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Curing kinetics of epoxy/SiO2 composites for microelectronic applications
Keywords: IC-Substrate, epoxy/SiO2 composites, curing kinetics
Epoxy resins, reinforced by SiO2 fibres or spherical filler particles are composite materials of key interest to answer the increasing demand of miniaturization and performance in advanced microelectronics [Gha2012]. Their thermo-mechanical, adhesive and dielectric properties make them the perfect candidates as insulating build up films in multi-layer printed circuit boards [Zha2014] and integrated-circuit substrates manufacturing. From b-stage films to the final insulating layers, the hardening of the resin matrix is typically achieved by thermally induced cross-linking reactions of the oligomers. The details of this curing process dictate the chemical resistivity during production, as well as the final properties of the composite [Lin2002] and must therefore be well understood and controlled. In this work, we present and discuss an in-depth study of the curing kinetics of a composite material relevant to the electronics industry. The curing kinetics of the epoxy matrix have been probed by in situ near-infrared (FT-NIR) spectroscopy and both isothermal and non-isothermal differential scanning calorimetry (DSC). Kinetic triplet parameters, namely the activation energy, the pre-exponential factor and f(α) as a description of the conversion dependence, have been elucidated using model fitting (nth-order autocatalytic) and isoconversional model-free kinetic methods, based on Friedman's and Ozawa-Flynn-Wall's equations [Vya2011], from both isothermal and non-isothermal experiments. For all applied methods consistent results have been determined for the apparent activation energy of the curing reaction. It has been found to increase with the degree of conversion, with a threshold-like behaviour at approx. 60-80%. On the other hand, the kinetic profiles show a concomitant slowing-down, in both isothermal DSC and FT-NIR experiments. The increase in activation energy, and thus the slowing-down of the reaction rate are assigned to a diffusion-controlled regime of the reaction. Furthermore, the viscoelastic properties of partially-cured composites have been characterized around the glass-transition temperature (Tg) by dynamic mechanical analysis (DMA) using rectangular torsion geometry. The viscoelastic parameters [Fer1980] and the apparent activation energy of the glass-transition have been determined and show drastic changes at the same extent of cure, assigned to a concomitant abrupt decrease of the free-volume. At this degree of conversion, the macroscopic network is built through interconnection of previously formed cross-linked nodules [Bah2015]. The increasingly efficient cross-links lead to the polymer chains being closer, thus restricting the molecular motion (diffusion-controlled regime). Moreover, this free-volume change is consistent with a change in swelling behaviour, posing a major influence on the crucial desmear step in the PCB manufacturing process. This study permits to propose an accurate and predictive multi-scale description of the curing kinetics of the composites. Finally, it is discussed how these kinetic models can be used to ensure proper handling of these materials, as well as to optimize their application in industrial production processes.
Lerys Granado, PhD Student
Atotech Deutschland GmbH
Berlin, Berlin

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