In-situ characterization of moisture absorption-desorption and hygroscopic swelling behavior of an underfill material

Moisture absorption and hygroscopic swelling behavior of an underfill material were measured in-situ using the sorption TGA and the DMA-RH techniques, respectively. Results showed that moisture diffusion can be well described by Fick's law when the relative humidity (RH) is 60% or less, and the diffusivity exhibits an Arrhenius temperature dependence with an activation energy of ~ 0.56 eV. At 85% RH, non-Fickian diffusion behavior becomes apparent in the latter stage of the diffusion process. Increase in the saturated moisture content (C
_sat
) at 60%RH was observed after exposure at 85°C 85%RH, which is attributed to hygrothermal ageing induced damage in the material. In-situ DMA-RH results showed that hygroscopic swelling strain is significant comparing with the thermal expansion of the material, and the swelling strain at 60°C 60%RH is equivalent to the thermal strain over a ΔT of 100°C for T <; T
_g
. The coefficient of hygroscopic swelling (CHS) was calculated and the results showed that CHS is temperature dependent, and it increases with the increasing temperature. At higher humidity condition (85%RH or higher), the swelling strain deviates from its linear dependence on C
_sat
and it increases faster than linear when C
_sat
increases, which may also be attributed to hygrothermal ageing induced damage in the underfill material.