Compliant Direct Attach Liquid Cooling

Abstract

Increasing device power and the desire to operate systems with higher ambient temperature for reduced cooling facility power are driving a need for improved electronic system thermal solutions. Liquid cooling is one area of significant effort toward this goal. To create a reliable system, however, most high-power device thermal solutions are implemented in a lidded configuration. This configuration requires that the generated heat transit two thermal interface materials (TIM) and the lid. A lower thermal resistance approach is direct attach, where the liquid cooled heat sink is coupled with a single TIM to the device silicon. Generally, such an approach makes significant demands of the TIM, requiring substantial mechanical compliance and robustness. One solution to reducing these demands is to move at least some of the compliance requirements into the heat sink by making the heat sink conformable. Prior work has shown one very effective but expensive demonstration of this approach. In this work we present a different compliant direct attach heat sink approach that shows promise to be high performance, reliable, and cost competitive. A thin, flexible cold plate with distributed pressure loading is combined with a pad TIM resulting in overall junction to water thermal resistance values as low as 19 C-mm2/W. The cold plates are constructed in a mesh configuration utilizing two different stacked sheet assembly processes with total cold plate thickness ranging from 2-5 mm. Some versions are post-processed to reduce pressure drop. Thermal resistance for these cold plates was measured utilizing a uniform power test vehicle. Thermal performance and pressure drop results were obtained across a range of coolant flows. Initial power-cycling data with up to 7000 full-power cycles showed no negative change in thermal performance, indicating the potential for good reliability. Model results for the base mesh structure were matched against the experimental results and used to predict junction temperature improvement relative to a lidded structure for power maps more representative of high-performance product. These results showed potential peak temperature improvement of 14 °C or more for one representative power map with a peak power density of ~3 W/mm2. Overall, this technology shows substantial promise for significant product temperature reduction at competitive thermal solution cost.