In situ moisture absorption and hygroscopic swelling behavior of an underfill material used for electronic packaging have been investigated using sorption thermogravimetric analysis (sorption TGA) and dynamic mechanical analysis (DMA) techniques under a controlled relative humidity (RH) environment. Results showed that moisture diffusion in this material can be described by Fick's diffusion law when the relative humidity is 60% or less, while at a high moisture level (85% RH), non-Fickian diffusion becomes apparent in the latter stage of the diffusion process. An increase in moisture absorption ability was observed after exposure at high temperature and high humidity level, which can be attributed to hygrothermal aging-induced damage in the material. DMA results revealed that hygroscopic swelling is significant comparing with the thermal expansion of the material. The coefficient of hygroscopic Swelling (CHS) was calculated and the results showed that CHS is temperature dependent, and it increases with the increasing temperature. (C) 2012 Elsevier B.V. All rights reserved.