Chandra S. Desai
Microchips are tiny and easily damaged; yet they function in some of the
world_s harshest environments.
In automobiles, airplanes and missiles, for instance, they are
repeatedly subjected to huge temperature changes. The chip may be
at 110 degrees when an airplane is on the runway, and well below
freezing at 30,000 feet.
Engineers call this "thermal cycling." Run the chips through enough of
these cycles and they will fail _ usually at solder joints
that connect the chip to the substrate (printed circuit board).
But how many cycles can a chip sustain before it breaks? And which
materials and types of connections will be the most reliable?
What combinations of thermal and mechanical stresses and strains can a
chip experience before it_s likely to fail?
Engineers don_t have a lot of basic data on the properties of these
solder joints _ or the solder that forms them _ to give
accurate answers to those questions. Until recently, in fact, not much
fundamental modeling has been done in this area.
A group of engineers at The University of Arizona in Tucson now is
working to change that and to vastly increase the
understanding of these important microchip connections. The researchers
include: Chandra S. Desai, Regents_ Professor of Civil
Engineering and Engineering Mechanics (CEEM), and principal investigator
and director of the project; CEEM Professor
Tribikram Kundu; and Professor John Prince of electrical and computer
engineering. Kundu and Prince are co-principal
Industry co-investigators include Mostafa Rassaian, of Boeing
Electronics in Seattle, and J. William Zwick and Marlene R.
Tomkins of Raytheon in Tucson.
The project focuses on:
o Modeling the thermal and mechanical response of materials in
electronic packaging and semiconductor systems.
o Developing a test device to study the response of microchip solder
joints to thermal and mechanical loading.
o Modeling fatigue failure under thermomechanical cyclic loading,
leading to improved design, reliability and manufacturing.
Laboratory testing is being done with a one-of-a-kind device that was
recently designed and fabricated in Desai_s laboratory. It_s
called a Thermomechanical Laser:Digital Image Correlation
(DAC)/Interferometry Device. It can cycle temperatures between ?55
and 160 degrees Celsius and has a displacement accuracy of about
0.000004 inches. Its force can be varied between plus and
minus 1,000 pounds in 0.1-pound increments.
"We are using the Thermomechanical:DAC/Laser Interferometry Device to
study how microcracking initiates and grows and
eventually fractures the joint," Desai says. "When this happens, the
connection is lost and the chip is ruined."
The group currently is using data gathered with this device to develop
and refine the new Distributed State Concept (DSC) Finite
Element Model that will predict the life span and reliability of solder
joints in microchips under various levels and combinations
of thermal and mechanical loading. Raytheon engineers recently performed
DSC computer analysis on a 313-pin BGA package.
The DSC predictions correlated closely with laboratory test data,
including cycles to failure.
"The DSC model will enable us to eliminate hardware testing and analysis
and replace it with computerized simulation that would
take only about 18 hours to run," said Marlene Tomkins, engineering
manager at Raytheon. "That would mean huge savings in
time and money. This modeling tool has the potential for application to
many of the joining materials used in our missiles."
The research is funded by the National Science Foundation under its
Grant Opportunities for the Academic Liaison with Industry
(GOALI) program. In addition to Boeing and Raytheon, Sandia National
Laboratory also collaborates on the project.
Technical collaborators include: Ephrain Suhir, of Lucent Technology;
Paul Mescher, of AMKOR Electronics; and Professor H.H.
Li, of Purdue University.