Hotspot remediation with anisotropic thermal interface materials

Shrinking chip feature sizes and increasing performance demands are resulting in non-uniform on-chip power dissipation. Sub-millimeter regions of high heat flux are developing with heat fluxes exceeding the average chip flux by a factor of six to ten and peak heat fluxes approaching 1000 W/cm². These "flux-spots" can yield locally high temperatures ("hotspots") and extreme thermal gradients which can degrade chip performance and compromise reliability. This paper will explore the potential for spreaders and thermal interface materials (TIMs) of anisotropic thermal conductivity to mitigate on-chip hotspots. When used together with an existing global cooling solution, such anisotropic materials, bonded directly to the back of the silicon chip, can conduct heat laterally away from the flux-spot and towards cooler areas of the chip that are subjected to lower heat flux. An available analytical solution for the temperature distribution in a perfectly attached bi-layer slab, subjected to a central rectangular heat flux region, is used to study hotspot remediation with such an orthotropic TIM/spreader. The parametric sensitivity of the hotspot temperature to the in- plane conductivity, TIM/spreader thickness, chip thickness, flux-spot size, and heat transfer coefficient are studied, along with the detrimental effects of an interfacial contact resistance.