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This paper reports on the improvement of thermal interfaces through the control of particle stacking during bondline formation. Particle stacking occurs in highly filled materials due to pressure gradients developing during squeeze flow over a rectangular surface, resulting in non-uniform interface properties and thick bondlines with a large thermal resistance. Nested surface channel designs are presented to create a uniform pressure drop as interface material flows across a rectangular surface. Reductions in thermal resistance of 2-3× compared with that of flat surfaces are demonstrated with similar reductions in bondline thickness and assembly pressure. We obtained thermal resistances as low as 2 Kmm 2 /W for thin bondlines (< 5 µm). Comparative powercycling results also demonstrate improved reliability against paste pump-out with nested channel interfaces. KeywordsThermal interface material, TIM, bondline, particle stacking, filtering, squeeze flow, nested channels, lid attach Particle-Filled Interface MaterialsParticle-filled materials are used extensively in the electronics industry to reduce thermal resistance between high-power devices and thermal-packaging components such as spreaders, heat sinks, and liquid cold plates. Thermal interface materials (TIMs) are usually dispensed onto a desired surface from syringes or applied as pre-forms with a subsequent squeezing step to mate the two surfaces and accommodate manufacturing tolerances. Because thermal interfaces consume 20-50% of the total thermal budget, a reduction in their resistance can extend the lifespan of current cooling solutions and further reduce junction temperatures.Thermal greases and adhesives are often made by mixing solid particles in bi-modal or tri-modal size distributions into a viscous matrix at high volumetric loading levelssometimes reaching 78% by volume [1]. At a critical percolation threshold loading of 35-45 v%, the effective conductivity increases significantly owing to random networks of particles in contact with each other [2]. Effective bulk thermal conductivities ranging from 2-5 W/mK are common in the industry, with the thermal performance influenced mainly by particle loading and size distribution. Depending on particle-size distributions and loading fractions, the resulting materials have highly nonlinear viscoelastic properties with a significant yield stress and a sheardependent viscosity that can require squeeze loads high enough to damage flip-chip solder ball arrays and crack chips and substrates.As particle-filled mixtures reach higher viscosities and yield stresses, an increase in bondline thermal resistance is observed because of the difficulty in achieving thin gaps close to the maximum particle sizes. In the industry often a tradeoff is made between thin gaps with low conductivity materials (low particle fill of small size range) or thick gaps with high conductivity materials (high particle fill with large size range) [3]. In addition to extremely high viscosities and yield stresses, another reason why highly fi...

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3D chip-stack packages are more difficult to cool than 2D chip packages due to additional thermal resistances in the heat flow path. The additional thermal resistances are due to the presence of the C4 joins between the chips, the BEOL wiring layers in each chip and the silicon thickness of the chips in the stack. In this paper we present an efficient lid design for a 3D flip-chip package that allows contact, through a thin thermal interface material (TIM) layer, of exposed chip regions of the lower chips in the 3D vertical stack.The efficient lid was assembled on to 3D thermal test vehicle packages and its thermal advantage over standard lid 3D packages was experimentally demonstrated. The packages were cross-sectioned to ensure that the assembly process yielded the correct TIM gaps. A thermal conduction model was calibrated to the experimental data and the stacked chipchip and the efficient lid TIM thermal resistances were extracted from the model. A sensitivity analysis was then conducted to identify the important parameters controlling the thermal performance of the 3D package.