Effects of Surface-Modified Alkyl Chain Length of Silica Fillers on the Rheological and Thermal Mechanical Properties of Underfill

Abstract

Due to the increased packaging density as well as the size reduction of the gap between the chip and the substrate in modern flip-chip technology, there is an increasing demand for the use of nanosized silica particles as fillers into underfill to further improve its comprehensive performance. However, silica nanoparticles (SNPs) cannot uniformly disperse into the polymer matrix and usually cause some negative issues, such as a dramatic increase in viscosity and a significant decrease in the glass transition temperature as a result of their excessive surface energy and surface hydrophilic silanol groups. Surface modification with alkyl chains is an effective way to facilitate a uniform dispersion of nanofillers in polymer matrix since suitable repulsive forces can optimize spacing among nanoparticles by decreasing interactions between nanoparticles and changing the surface property of nanoparticles from hydrophilic to hydrophobic. In this paper, SNPs surface-modified by alkyl groups of different lengths were prepared and used as fillers to study the role of the surface alkyl chain length in the rheological and thermal–mechanical properties of underfill nanocomposites. It was found that surface alkyl chains could improve the dispersion of SNPs in epoxy matrix, and thus contributed to decrease the viscosity and enhance the glass transition temperature of underfill nanocomposites. With an increase in the surface alkyl chain length, the viscosity of underfill nanocomposites increased and the glass transition temperature shifted to a higher temperature. This paper sets the way toward the choice of an alkyl chain of proper length as filler surface functional group to improve the comprehensive performance of underfill.