Raschid J. Bezama

A Practical Implementation of Silicon Microchannel Coolers for High Power Chips

This paper describes a practical implementation of a single-phase Si microchannel cooler designed for cooling very high power chips such as microprocessors. Through the use of multiple heat exchanger zones and optimized cooler fin designs, a unit thermal resistance 10.5 C-mm2 /W from the cooler surface to the inlet water was demonstrated with a fluid pressure drop of <35kPa. Further, cooling of a thermal test chip with a microchannel cooler bonded to it packaged in a single chip module was also demonstrated for a chip power density greater than 300W/cm2. Coolers of this design should be able to cool chips with average power densities of 400W/cm2 or more

A Graphite Foams Based Vapor Chamber for Chip Heat Spreading

A vapor chamber using high thermal conductivity and permeability graphite foam as a wick has been designed, built, and tested. With ethanol as the working fluid, the vapor chamber has been demonstrated at a heat flux of 80 W/cm 2 . The effects of the capillary limit, the boiling limit, and the thermal resistance in restricting the overall performance of a vapor chamber have been analyzed. Because of the high thermal conductivity of the graphite foams, the modeling results show that the performance of a vapor chamber using a graphite foam is about twice that of one using a copper wick structure. Furthermore, if water is used as the working fluid instead of ethanol, the performance of the vapor chamber will be increased further. Graphite foam vapor chambers with water as the working fluid can be made by treating the graphite foam with an oxygen plasma to improve the wetting of the graphite by the water.

High Performance and Sub-Ambient Silicon Microchannel Cooling

High performance single-phase Si microchannel coolers have been designed and characterized in single chip modules in a laboratory environment using either water at 22°C or a fluorinated fluid at temperatures between 20 and −40°C as the coolant. Compared to our previous work, key performance improvements were achieved through reduced channel pitch (from 75 to 60 microns), thinned channel bases (from 425 to 200 microns of Si), improved thermal interface materials, and a thinned thermal test chip (from 725 to 400 microns of Si). With multiple heat exchanger zones and 60 micron pitch microchannels with a water flow rate of 1.25 lpm, an average unit thermal resistance of 15.9 C-mm2 /W between the chip surface and the inlet cooling water was demonstrated for a Si microchannel cooler attached to a chip with Ag epoxy. Replacing the Ag epoxy layer with an In solder layer reduced the unit thermal resistance to 12.0 C-mm2 /W. Using a fluorinated fluid with an inlet temperature of −30°C and 60 micron pitch microchannels with an Ag epoxy thermal interface layer, the average unit thermal resistance was 25.6 C-mm2 /W. This fell to 22.6 C-mm2 /W with an In thermal interface layer. Cooling >500 W/cm2 was demonstrated with water. Using a fluorinated fluid with an inlet temperature of −30°C, a chip with a power density of 270 W/cm2 was cooled to an average chip surface temperature of 35°C. Results using both water and a fluorinated fluid are presented for a range of Si microchannel designs with a channel pitch from 60 to 100 microns.Copyright

Microjet Cooler with Distributed Returns

This paper details the technology elements developed to design and manufacture a liquid microjet array cooling device for the thermal management of very high power dissipating electronic chips. Multilayer ceramic technology (MLC) is used to build the cooling device with micron-size jet arrays, which includes a distributed return network for the spent fluid. Intertwined microchannel flow networks inside the cooler body distribute the flow in and out of the device. A cooler with 1600 jets and 1681 interstitial returns for the drains built using glass ceramic material is discussed. When tested with an 18 mm heated silicon chip and an average convection coefficient of 0.052 MW/m2K, the device demonstrated a cooling capability greater than 2.5 MW/m2 with a water pressure drop of < 70 kPa. Further extension of the cooling capability to greater than 6 MW/m2, as predicted by the simulation, is also discussed.

Silicon Microchannel Cooling for High Power Chips

In this work, single-phase Si microchannel coolers have been designed and characterized for cooling very high power density chips in single-chip modules (SCMs) in a laboratory environment. The average heat transfer coefficient was determined for a wide range of microchannel designs. Through the use of multiple heat exchanger zones and optimized cooler fin design, an average unit thermal resistance of 16.2°C·mm2/W between the chip surface and the inlet cooling water was demonstrated for an Si microchannel cooler attached to a chip with Ag epoxy in an SCM. Very good uniformity from SCM to SCM (±2%) and within an SCM (±5%) was achieved. Further, cooling of a thermal test chip with a microchannel cooler bonded to it and packaged in an SCM was also demonstrated for a chip power density greater than 400 W/cm2. Coolers of this design should be able to cool chips with average power densities of 500 W/cm2 or more.

A Vapor Chamber Using Graphite Foams as Wicks for Cooling High Heat Flux Electronics

A vapor chamber using high thermal conductivity and permeability graphite foam as a wick has been designed, built and tested. With ethanol as the working fluid, the vapor chamber has been demonstrated at a heat flux of 80 W/cm2 . The effects of the capillary limit, the boiling limit, and the thermal resistance in restricting the overall performance of a vapor chamber have been analyzed. Because of the high thermal conductivity of the graphite foams, the modeling results show that the performance of a vapor chamber using a graphite foam is about twice that of one using a copper wick structure. Furthermore, if water is used as the working fluid instead of ethanol, the performance of the vapor chamber will be increased further. Graphite foam vapor chambers with water as the working fluid can be made by treating the graphite foam with an oxygen plasma to improve the wetting of the graphite by the water.Copyright

Ceramic Microjet Cooling Device

This paper details the technology elements developed to design and manufacture a liquid microjet array cooling device, for thermal management of very high power dissipating electronic chips. Multilayer ceramic technology (MLC) is used to build the cooling device with microns size jet arrays, which include distributed return network for the spent fluid. Intertwined microchannel flow networks inside the cooler body distribute the flow in and out of the device. A cooler with 1600 jets and 1681 interstitial returns for the drains built using Glass Ceramic material is discussed. The device when tested with an 18 mm heated silicon chip and an average convection coefficient of 0.052 MW/m2 K demonstrated a cooling capability greater than 2.5 MW/m2 , with a water pressure drop of < 70 kPa. Further extension of the cooling capability to greater than 6 MW/m2 , as predicted by the simulation is also discussed.Copyright