Viscoelastic analysis for numerical modeling of IC assembly processes are generally non-linear and require extensive time and computational resources when compared to a linear elastic analysis. Experimental identification/approximation of the viscoelastic properties (in terms of the Prony series) of any polymeric material is also an exhaustive effort. These drawbacks in experimental procedures and modeling activities have forced us to explore the possibility of approximating viscoelastic response of a polymer with an appropriate linear-elastic model. This paper discusses the impact of different approximation methodologies for a viscoelastic material most commonly used in electronic packaging — the underfill material in flip-chip technology. The modeling methodologies discussed here include the use of long term modulus, short term modulus and other “effective” stiffness measures to approximate the response of underfills during assembly processes. The package response, from each of these models, has been compared to the package response from a linear viscoelastic analysis assuming underfill materials to behave as Maxwell solids. It has been observed that the short term modulus consistently over predicts the ILD (interlayer dielectric in the silicon backend) peel stress for various underfill materials. In addition, the paper also explores the existence of an “effective” stiffness that can be used in lieu of a full fledged viscoelastic analysis. The ability to estimate this “effective” stiffness using available temperature dependent modulus data (from DMA tests) is also discussed. The applicability of this effective metric to different underfill materials has also been explored. In conclusion, this study highlights the need for accurate characterization of polymeric materials so that numerical predictions can provide realistic risk assessments for future packaging technologies.

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