This paper focuses on the design and parametric numerical study of a hybrid heat sink combining a liquid thermal interface with an array of synthetic jet actuators for 3-D chip stack cooling. The air-side heat sink exploits enhanced localized heat transfer achieved via a central array of synthetic jet actuators. The key focus of this paper is the numerical simulation of the dielectric liquid interface used to efficiently transmit the heat from the high-power 3-D stacked electronics to the hybrid heat sink base. The coupled natural convection in the fluid and conduction in solid spreaders sandwiched between the tiers of the stack form a novel efficient, passive, and scalable thermal management solution for 3-D stacked die structures. It is shown that this heat sink with a footprint of 76-mm square × 51-mm height can dissipate a total of 41 W of heat/power from the stack for a 44°C average chip temperature rise above ambient (an Rja of ~ 1.06 K/W obtained passively).