VOLUME 1 ISSUE 12 JANUARY 2008 THE NEWSLETTER FOR THE THERMAL MANAGEMENT OF ELECTRONICS FUTURE COOLING In this issue: Thermal Management by Immersion Cooling 1 Future Cooling PROPERTY FC-87 FC-72 FC-77 H2O Boiling Point @ 1 Atm (°C) 30 56 97 100 -3 3 Density x 10 (kg/m ) 1.63 1.68 1.78 0.1 -3 TJ ColdSpecific Heat plate x 10 (Ws/kgK) 1.09 1.09 1.17 4.18 Thermal Conductivity (W/mK) 0.05 0.05 0.06 0.63 7 Thermal Minutes Dynamic Viscosity x104 (kg/ms) 4.20 4.50 4.50 8.55 -4 Heat of Vaporization x10L (Ws/kg) 8.79 8.79 8.37 243.8 Surface Tension x103 (N/m) 8.90 8.50 8.00 58.9 Thermal Coefficient of Expansion x 103 (K-1) 1.60 1.60 1.40 0.20 Dielectric Constant 1.71 1.72 1.75 78.0 14 Thermal Analysis Table 1. Thermal Properties of Fluorocarbon Fluids and Water [2]. The term ‘immersion cooling’ implies The use of liquids is an attractive propo­ that a device is plunged entirely into a sition for heat transfer.Liquid bath It provides three fluid. Following this course, traditional broad spectra and distinct benefits: Thermalair cooling could be considered immer­ TJ-Junction • High heat transport capability 19 Thermal Fundamentals sion cooling because an entire system Insulationis immersed in air, a gaseous fluid. Finned enclosure temperature • No need for a secondary medium However, immersion typically refers to a for removal of the between the heatheat source and the sink high power system that is immersed in some type of inert fluid, such as mineral • Effective heat spreading on a larger oil or a 3MTM FluorientTM Electronic Fluid, surface area e.g. FC-77. Although water is the best 25 Cooling News fluid for heat transfer, there are many The residual benefits of having the heat others on the market for use in electron­ source immersed in a coolant are also ics cooling by immersion or in cold plate significant. For example, by immersing applications [1]. To gain an appreciation the electronics in a liquid bath, the ther­ for the thermal transport capabilities of mal resistance between the heat source different fluorinert fluids, it is instructive and the sink is eliminated, as shown in to compare their properties with water. Figure 1. 26 Who We Are Table 1 shows such a comparison. Thermal Insulation (top and bottom) Ta Ta TJ P1 P2 TB θb1b2 Tb1 Tb2 © ADVANCED THERMAL SOLUTIONS, INC. 2007 | 89-27 ACCESS ROAD NORWOOD, MA 02062 USA| T: 781.769.2800 WWW.QATS.COM PAGE 1 Cold plates PROPERTY FC-87 FC-72 FC-77 H2O Boiling Point @ 1 Atm (°C) 30 56 97 100 Density x 10-3 (kg/m3) 1.63 1.68 1.78 0.1 Specific Heat x 10-3 (Ws/kgK) 1.09 1.09 1.17 4.18 Thermal Conductivity (W/mK) 0.05 0.05 0.06 0.63 4 Dynamic Viscosity x10 (kg/ms) 4.20 4.50 4.50 8.55 -4 Heat of Vaporization x10L (Ws/kg) 8.79 8.79 8.37 243.8 3 Surface Tension x10 (N/m) 8.90 8.50 8.00 58.9 3 -1 Thermal Coefficient of Expansion x 10 (K ) 1.60 1.60 1.40 0.20 Dielectric Constant FUTURE COOLING 1.71 1.72 1.75 78.0 are used in the boiling mode or single phase, the heat transfer coefficient is substantially larger than from other cooling modes. The cooling capability is clearly demon­ strated by Figures 3 and 4 [3]. Figure 3 shows the cooling capacity of 50 W/cm2 in natural convection with moder­ Liquid bath ate fluid temperature rise. If forced convection is used, the cooling capacity is increased appreciably at a lower temperature rise. 1000 Finned enclosure F C -72 for removal of the Tf = 20 °C heat . 100 2 C HF q" = 50 W /c m g n i l i g o lin b i o e b t o N Figure 1. Immersion Cooling of Electronics in a Passive Mode, 10 ea l 2 c Providing a Direct Path Through the Liquid from the Source to u the Sink. N n T = 85 °C Figure 1 shows a system where natural convection within o s ti ec q" ( W/cm ) v an enclosure is used for cooling the devices on a double- n 1 o 2 C c ° sided PCB. If forced convection or boiling had been id u c m iq / used, the thermal transport capability would have been l l W a .1 r 0 tu significantly larger. Subsequently, higher heat dissipation a = N h from the electronics could have been achieved. Figure 2 demonstrates such a capability by comparing different Ta Ta .1 heat transfer coefficients. n io c t ve n o Air c P1 P2 ir θ a Fluorochemical Naturalb1b2 d e WaterLiquids Convection rc Fo Tb1 Tb2 .01 1 1 0 100 1000 Air S ingle-Phas e Ts - T f (°C ) Fluorochemical Forced Liquids Water Convection Figure 3. Comparison of Natural and Forced Convection in Fluorochemical B oiling Fluorinert FC-72 [3]. WaterLiquids In another example, Mudawar shows the thermal trans­ 0.0001 0.001 0.01 0.1 1 10 100 1000 port capacity of the same coolant, FC-72, in a boiling h (W/cm2K) phase in different modes [3]. Jet impingement, mini-chan­ Figure 