvements.  The objective of the ongoing research is to design and develop stress-gradient micro-springs, which are fabricated in a sputter deposition and lithography process, for probing and packaging of microelectronics.

The goal of the thesis is to develop a modeling methodology for understanding the thermo-mechanical behavior of micro-springs and to develop design and processing guidelines their use in probing and packaging.  The primary challenge in designing micro-springs for probing is to ensure sufficient contact pressure is generated at the tip to break through bonding pad surface oxides.  The second challenge is to allow enough spring flexure to accommodate topological variation in the probing surface. Analytical and numerical models have been developed to maximize probing force and to understand and reduce anomalies in micro-springs.  A model of a sputter deposition planetary system has been developed to achieve uniform deposition across the substrate.  Results have been validated against experimental data.

In the area of packaging, two- and three-dimensional models have been developed to examine solder fatigue to ensure that sufficiently compliant micro-springs will accommodate the thermal mismatches induced during qualification. The effects of substrate, solder, spring geometry, and spring orientation have been systematically studied to maximize the micro-spring package reliability. In addition, scale effects in plastic deformation of the solder have been investigated