Award Abstract #0081796
FRG: Modeling of Microstructure Evolution & Thermomechanical Fatigue in Lead-Free Solder Joints

NSF Org:	DMR Division of Materials Research
Initial Amendment Date:	July 26, 2000
Latest Amendment Date:	May 1, 2002
Award Number:	0081796
Award Instrument:	Continuing grant
Program Manager:	Harsh Deep Chopra DMR Division of Materials Research MPS Directorate for Mathematical & Physical Sciences
Start Date:	September 1, 2000
Expires:	August 31, 2004 (Estimated)
Awarded Amount to Date:	$569541
Investigator(s):	K. Subramanian subraman@egr.msu.edu (Principal Investigator) Thomas Bieler (Co-Principal Investigator)
Sponsor:	Michigan State University CONTRACT AND GRANT ADMINISTRATIO EAST LANSING, MI 48824 517/355-5040
NSF Program(s):	METAL & METALLIC NANOSTRUCTURE
Field Application(s):	0106000 Materials Research
Program Reference Code(s):	AMPP, 9232, 9161, 1589
Program Element Code(s):	1771

ABSTRACT

This grant focuses on characterizing the microstructural evolution and consequent changes in mechanical properties that result from thermomechanical fatigue for four Sn-Ag based solders. The goal is to develop microstructurally based phenomenological models that can be used to predict the onset of failure. The experimental plan uses component-scale solder joint specimens that allow effective monitoring of microstructural and mechanical property changes as a function of the number of real-time thermomechanical cycles. The thermomechanical cycling includes short times at elevated temperatures and long times at cold temperatures to allow the effects of low temperature creep to be evaluated, a condition not considered by the commonly used but non-conservative accelerated testing practice of the electronics industry. Specimens will also be cycled in industrial accelerated testing facilities to examine how different rates and magnitudes of heating and cooling affect structural damage accumulation. Orientation imaging is used to evaluate structural changes in crystal orientation and misorientation. The size distribution and shapes of second phases are also determined. Mechanical properties are measured using nanoindentation and destructive testing techniques that include stress-relaxation, creep-rupture, and constant strain rate tests to evaluate corresponding failure strains. Modeling is based upon a combination of the Dorn/Garafolo sinh equations for creep-plasticity coupled with threshold stresses based upon growth rate kinetics of particles and microstructure as modified by stress/strain history.

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Alternatives to leaded solders used in electronic applications are being sought to address environmental concerns and to accommodate the policies unfolding in several foreign countries that will affect the global marketing of US electronic products. Eutectic Sn-Ag solder is a recommended alternative solder, aimed at higher temperature service conditions. Solders used in severe service environments, such as automotive, aerospace, and military environments, can experience thermal excursions that range between -50 to 180 degrees C. Such thermal cycles cause severe thermal stresses due to the thermal expansion mismatch between soldered components and the substrates. This thermomechanical fatigue process damages solder joints, and more than 70% of electronic system failures are due to failed solder joints.

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