Underfill Filler Settling Effect on the Adhesive Force of Flip Chip Packages

As microelectronic packaging technology is developed toward miniaturization and high density, flip chip is one of the most accepted packaging forms at present. In order to solve the reliability problem caused by the mismatch of thermal expansion coefficient between the silicon substrate and other structures in the flip chip, the underfill is used to solve the stress-strain problem caused by the thermal mismatch. However, due to the gravity, the filler of underfill is prone to settle down during the filling and the curing process. This study focuses on the effect of the filler settling on the adhesive strength of underfill. A simplified sample of sandwich structure was used to mimic the microstructure around the corner of the copper pad and the tin bump in the flip chip. The underfill was filled and cured in the gap (100 μm) between the copper and tin blocks. Three patterns of filler settlement were produced to fill the copper/tin gap by a special experimental method. The adhesive strength of the underfill with three filler settling patterns is measured by the shearing test (Dage 4000) at room temperature and 160°C, respectively. The results indicate that the filler settling has a significant effect on the adhesive strength of underfill. The adhesive strength decreases dramatically when segregation is formed. Therefore, it is important to ensure the uniform filler distribution of the underfill for the reliability improvement of the chip.