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Formulation of percolating thermal underfills by hierarchical self-assembly of micro- and nanoparticles by centrifugal forces and capillary-bridging.
Keywords: percolating thermal underfill, capillary-bridging, hierarchical self-assembly
Silica filled capillary underfill materials are used for many years in flip-chip packages, supporting their mechanical and chemical integrity. In 3D chip stacks heat transfer through the underfill material is an additional aspect to consider. It improves the heat dissipation to the heat sink. Hence, alumina particles were introduced. The particle loading is unfortunately limited below the percolation threshold due to the soaring viscosity. This results in moderate thermal conductivities of < 1 W/m-K, not sufficient for future high-performance chip stacks. Therefore, we propose a novel sequential gap-filling methodology using centrifugation to achieve percolation between micron-sized filler particles. The resulting heat transport is now limited by the point contacts between the fillers. Directed self-assembly of nanoparticles by capillary-bridging is further proposed, to achieve an areal contact - so called necks - between the fillers. Finally, an epoxy resin is injected into the pores of the hierarchical particle assembly. Thermal conductivities of up to 3.8 W/m-K could be demonstrated. We will report on the formation of hierarchical thermal underfills. The optimal centrifugal disk geometry resulting in high filling dynamics is governed by particle kinetics. Filler particle packing densities of > 45 vol. % were achieved on microprocessor package test sites. The meniscus evolution of the injected nano-suspension, defining the capillary-bridges between the fillers during the solvent evaporation is visualized for hydrophobic and hydrophilic surfaces. Uniform neck formation, based on dendritic air penetration into the filler bed is achieved at evaporation temperatures above 60C. Bi-modal nano-suspensions of alumina and polystyrene particles were finally applied to form mechanical stable necks. The alumina particles of 300nm size assemble in dense packing with 100nm polystyrene nanoparticles in interstitial positions. The thermoset material becomes viscous and bridges between the dielectric nanoparticles upon annealing at 150C and thus results in stable necks.
Thomas Brunschwiler,
IBM Research - Zurich
Rüschlikon, ZH

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