Abstract
In the present study, several experiments were performed to investigate the fundamental characteristics of moisture behaviors in various polymer materials and the interactions of water molecules with polymer matrix. Moisture weight gain tests were performed with different packaging materials. Both Fickian and non-Fickian diffusion behaviors were observed. The mold compound under investigation showed stronger non-Fickian absorption kinetics than the underfill. For most polymer materials in electronics packaging, saturated moisture concentration does not depend on temperature but relative humidity only as long as the temperature is far below the glass transition temperature. However, the saturated moisture content may increase significantly with temperature when the temperature is across the glass transition temperature. There are two distinct diffusion mechanisms involved in the transport of moisture: transfer across surface and transfer through bulk. Water sorption appears to be different from moisture sorption. Hydrophobic film can prevent water liquid molecules from penetrating through the surface. However, this has never been effective for the water vapor transmission through hydrophobic materials. Further in this study, mercury intrusion method was introduced to characterize the pore size and porosity of various materials. For most polymer materials, the free volume or pore sizes are in nano-meter range though the free volume fraction is usually in the range of 1% to 5%. Significant voiding can be developed at reflow process for soft films. An approximate estimation of free volume fraction using weight gain data was proposed. Furthermore, the moiré interferometry technique was employed to study the aging effect of hygroscopic swelling. It was found that hygroscopic swelling is coupled with viscoelastic deformation. At last the mathematical descriptions of moisture phase transition with temperature and the governing equations for a deforming polymer with moisture effect are presented in this paper.
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