Structure-property relationships between microscopic filler surface chemistry and macroscopic rheological, thermo-mechanical, and adhesive performance of SiO2 filled nanocomposite underfills

SiO2 nanoparticles are attractive components for formulating highly filled underfill adhesives in the electronic packaging field. However, achieving uniform dispersion of inorganic SiO2 nanoparticles into an organic polymer matrix to obtain desired rheological and thermo-mechanical properties remains a significant challenge in nanocomposites engineering. To address the issue, previous studies mainly focused on tailoring the filler morphology and size, volume fraction and filler distribution. Here, in this work, in-situ modification of SiO2 nano particles by using organosilanes with different functional groups was conducted. We demonstrated the structure property relationships between microscopic surface state of SiO2 nanofillers and macroscopic rheological, coefficient of thermal expansion (CTE) and adhesive properties of the resulting amine curing epoxy-based nanocomposite underfills. Our experimental results show that the modification of filler surface effectively enables the decrease of viscosity of underfill, and methacryl-terminated silane exhibits the highest efficiency among various organosilanes. Moreover, surface modification improves the adhesion strength between the underfill and substrate evidently, and nonpolar groups function better than their polar counterparts. In addition, modification with nonpolar methacryloxy and phenyl groups is capable of reducing CTE of nanocomposites effectively while other groups have no significant contribution to CTE reduction. These properties improvement can be attributed to the alteration of interfacial compatibility and adhesion in the SiO2 nanocomposites. Overall, our work provides a useful guideline for the rational surface chemistry design of SiO2 filler and optimizing the overall performance of underfill adhesives.