Molecular rigidity and surface tension effect of silane coupling agent on the performance tailoring of underfill

With the miniaturization of the electronic devices, a continuously narrowed down pitch size between the solders is necessary for the advanced flip chip packaging, which results in the application of nano silica fillers in the underfill. Due to the large specific surface area, under the high filling ratio >50 %, the nano-silica filler without any surface modification is prone to aggregate, and the viscosity of the underfill will be increased dramatically in an exponential order, and therefore leads to the failure of the packaging material. In order to reduce the cohesion between silica fillers and improve the interface adhesion between silica and the resin matrix under the large specific surface area, the surface of nano-silica needs to be modified or grafted with the silane coupling agent (SCA), which is always effective in improving the wetting ability of fillers in the resin matrix. The SCA plays a role of molecular bridge between the silica and resin due to the SiO2 reactive siloxy and resin reactive functional groups in the molecular structure. However, the optimum SCA in the underfill is always determined empirically based on huge of experiments. Actually, the molecular rigidity as well as the surface tension of the SCA should play a critical role in the performance tailoring of electronic packaging materials. In this paper, four epoxy SCA, 2-(3, 4-epoxycyclohexyl) ethyl alkyl trimethoxysilane (KBM303), 2-(3, 4-epoxycyclohexyl) ethyl alkyl triethoxylsilane (KH614), 3-glycidyl ether oxypropyl trimethoxysilane (KBM403), 8-glycidyl ether oxyoctyl trimethoxysilane (KBM4803), are used to perform the surface modification of nano-silica fillers. The surface tension of SCA is measured by surface tensiometer. The surface modified silica is characterized by diffuse FTIR, thermal gravimetry analyzer (TGA), SEM, and DLS particle size analyzer. The characterization reveals that the SCA can be successfully grafted onto the silica surface, and the surface modification can effectively improve the dispersibility of the filler. The viscosity, flow length and thermo-mechanical tests of the underfill are fully characterized, and the results show that the best performance can be achieved by utilizing the nanosilica filler modified with the SCA, occupying the resin matchable surface tension and the proper molecular rigidity. This work should be of great importance for the design of high-performance underfill for the advanced electronic packaging.