Enhanced centrifugal percolating thermal underfills based on neck formation by capillary bridging

We present a methodology for the formulation of percolating thermal underfills (PTUFs) with enhanced thermal conductivity for efficient heat dissipation between dies in 3D chip stacks. The methodology is based on the centrifugal filling of micron-sized powders in a confined space (defined by a solder ball array) to form a percolating particle bed, and on the formation of enhanced thermal contacts between particles and contacting surfaces, through the directed self-assembly of nanoparticles around the contacts (i.e. neck formation). The resulting composite material is characterized in terms of the fill fraction and its corresponding thermal conductivity with and without the formation of enhanced particle contacts. For underfills (UFs) formulated without enhanced contacts and using boron nitride, graphite or diamond powders, we have found thermal conductivity values ranging from 1.8 to 2.5 ± 0.1 W/m-K. The formation of enhanced particle contacts using silver nanoparticles dispensed in a 4.8 vol% suspension further increases the thermal conductivity to 3.8 ± 0.3 W/m-K; representing an increase of nearly one order of magnitude compared to silica laden capillary underfills (i.e. ~ 0.4 W/m-K). The thermal conductivity of all samples was measured using our in-home thermal conductivity tester. The increase in the thermal conductivity is related to thermal percolation resulting from the very high volumetric fill fractions (i.e. >; 60 vol%) reached with the proposed approach and to the reduction in the thermal resistance at contact locations by the silver necks. Furthermore, the present methodology is relatively insensitive to the shape and size of particles used, offering a great flexibility in material selection and quality (not acceptable for capillary-based underfills); and could enable efficient heat removal in future 3D chip stacks, flip-chip on board assemblies for mobile applications.