Cooling of a stacked multichip module - tests and validations

In this paper, the experimental results for a stacked Multichip Module (MCM) designed for single phase direct liquid cooling using FC-72 are presented and discussed. The stacked MCM consists of three thermal test chips, each attached on a carrier, which is stacked vertically, and the whole mounted on a substrate. It is first tested under natural convection in air, to establish the minimum acceptable conditions it can operate. A direct liquid cooling scheme was then applied to the stacked MCM, which is covered by a plastic casing, and the dielectric coolant is forced to flow over the three chips. The thermal resistances for a single chip powered and three chips powered conditions are shown for different flow rates. The pressure drops between the inlet and outlet for different flow rates are also shown and discussed. The characteristics of heat dissipation in a 3D stacked package can hence be deduced. The maximum limit on the package power currently tested (4W) is however quite limited because of the high flow bypass and relatively low flow rate. Further analysis indicates that by reducing the bypass, increasing the flow rate and using better coolants, a maximum power of 40W is achievable for single phase liquid cooling. Higher cooling potential may require two-phase flow conditions.