2. Heat Transfer Coefficients for Different Coolants and nel and low-speed cavity flow are compared not only with Cooling Configurations. each other, but against conventional conduction cooled chassis. From Figure 2, it is quite evident that when fluorocarbons © ADVANCED THERMAL SOLUTIONS, INC. 2007 | 89-27 ACCESS ROAD NORWOOD, MA 02062 USA | T: 781.769.2800 WWW.QATS.COM PAGE 2 Air Fluorochemical Natural WaterL iquids C onvection Air S ingle- Phase F luorochemical Forced L iquids Water C onvection F luorochemical B oiling WaterL iquids 0.0001FUTURE 0.001 COOLING 0.01 0.1 1 10 100 1000 h (W/cm 2K) Jet inpingement BTPFL-C3 Sub-cooled Boiling FC-72 Mini-channel flow BTPFL-C2 Sub-cooled Boiling FC-72 Low-speed Cacity Flow BTPFL-C1 Sub-cooled Boiling FC-72 Air Force Liquid Cooled Frame Module Channel Flow, Single Phase Indirect Liquid Cooling Polyalphaolefin (PAO) Convection Edge Cooled Modules Conduction Cooling 0 2 4 6 8 10 12 14 16 18 20 SEME - PEAK COOLING RATE (kW) Figure 4. Comparison of Boiling FC-72 in Three Different Transport Modes vs Conventional Cooling [3]. Figure 5. Heat Flux (W/Cm2) as a Function of “Wall Superheat” or the Surface-To-Liquid Temperature Difference for a Typical Figure 4 describes thermal transport as a function of dif­ Fluorocarbon Coolant [5]. ferent heat transfer modes in an immersion configuration. Such a high heat transport capability is attractive for electronics cooling when dealing with high power devices Figure 4 clearly shows that a fluid’s flow delivery method or an aggregate of lower power devices concentrated in a can have a significant impact on its thermal transport small area. Perhaps the most famous immersion cooling capability, e.g., jet impingement, while allowing it to boil application on the market is the Cray-1 computer. Intro­ in a particular regime. When compared to conventional duced during the 1970s, its unique electronics packag­ cooling, there is an order of magnitude change which ing, dictated by the need for the highest communication corroborates the heat transfer coefficient data shown in speeds between different devices, required novel cooling Figure 2. to make the system operational. This fact suggests that the highest heat transfer can be achieved when the fluid boils. The boiling mode also provides another advantage: a constant component tem­ perature that positively affects device reliability. However, there are no gains without penalties. Along with the pack­ aging challenges to gain such high levels of cooling, ther­ mal overshoots may occur when boiling begins. This is because, in these applications, the fluids tend to be low in surface tension and viscosity. This could create rapid dry out as a result of the liquid-to-vapor transition. Over the past decade, researchers have contributed many articles to the physics of boiling heat transfer and temperature overshoot. Such detail is not the scope of this article, but the reader should be aware of the potential challenges encountered by cooling with boiling heat transfer [4]. Figure 6. Cray Super Computer with Cooling Tower in the Background. It is helplful to also note that both natural and forced convection can be rather effective for electronics cooling. In this system all PCBs were installed horizontally and Thermal transport capability may be best described by the entire system was immersed in coolant. Stacks of 2 Figure 5, where the heat flux (W/cm ) is presented as a electronic module assemblies were cooled by a parallel function of “wall superheat” or the surface-to-liquid tem­ forced flow of FC-77. Each module assembly consisted perature difference for a typical fluorocarbon coolant [5]. of 8 printed circuit boards on which were mounted arrays © ADVANCED THERMAL SOLUTIONS, INC. 2007 | 89-27 ACCESS ROAD NORWOOD, MA 02062 USA | T: 781.769.2800 WWW.QATS.COM PAge • Fully portable and automated • Embedded PC with touch screen • Temperature Range from -30°C to 150°C (±1 °C)* • Velocity from 0 to 50 m/s (1 0,000 ftlmin) (± 2%) • Available with up to 32 channels • Ethernet and CD-RW enabled • Automated data acquisition & reduction • Up to 32 channels • Temperature Range from -30°C to 150°C (±1 °C)* • Velocity from 0 to 50 m/s (1 0,000 ft/min) (± 2%) • stageVIEW™ thermal analysis software • Available with 4 or 8 channels • Temperature Range from -30°C to 150°C (±1 °C)* • Velocity from 0 to 50 m/s (10,000 ft/min) (± 2%) • stageVIEW™ thermal analysis software • Design allows continuous repositioning and reading • Measures temperature and velocity • Narrow and low profile minimizes disturbance • Temperature Range from -30°C to 150°C (±1 °C)* ADVANCED IS THERMAL SOLUTIONS, INC.