In response to some exchange of information among the Technet subscribers on creep mechanisms in solders, I would like to inform this group that we, in the Department of Mechanical Engineering at IUPUI (Indiana University Purdue University Indianapolis), have been involved with FEM modeling of solder joints and their fatigue life predictions under cyclic thermal loads since 1991.  We are continuously searching for reliable fatigue data on eutectic solder joints.  We will appreciate it if anyone directs us to some available data on this which we can use.

	So far we have developed a Dorn type constitutive model for creep and stress relaxation behavior of solders using creep constants provided for eutectic solders by a number of researchers, including Lam et al. ("Superplastic Behavior of Pb-Sn Eutectic Alloy," Materials Science and Engineering, Vol 40, 1979).  Our experience has shown that creep and relaxation phenomena are very important to include in the models for determining the behavior reasonably well.  

We have also worked with the following fatigue life prediction methods:

i) 	Maximum creep strain range model (e.g., Coffin-Manson, Engelmaier, etc.) and

ii) 	Energy-partitioning models (e.g., Das Gupta and Barker in CALCE) 

Our experiences so far have shown that:

a)	Creep and stress relaxation phenomena are very important in predicting the behavior.

b)	None of the fatigue life prediction methods are reliable when maximum strain and/or stress values of the FEM models are used at stress and strain singularity points for fatigue life predictions (i.e., solder-chip-board interfaces).  This is because, as may be expected, the FEM results at singularity points continuously increase with mesh refinement, e.g., Akay, et al. ("A Finite Element Study of Factors Affecting Fatigue Life of Solder Joints," ASME Journal of Electronic Packaging, Vol. 116, 1994, pp. 265-273 and "A Finite Element Study of Fatigue Life Prediction Methods for Thermally Loaded Solder Joints," Advances in Electronic Packaging, Edited by Engel, et al., ASME, EEP-Vol. 4-2, 1993, pp. 1063-1070).  To alleviate this difficulty, we have developed a volume-weighted averaging technique for stresses and strains over the entire solder geometry.  With this, the energy method gave more reliable predictions which significantly improved the reliability and practicality of the FEM approach in this area.  

c)	The energy method was further improved by correlating the average dissipated energy in one cycle of stress-strain hysterysis loops with some experimentally available fatigue data on solder joints (including some from Jet Propulsion Laboratory) as described in Akay, et al. ("Fatigue Life Predictions for Thermally Loaded Solder Joints Using a Volume-Weighted Averaging Technique," ASME Journal of Electronic Packaging, 1997 - to be published).  This way, a new set of fatigue constants were found for the volume-weighted averaging of the energy method.  We are interested in improving these constants as we have access to more experimentally available data. 

d)	We would like to point out that these constants are dependent on the particular creep model and the set of creep constants used for the correlations, which show a significant variance in the literature.  Hence the ones we obtained are reliable when Dorn's model with a set of creep properties provided by Lam, et al. are used.  However, by going through our correlation procedure, it is possible to determine a new set of fatigue constants, when other models and creep properties are chosen.

e)	Presently we work with an electronic packaging company to obtain some more experimental data on accelerated tests of their resistors.  We are also in search of reliable experimental data of others too so that we can model them with FEM and obtain improved correlations.  We will appreciate it if any solder joint test data is made available to us by the members of this group.  

	Some results and discussions on our recent activities will be presented at the forthcoming TSM Meeting in Orlando (February 9-13, 1997) as well as INTERpack'97 meeting in Hawaii (June 15-19, 1997).  Additional information on the research conducted in our CAD/CAM laboratory is available on the web site: http://www.engr.iupui.edu/cfdlab/cad_cam.html.


Hasan U. Akay
Professor of Mechanical Engineering
IUPUI

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