M.S. Thesis Presentation by Mudasir Ahmad
Friday, May 4, 2001

(Dr. Suresh Sitaraman, advisor)

"Structural Thermal-Electric Modeling and Analysis of Micro-Springs for Microelectronic Probing and Packaging Applications"


As a result of the continuous reduction in feature size and an increasing demand for higher performance at lower costs to meet the needs of future electronic devices, there is an ever increasing search for innovative and reliable interconnect technologies to meet these needs.  The International Technology Roadmap for Semiconductor (ITRS), predicts that interconnect pitch sizes will be less than 40 µm by 2014, and in select opto-electronic applications, could be as small as 10 µm.  Existing interconnect and probing technologies are not capable of achieving this fine pitch requirement, even with dramatic improvements.  The objective of the ongoing research is to design and develop stress-gradient micro-springs fabricated with sputter deposition and micro-lithography techniques leveraged from silicon chip manufacturing.  These fine pitch micro-springs will be used both for packaging and for wafer level test and burn-in operations.

The goal of the thesis is to develop a modeling methodology for understanding the behavior of micro-springs and to develop design and processing guidelines their use in probing and packaging.  For probing applications, these micro-springs will serve as current carriers and will heat up as a result of Joule Heating.  An electric current of about 200mA is passed through these micro-springs.  Due to their extremely small dimensions, the electrical resistance of these micro-springs will be higher than in conventional probes.

Coupled thermal-electric finite element models have been developed to understand the thermal contours developed across these springs, and to determine the maximum temperature reached.  In addition, the role of scale effects on the thermal conductivity of the spring material is studied and incorporated in the model.  The electrical contact resistance between the micro-spring and the bonding pad, and its effect on the maximum temperature is outlined.  The results are compared with those obtained from experimental data.

Contact models and submodels have been developed to simulate the establishment of contact, sliding and indentation resulting in plastic deformation of the pad.  Comparisons of the resulting scrub mark are made with experimental measurements using Focused Ion Beam images to determine the role of friction in the contact mechanics.  The stresses at the spring tip and base are also determined.  Finally, mechanical fatigue experiments are also conducted to determine the fatigue life of the springs.