Thermal Technology for Cooling Electronics Koichi Mashiko 1 Thin heat pipe modules are widely used for cooling the CPU of a PC or electronic devices. To meet customer demands for improvement in thermal performance and reduction in size and height, we have developed an ultra-thin (1mm) heat pipe. In addition, we have worked on the development of an ultra-thin (1mm) piezo fan included in the cooling module. Another product we developed is a water-cooled module having a special channel structure for a power semiconductor device of a power converter used for wind or solar power generation requires a water-cooled system to help increase the converter capacity. As the power consumption of datacenters or super computers has been increasing, increases in efficiency of cooling facilities and reductions in power consumption are absolutely necessary. So far, the air cooling system has been predominantly adopted in this field, but currently water cooling system is being reconsidered as an efficient cooling system. We also completed the development of a cold plate unit, which has a micro channel structure, for cooling super computers. After making the prototypes, the performance of the system met customer demands, and our products have contributed not only to the highest calculation speed but also to reductions in the power consumption of CPUs. Shipment of product ( 1 Mil/Y) 1 9 8 7 6 5 4 3 2 1 PC Fig. 1. Shipment number prediction (worldwide) of PC, Tablet and Smartphone 1 Thermal Technology Division Tablet and Smartphone (Prediction) 29 21 211 212 213 214 Year [US-IDC] 1. Introduction Cooling modules assembled from a heat pipe and a fan have become widely used as an increase in the performance of a CPU used in a note PC has resulted in increases in power consumption with increases in clock frequencies. 1) Figure 1 shows the progression of the shipment number (worldwide) of PCs, tablets, and smart phones. The number of notebook PCs is increasing, but that of thinner note PCs, in particular, is increasing. The heat dissipation of the CPU is approximately 15 W, and that of tablet is approximately 5 W. The cooling modules for these CPUs are required to be thinner and lighter in weight. To meet the requirements for the cooling modules, we continuously put our efforts into developing thinner heat pipes as well as higher performance heat pipes. 2) In addition, with downsizing of electronic equipment, we have developed smaller size heat pipes and are looking for their uses in new fields. On the other hand, water cooling system has become necessary instead of conventional air-cooling system in particular for cooling power modules and data-centers that of high power, high performance and compact. Power consumption at data-center will always be increasing due to the demand of faster and more data processing. Figure 2 shows the trend of power consumption estimated by Green IT Promotion Council. Power Consumption (Bill Kwh/Y) 8 6 4 2 (Source: Green IT Promotion Council) 8 Power consumption Exhaust Quantity 6 4 2 CO2 Exhaust Quantity (Mil Ton/Y) 25 215 225 235 245 Year Fig. 2. Estimated trend of the power consumption in datacenters (Green IT Promotion Council) 7
Abbreviations, Acronyms, and Terms. CPU Central Processing Unit PUE Power Usage Effectiveness IGBT Insulated Gate Bipolar Transistor According to the simulation, if the use of IT apparatus increases at the current pace, the amount of power consumption of the apparatus in 225 is expected to be approximately over 5 times that of 25, or 24 Billion kwh. The power consumed at the facilities for cooling CPUs and IT apparatus accounts for 8% of the total power consumption. Therefore, cutting down on the power consumption has become an important task for data-centers. The air cooling system of data-centers shown in Fig. 3, has a problem that hot spots occur during cooling. For air cooling, its cooling efficiency is insufficient because a CPU is cooled by air, which has a low heat capacity and thus requires higher air flow and therefore higher power consumption. So, the water cooling has been reconsidered as a more effective means instead CRAC Rack Rack Rack Ceiling return option Rack CRAC Vents Vents Room Chilled Water Supply Fig. 3. Air cooling system of data-center of the conventional air-cooling. 2. Thermal Technology for Cooling Electronics Figure 4 shows applications of our thermal products. The main thermal products include heat pipes, and the modules, heat sinks, vapor chambers, heat spreaders, and cold plates. These products are widely used, especially for cooling PCs and electronics. With the trends of thinner and compact assemblies in a PC, a thinner heat pipe module is required. On the other hand, an effective and high performance water cooling module is required for a data-center cooling system. Our newest products and their typical applications are described as follows: 2.1 Micro Heat Pipe for Cooling PC 2) With the trend of compact cooling in PCs, there has been a growing demand for a thinly pressed micro heat pipe. Figure5 shows the thickness and the heat dissipation of micro heat pipe, which are required for PCs and mobile devices. For high performance notebook PCs, a heat pipe requires heat dissipation of 3 to 5 W and 1.5 mm thickness. If the heat pipe is used for cooling Note PC, the heat pipe is installed in a horizontal position. In this case, the maximum heat transfer rate (Qmax) of the heat pipe is expressed in the following formulas, (1) and (2), which include the factors of (QL)c-max and (Leff). Cooling Electronics Heat Pipes Snow melting & Environment Heat Pipes module Automotive Hat Sink & Heat Spreader Power/Energy Data Center/Server Vapor chamber & Cold Plate Aviation Fig. 4. The field of application of the thermal product Fujikura Technical Review, 213 71
