Effect of system and operational parameters on the performance of an immersion-cooled multichip module for high performance computing

Immersion cooling is making a resurgence for use in server applications driven by an increase in chip densities and the need for reduction in overall data center power usage. This paper focuses on pool boiling characteristics from an array of heaters which simulate electronic chips on a vertically-oriented printed circuit board. A pool boiling study was conducted on an array of four bare die using two different dielectric fluids; namely Novec 649 and HFE 7100, both with low global warming potential (GWP). Tests were conducted at two die spacings; 25mm and 10mm under two different pool conditions: saturated and 15°C subcooled representing a start-up transient. Data were collected for increasing and decreasing heat flux cycles. The flux dissipated was found to be 14.6 W/cm² and 14.2 W/cm² with Novec 649 and HFE 7100 respectively for the test board with 10mm die spacings under subcooled conditions. These values are recorded at self-imposed maximum surface temperatures that ensured operation well below the critical heat flux. These flux values are quite high especially since they are attained without the need to modify the surface or add heat sinks. In an effort to increase thermal performance, tests were also conducted on dies spaced 25mm apart augmented with two different enhanced heat sinks featuring microporous and microfinned surfaces. Enhanced heat sinks performed better than bare die and the flux dissipated was found to be 18.87 W/cm² and 18.31 W/cm² using Novec 649 and HFE 7100 respectively under subcooled conditions. Additionally, these values were achieved at surface temperatures 15°C lower than the surface temperatures recorded by the bare dies for dissipating the same amount of heat. High speed images were obtained to provide a better understanding of nucleation characteristics.