On the moduli of viscoelastic materials

The moduli of viscoelastic materials are dependent on temperature and time. Polymeric materials are widely used in electronic packaging as encapsulation, underfill and die attach. The viscoelastic model offers better representation of material performance than the linear elastic model. It is important to provide the viscoelastic properties for polymeric materials in electronic packages for finite element analysis (FEA) modeling. In this paper, different concepts for describing the moduli of viscoelastic materials have been reviewed. Analysis on how to correlate different types of tests for the determination of different moduli of viscoelastic materials was performed. Results obtained from a constant-strain-rate tensile test were employed to illustrate the applicability of the elastic modulus concept. It was found that the tangent modulus or the differential of the stress-strain curve from constant strain rate test is equivalent to the relaxation modulus from a stress relaxation test. The plot of σ / α versus ε / α is no longer dependent on the strain rate for linear viscoelasticity. The difference between the moduli behavior of viscoelastic materials and that of linear elastic material was also discussed.