Heat Dissipation (W) Heat Dissipation (W) 4 3 2 1 5 4 3 2 1 Ultra Thin PC Note PC Tablet, Smart phone.5 1 1.5 2 2.5 Thickness of heat pipe T (mm) Thinner Heat Pipe Fig. 5. Required performance (the thickness and heat dissipation ) for cooling thin notebook PC and Tablet.5 1. 1.5 2.. 2.5 Thickness heat pipe (mm) D4_Qmax D4_Resistance D5_Qmax D5_Resistance D= 6 mm D= 5 mm D= 4 mm Fig. 6. Relation between thickness and maximum heat transfer rate (Qmax) in each size of MHP Qmax=(QL)c-max/Leff (1) Leff=L - (Le + Lc)/2 (2) Here, Qmax : Maximum heat transfer rate of heat pipe [W], (QL)c-max [W-m] : Factor of maximum heat transfer of heat pipe with capillary wick structure Leff: Effective length of heat pipe L: total length of heat pipe [m] Le, Lc: evaporator, condenser length of heat pipe [m] In a conventional heat pipe, when it is pressed to less than 1.5 mm in thickness, there is a problem that the thermal performance decreases due to insufficient space for vapor flow. Then, a new internal wick structure and surface treatment have been devised for improvement. Figure 6 shows the thermal performance when the heat pipes of 4, 5, and 6 mm in diameter are pressed to 1 to 2 mm in thickness. The results show that with the new internal structure, each heat pipe of 4, 5, 6 mm in diameter ensures heat transfer rates of 1, 13, 2 W even if each of them is pressed to 1.2 mm in thickness. 2.2 Ultra-thin Piezo Fan A compact fan is also used for cooling a CPU of a notebook PC, but conventional DC brushless fans have limitations in thickness up to 3 mm. So, we have developed a 1 mm ultra-thin piezo fan. Figure 7 shows 1..8.6.4.2 Thermal Resistance, R=(Te- Tc)/Q (C/W) D6_Qmax D6_Resistance Ta 4 mm AC Power Top view of DCJ 4 mm Cross section view of DCJ Compression/Expulsion volume of Air Expansion/Ingestion volume of Air Principle Piezo device Blade Fig. 7. Structure and principle of piezo fan 42 mm DCJ Heater Temperature Th (degreec) T air-out 12 1 8 6 4 2 3 mm Heat Sink Fig. 8. Example of cooling performance of piezo fan combined with thin heat pipe the structure of this piezo fan and the principle of operation. The circular piezo device is bonded to the square metal blade, and then the same two parts are bonded together. This structure enables a total thickness of 1 to 1.5 mm. Figure 8 shows an example of assemblies with a thin heat pipe and the piezo fan. This cooling module has been developed to be used for cooling ultra-thin notebook PCs and tablets. Th Thin heat pipe (1.5 mm) Heater (12 11) Heat Spreader Thermal performance Ambient 25 C 5 1 12 15 Input power (W) 72
2.3 Cooling module for power electronics (IGBT) In recent years, as wind power systems and photovoltaic power generation systems are growing in demand, there are growing demands for high-power semiconductors used in electricity converters to convert DC to AC. As shown in Fig. 9, direct contact cooling method is applied to cooling IGBT instead of conventional indirect cooling method. Figure 1 shows an example of water cooling products for IGBT. The heat dissipation is 2.5 kw at one side (approximately 2 W/ cm2 of heat flux). c) Indirect Cooling (Water/Air cooling) b) Both side Cooling c) Direct Cooling (Water cooling) (Water/Air cooling) Fig. 9. Comparison of cooling method of IGBT module Cooling module for IGBT Cooling module for IGBT Fig. 1. Application of cold plate for cooling power module of power semiconductor 3. Thermal Technology in data-center cooling 3)4)5) 3.1 Trends of data-center cooling technology With increases in CPU heat dissipation, a hot spot problem occurred in the air cooling system of datacenters, which increased power consumption for cooling sufficiently. Recently, data-centers have more effectively arranged equipment and control systems of the facilities according to their heat generation. The Green Grid recommends PUE (Power Usage Effectiveness) to data-centers as the index. PUE=P(total)/P(IT) (3) Here, P(total): Total power usage of data-center, P(IT): Power usage of IT equipment of data-center In Table 1, the comparison between air cooling system and water cooling is shown. The water cooling is confirmed more effective than air cooling to meet the demands for high density assembly and performance of the servers at data-centers. 3.2 Cold plate technology for CPU of cooling super computer The faster the calculation speed of a super computer is, the more water cooling is needed for cooling its CPUs because the system has high PUE. To respond to the requirements for high performance (thermal resistance of.5 C/W) and mechanical strength (against inner pressure 1 MPa) of a cooling module for high density assembly of data-center, we built a cold plate with a micro channel heat transfer surface. The micro channel cold plate has much more surface areas for heat transfer and higher heat transfer coefficient compare to conventional cold plate consist of pin fins. Table 1. Comparison between air cooling and water cooling of data center Advantage Disadvantage and Countermeasure Cooling system and PUE (Power Usage Effectiveness) Air Cooling Easy to assembly (Changing position, restructure) No piping of water Need the space of heat sink module. Hot spot and limitation of cooling capacity. PUE=1.8-2. Heat sink Rack CPU Hot air Air mover Air conditioner Cold air Water Cooling High cooling capacity High density of assembly Saving Power Need water piping, Leakage of water PUE=1.4-1.8 Cold water Cold Plate CPU Rack Hot water Chiller Pumping unit Fujikura Technical Review, 213 73
