Characterization of the viscoelastic properties of an epoxy molding compound during cure

In the electronics industry epoxy molding compounds, underfills and adhesives are used for the packaging of electronic components. These materials are applied in liquid form, cured at elevated temperatures and then cooled down to room temperature. During these processing steps residual stresses are built up resulting from both cure and thermal shrinkage. These residual stresses add up to the stresses generated during thermal cycling and mechanical loading and may eventually lead to product failure. The viscoelastic properties of the encapsulation material depend highly on temperature and degree of cure. This paper investigates the increase of elastic modulus and the changes in the viscoelastic behavior of an epoxy molding compound, during the curing process. This is done using the shear setup of a Dynamic Mechanical Analyzer DMA-Q800. The cure dependent viscoelastic behavior is determined during heating scans of an intermittent cure experiment. In such an experiment the material is partially cured and then followed by a heating scan at 2 degrees C/min. During this heating scan continuous frequency sweeps are performed and the shear modulus is extracted. The Time-Temperature superposition principle is applied and the viscoelastic shear mastercurve is extracted. Analyzing the shear modulus, the cure dependent viscoelastic material behavior was modeled using the cure dependent glass transition temperature as a reference and a cure dependent rubbery modulus. It is shown that partial curing would increase the glass transition temperature and rubbery shear modulus. It also shifts the viscoelastic mastercurve to the higher time domain. Taking T-g as the reference temperature for different heating scans, the mastercurves collapse to one graph. In addition, using a Differential Scanning Calorimeter (DSC), the growth of the glass transition temperature. T-g(DSC), with respect to the conversion level is obtained. These values are coupled to the values of glass transition temperature in DMA apparatus, T-g(DMA), for calculating the conversion level at each step of curing process in shear mode test. (C) 2012 Elsevier Ltd. All rights reserved.