Thermo-mechanical assessment of copper and graphite heat spreaders for compact packages

Abstract

Graphite-based materials have been proved to be an enhancement over copper heat spreaders when directly integrated at the silicon level. However, not only they outdo copper in terms of thermal performance, but their in-plane CTE (coefficient of thermal expansion) is much closer to that of silicon. As a result, mechanical stress due to deformation mismatches is reduced during thermal cycling. Thus, thermal interface thickness can be reduced to optimize the heat flow from the hotspot. Heat management of 3D structures implies several challenges. The silicon die and intertier thickness are limited by the vertical connexions height in these heterogeneous stacks. These constraints will also imply strongly thinned heat spreaders and thermal interfaces in a future intertier implementation. This work investigates the thermo-mechanical constraints of integrating a heat spreader at the die level. Copper and PGS (pyrolytic graphite sheet) heat spreaders are compared. Their deformation subjected to thermal cycling is measured experimentally via Thermo-Moiré measurements. The differences in the deformation between the silicon die, molding and the substrate are also measured.