Viscoelastic Characterization of an Epoxy-Based Molding Compound
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V. H. KENNER,, M. R. JULIAN, C. H. POPELAR AND M. K. CHENGALVA Department of Aerospace Engineering, Applied Mechanics and Aviation The Ohio State University Columbus, OH 43210 ABSTRACT This paper describes the viscoelastic characterization of a highly filled epoxy molding compound commonly used in electronic packaging applications. Both stress relaxation tests and constant strain rate tensile tests were conducted. The material was found to be nonlinear in its viscoelastic behavior and to be amenable to horizontal shifting to form master curves. A representation of the master stress relaxation curves in terms of a Prony series is given, and the use of this representation illustrated in the context of both linear and nonlinear representations of the viscoelastic behavior to predict the results of the constant strain rate experiments.
INTRODUCTION The evolving and ever more stringent demands on the performance and reliability of electronic packages has necessitated a corresponding increase in the sophistication of the tools utilized in the mechanical aspects of package design and reliability analysis. Since molding compounds typically exhibit viscoelastic behavior and are subjected to significant temperature changes both in the fabrication process and in service, an understanding of their temperature and time-dependent behavior is necessary for careful design or failure analysis.
The investigation
described here was undertaken both to illustrate the application of available techniques of viscoelastic characterization to a practically important molding compound and to provide data for an important package component. A pivotal criterion for the usefulness of characterization as is delineated in this paper is the use to which the data can be put in actual stress analyses of packages. To this end, the viscoelastic data collected in the present investigation are represented in a form which is easily amenable to utilization in analysis tools. Illustration is made, using data collected from constant strain rate tensile tests, of the limits of the application of linearly viscoelastic theory for the presently examined molding compound. A nonlinear representation of the relaxation data has been developed and its use in the commercially available finite element analysis code ABAQUS is also illustrated, again by predicting the results of constant strain rate tensile tests.
EXPERIMENTS The molding compound used in this investigation was manufactured by Nitto-Dinko and bears the designation MP7100. It is an epoxy-based material which is highly filled with silica particles. Further details of its makeup are not available, although it has seen broad application in electronic packaging. 97 Mat. Res. Soc. Symp. Proc. Vol. 390 ©1995 Materials Research Society
Molded dog bone type WV tension specimens conforming to ASTM D638-91 which had a rectangular cross-section with nominal dimensions of 6.35 mm by 3.18 mm in the test section were made available by Intel Corporation. The specimens were subjected to a post mold cure at 175°C for
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