The Particle Interaction Analysis for Nanoparticles in Underfill for Flip-Chip Packaging-

The introduction of inorganic nanoparticles into polymer matrix improves the performance of the underfill in flip-chip packaging, for example it improves the mechanical properties of the polymer and reduces the coefficient of thermal expansion (CTE). However, the nanoparticles are not always compatible with the polymer resin and hard to be well dispersed in the colloidal suspension. The phenomenon is caused by the very small diameter, the large specific surface area, and the outstanding surface energy of nanoparticles. As a result, nanoparticles are easy to interact with each other and form agglomerates in the colloid, and lead to mobility problem. To resolve this problem, this paper investigated the particle interaction for raw nanoparticles and surface modified nanoparticles in epoxy resin, and distinguished the functional group impact on the particle interactions. The Fourier transform infrared spectroscopy (FT - IR) is used to characterize the specific chemical groups of modified nanoparticles. The FI-IR result verified that silane coupling agents (SCA) were grafted onto the nanoparticles. Raw nanoparticles and surface modified nanoparticles were suspended into epoxy resin to investigate the rheological behaviour. The rheological flow curves were well fitted by the Herschel-Bulkley model. The dynamic rheology of these colloidal suspensions was examined as well. According to the frequency spectrum, the elastic modulus (G') dominants over the whole angular frequency range for raw nanoparticle epoxy resin colloidal suspension. N anoparticles interact with each other via hydrogen bonding and van der Waals attraction strongly in the suspension. However, surface modification for nanoparticles reduced the fluidity according to the rheological flow curves. The dynamic rheological property demonstrates that surface modification improves the elasticity of the suspension, which means van der Waals attraction between nano-silica particles becomes stronger while the particle-resin interaction is weaker for the SCA modified nano-silica suspension.