We built a cold plate with the upper part of a micro channel brazed to the inside of the cover with the cover brazed to the base. We adopted a special machining method to manufacture micro channel fins. In Fig. 11, the final design of the cooling unit is shown. We completed the unit consisting of eight cold plates, pipes and couplers, which were brazed together, in cooperation with a customer who is in charge of designing. This cold plate requires the reliability of mechanical strength (1 MPa) as well as that of high thermal performance. Figure 12 shows a photo of the rack of a super computer manufactured by our customer. This rack is composed of 3 units of system boards, at each end of which water cooling piping is placed. In the evaluations by our customer, this cold plate could decrease the temperature of the CPU drastically and also the power consumption of CPU. Fig. 11. Cooling Unit with cold plates Cold plate (8 pieces) One touch Coupler Manifold Copper tube Transportation plate (aluminum) 3.3 Proposal of energy conservation in data-center 5)6) In the above, cooling technology of CPU is described, on the other hand, an energy conservation of cooling supply source in server or data-center is also important item. The countermeasure is based on each cooling system. In Table 2, the comparison of cooling method and installation condition between Japan and the USA of cooling system of data-center is shown. In the USA, large scale data-centers are installed outside of city, and the cooling system is mostly water cooling. On the other hand, in Japan, most of data-centers are installed in city region, and cooling system is air cooling. For energy conservation in cooling source of water cooling system, we are proposing a system that utilizing the natural cold energy for cooling. Figure 13 shows the principle of our proposal by using heat pipes. One of our proposals is that heat pipes are cooled and water is iced in winter season, cold energy is stored then the cold energy is used for cooling source for back up source. This concept proposal will be compact and occupied less space than conventional water storage tank. Another proposal is pre-cooling of Table 2. Comparison of cooling method and installation condition of data-center between Japan and USA Place Japan USA Installation site Inside of city Outside of city Space Limited space Large space Cooling method Mainly air cooling Air cooling and water cooling Shut down Few Sometimes Earthquake Often Few Data center Fig. 12. Photo of rack of Super computer equipped cold plate units Cooling Tower HPHE 25 C 2 C C Plate Hx 32 C 22 C 25 C15 C Cold Plate 32 C 6 C CPU 22 C 15 C Natural cold energy T a = C 2 C 27 C Heat Pipes 2 C 25 C Plate Heat Chiller Exchanger Heat Pipe Heat Exchanger (HPHE) Pre-Cooler Fig. 13. Cooling system of Data center utilizing natural cold energy with heat pipes 74
cooling water of chiller by a heat exchanger of heat pipes. Another proposal is pre-cooler which a heat exchanger of heat pipe type is installed in front piping flow line of chiller for cooling water. This purpose is to reduce the cooling load of the chiller, thus reduce the power consumption, by utilizing natural cold energy. It will be applicable to apply this system in North America, where the freezing index is available and the space for installation of heat pipes is also available in large scale data-center. On the other hand, if the temperature of inside of data-center is relaxed to be higher temperature in air cooling system, heat exchanger (air-air) of a heat pipe type will be possible to apply in air cooling data-center with reasonable cost. 4. CONCLUSION With the trend and need of thinner solution in PC and electronics cooling, thinner heat pipe and piezo fan will be needed. Also, for meeting the requirement of high density assembly and high thermal performance in power semiconductor and server of super computer, cold plate technology with micro channel structure has been developed and manufactured. We will continue to develop new products to expand the sale market, and thermal technology will be expanded with many applications. References 1) M. Mochizuki et al Current and future of cooling high performance CPU Fujikura Technical Journal, Apr, 27, p33- p43 2) Y. Kawahara, et al.: One Millimeter Heat Pipe and Application to Cooling Module for Electronic Devices, Fujikura Technical Review, Vol.4 (211) 3) M. Mochizuki, et al.: Advanced Micro-channel Vapor Chamber For Cooling High Power Processors, InterPACK 7, 27 4) Y. Horiuchi, et al.: Micro Channel Vapor Chamber for High Heat Spreading, 1 th Electronics Packaging Technology Conference, 28 5) K. Mashiko, et al.: Application of Micro-channel Fin of Liquid Cooling System and Vapor Chamber, InterPACK 9, 29 6) M. Mochizuki, et al.: Endless Possibilities Use of Heat Pipe for Global Warming Reduction, 1 th International Heat Pipe Symposium, 211 Nov. Fujikura Technical Review, 213 75