A high thermally conductive underfill material is key for the efficient removal of heat generated by a 3-dimensional chip stack. Improved thermal properties are achieved by creating a percolating microparticle network within the composite underfill material. In this work, the directed assembly of nanoparticle necks formed by capillary bridging is investigated in order to improve the thermal transport in microparticle to microparticle contacts. The morphology of the formed necks using different alumina nanoparticle sizes and distributions, as well as a sol-gel binding system are characterized. High density and defect free nanoparticle necks were formed by using a mixture of small (28 - 43 nm) and large (200 - 300 nm) nanoparticles. The formation of such necks in the percolating alumina microparticle network increased the thermal conductivity of the underfill material from 1 W/mK without necks to 2.4 W/mK, a 2.4 × improvement in thermal conductivity.