Dual stage modeling of moisture absorption and desorption in epoxy mold compounds

A dual stage diffusion model is developed in this paper for both absorption and desorption processes. Both stages in moisture absorption and desorption, i.e., Fickian and non-Fickian process, are described mathematically using a combination of Fickian terms. Absorption and desorption tests are also conducted on six distinct commercial epoxy mold compounds (EMCs) used in electronic packaging. For absorption, the samples are subjected to 85 °C/85% relative humidity and 60 °C/85% relative humidity soaking, respectively. Desorption conditions are above glass transition temperature at 140 °C and 160 °C. The dual stage models generate reasonable results for the diffusive properties and display outstanding experimental fits. All six compounds show strong non-Fickian diffusion behaviors, which are further verified by the experiments with different thicknesses. For absorption, while Fickian diffusion is dominant in the beginning of process, non-Fickian mechanism plays a large role with time increasing. Saturated moisture concentration associated with Fickian-stage diffusion appears to be independent of temperature under the tested conditions. For desorption, higher temperature leads to less percentage of the permanent residual moisture content in most compounds. At 160 °C, 90% of the initial moisture for all samples is diffused out within 24 h, following an approximate modified Fickian diffusion process. The dual stage model developed in this paper provides a mathematical formulation for modeling anomalous moisture diffusion behavior using commercial finite element analysis software.