Exposing the composite polymeric casing of integrated circuits (ICs) to water results in miniscule water absorption, followed by its permeation throughout the packaging, and component damage. Studying water penetration and solubility mechanisms in the IC casing is crucial for understanding water-related damage mechanisms and protection against them. The main analytical challenge, hereafter, stems from the need to study miniscule water amounts (< 0.5 wt %) capable of penetrating the casing, despite its hydrophobic nature. In this article, a TGA has been employed to study the water uptake kinetics in the casing, and to decipher the related water penetration mechanisms. High-resolution measurements of water adsorption and desorption profiles were performed, followed by calculations of the related activation energies and solubility enthalpies. These data were correlated with a relatively new model that assigned the primary locales of the adsorbed water to the compatible filler-polymer interface. Thus, water permeability is related to molecules hopping between these sites. Finally, we have shown that for the IC casing in our study, the activation energy of water permeation is related to the binding energy of H2O onto the Si-O-Si groups at the fused-silica-filler surface, where they desorb, hop, and reabsorb. (c) 2006 Wiley Periodicals, Inc.