Characterization and Modeling of Moisture Absorption of Underfill for IC Packaging

Underfill is a highly particle filled epoxy polymer used in flip chip to fill the gap between the leadframe and die or between die and die. In order to be able to flow in the thin gap, the silica filler particle must have a diameter below 10 mum. This epoxy underfill material mechanically couples the chip and substrate and decreases the stress in the solder joints, therefore enhancing solder fatigue life. Good adhesion between the epoxy underfill and the surface of solder ball, silicon chip, and substrate is necessary to minimize stress in a package. Use of the non-hermetic material does raise a potential concern, i.e. moisture induced interfacial delamination and moisture expansion may cause failure. In this study, moisture absorption and desorption properties were investigated, such as moisture absorption equilibrium weight gain and diffusion coefficients at different temperature and different humidity are tested and characterized, moisture absorption and desorption kinetics are tested and fitted.