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Stresses in Polyimide Films during Cure and Thermal Cycling

Published online by Cambridge University Press:  15 February 2011

David J. Monk ,
Yongxiang He  and
David S. Soane
David J. Monk
Affiliation:
Department of Chemical Engineering, 201 Gilman Hall, University of California, Berkeley, CA 94720, (510) 642–0958, FAX (510) 642–7798
Yongxiang He
Affiliation:
Department of Chemical Engineering, 201 Gilman Hall, University of California, Berkeley, CA 94720, (510) 642–0958, FAX (510) 642–7798
David S. Soane
Affiliation:
Department of Chemical Engineering, 201 Gilman Hall, University of California, Berkeley, CA 94720, (510) 642–0958, FAX (510) 642–7798
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Abstract

Several electronics packaging schemes involve polymer/inorganic interfaces, including: dual-in-line packages, tape automated bonding and multilayer interconnects. Typically, the thermal expansion coefficients are disparate, so these interfaces often cause high stress. Therefore, a phenomenological model describing transient stresses in spin-coated polyimide films was developed. The model is based on linear viscoelastic theory, and it accounts for shrinkage caused by solvent evaporation and imidization, viscoelastic relaxation, and thermal expansion mismatch. Strains have been defined from three mechanisms: thermal expansion mismatch, chemical curing, and solvent evaporation. Stress is, then, calculated by using the Classical Maxwell Model with one element. The concept of free volume is used throughout the model to estimate viscosity, modulus, and other quantities related to calculating strains. Model predictions for stress as a function of temperature during film cure and thermal cycling are fit with experimental data obtained from a bending beam apparatus.

Stress has been estimated by using the thin film approximation of the Timoshenko bilayer stress equation. Experimental data agree well with wafer bowing stress measurements. Although the technique does not yet take into account changing polyimide thickness during curing, the results still show qualitative curing dynamics. This preliminary study revealed good agreement between predicted and observed effects of material properties on stresses developed during cure and thermal cycling. Specificially, an unexpected high in-cure stress was observed for a standard low CTE polyimide. High stresses during curing can be as detrimental to an electronics device as high stress during device operation, so this technique may be useful when screening polyimides and/or prescribing curing schedules. Future work will improve the predictive capability of the model.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 264: Symposium G – Electronic Packaging Materials Science VI , 1992 , 175
Copyright
Copyright © Materials Research Society 1992

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