Efficient heat removal from integrated circuits arranged vertically in 3-D chip stacks requires thermally conductive underfill materials. The low-heat-transport performance of traditional capillary underfills can be improved by percolating the thermal conductive filler particles. We increased the thermal path by adding quasi-areal contacts using nanoparticle assemblies directed to the contact points of the percolating filler particles. We studied the formation and thermal effect of such nanoparticle neck assemblies in the filler-particle contact points using aqueous suspensions containing nanoparticles of different sizes, size distributions, and concentrations. An optimized binary mixture of small (28-43 nm) and large (200-300 nm) nanoparticles results in dense and defect-free neck assemblies. A neck-enhanced percolating thermal underfill (PTU) with a thermal conductivity as high as 2.4 W/mK was achieved using alumina filler and nanoparticles. Compared to a PTU, the addition of nanoparticle necks resulted in a more than twofold improvement in thermal conductivity.