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Efficient thermal management of large vertically stacked integrated circuits (ICs) requires a material with high thermal conductivity inside the micrometer-sized gaps between the IC dies. Such an underfill material can be obtained by adding thermally conductive filler particles at high loadings to the adhesive matrix material. However, viscosity requirements for the state-of-the-art capillary-driven filling of particle-loaded adhesives limit the particle fill fraction to values lower than those necessary to reach true percolation. Accordingly, heat transfer through the composite material is dominated by conduction through the adhesive matrix material. We propose using an alternative, sequential filling method to achieve percolation with particles down to 1~\mu \textm in diameter. The percolating thermal underfill (PTU) can be achieved by a centrifuge-assisted filling of micrometer-sized particles, followed by a backfilling of a low-viscosity epoxy with long open time acting as matrix material, and a final thermal curing step. In this paper, we present and discuss the fabrication and relevant process parameters of the PTU composite material in chip stacks with critical dimensions below 30~\mu \textm . The wet dispensing of alumina particles with a diameter distribution of 1– 15~\mu \textm is proposed for overcoming the agglomeration observed for dry particle filling. Thermal conductivities of up to 3 W/( \ m\cdot \ K ) for the underfill system with a particle fill fraction of 61% were achieved, which is three times higher than those of commercially available capillary thermal underfills. Critical parameters in the formation of the percolating composite, such as the choice of filler material and its refinement, as well as the properties of the particle bed and the process parameters for the centrifugal filling, epoxy capillary backfilling, and composite curing are addressed. [ABSTRACT FROM PUBLISHER]

Heat dissipation from three-dimensional (3D) chip stacks can cause large thermal gradients due to the accumulation of dissipated heat and thermal interfaces from each integrated die. To reduce the overall thermal resistance and thereby the thermal gradients, this publication will provide an overview of several studies on the formation of sequential thermal underfills that result in percolation and quasi-areal thermal contacts between the filler particles in the composite material. The quasi-areal contacts are formed from nano-particles self-assembled by capillary bridging, so-called necks. Thermal conductivities of up to 2.5W/m K and 2.8W/m K were demonstrated experimentally for the percolating and the neck-based underfills, respectively. This is a substantial improvement with respect to a state-of-the-art capillary thermal underfill (0.7 W/m K). Critical parameters in the formation of sequential thermal underfills will be discussed, such as the material choice and refinement, as well as the characteristics and limitations of the individual process steps. Guidelines are provided on dry versus wet filling of filler particles, the optimal bimodal nanosuspension formulation and matrix material feed, and the over-pressure cure to mitigate voids in the underfill during backfilling. Finally, the sequential filling process is successfully applied on microprocessor demonstrator modules, without any detectable sign of degradation after 1500 thermal cycles, as well as to a two-die chip stack. The morphology and performance of the novel underfills are further discussed, ranging from particle arrangements in the filler particle bed, to cracks formed in the necks. The thermal and mechanical performance is benchmarked with respect to the capillary thermal and mechanical underfills. Finally, the thermal improvements within a chip stack are discussed. An 8- or 16-die chip stack can dissipate 46% and 65% more power with the optimized neck-based thermal underfill than with a state-of-the-artcapillary thermal underfill. [ABSTRACT FROM AUTHOR]