Thermal Performance of Thermoelectric Cooler (TEC) Integrated Heat Sink and Optimizing Structure for Low Acoustic Noise / Power Consumption

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1 Thermal Performance of Thermoelectric Cooler () Integrated Heat Sink and Optimizing Structure for Low Acoustic Noise / Power Consumption Masami Ikeda, Toshiaki Nakamura, Yuichi Kimura, Hajime Noda The Furukawa Electric Co., Ltd Higashi-yawata 5-1-9, Hiratsuka, Japan, masami@ch.furukawa.co.jp, Phone: Ioan Sauciuc Ph.D., Hakan Erturk Ph.D Intel Corporation, 5 W. Chandler Blvd., Chandler, AZ, USA, Abstract In this paper, authors are proposing the idea of applying thermoelectric cooler () to CPU cooling. The proposed cooling system is a no moving parts apparatus that can improve thermal performance by keeping same form factor. This system will accommodate an increase in the CPU thermal design power and/or lower the noise of the cooling solution. The thermal performance of this proposed device will be presented together with the optimization for low acoustic noise and low power consumption. As a result, we found out that Hybrid structure, which was composed of an integrated combination of integrated heat sink and heat pipe remote heat sink, could reduce acoustic fan noise and required power consumption to cool the CPU. In one particular case, we succeeded to develop a compact cooling device which had a capability of CPU cooling for 13 W at acoustic fan noise less than 4 db, with COP=1.8. Keywords Thermoelectric, Electronics cooling, Low noise cooling device, Low power consumption, Compact cooling device, Hybrid structure 1. Introduction Last two decades have seen a steady increasing in microprocessor performance as silicon technology continues to scale in accordance with Moore s Law. This increasing performance resulted in microprocessor thermal design power increasing [1]. At the same time, because of increasing demand of home-use personal computer in which high performance microprocessor is built, low acoustic noise cooling system is getting focused on. Conventionally, in order to overcome this increasing design power, some key cooling technologies such as heat pipe, radiation fin or fan, was being adopted, and heat pipe remote heat sink [2] was proposed as one of the solution. And, so far, combination of this heat pipe remote heat sink and large fan is considered also one of most suitable solutions to reduce acoustic noise even compared to liquid cooling. Due to CPU reliability requirements and/or possibility of thermal design power increases it is assumed that it might be too difficult to stay within conventional air cooling and in compact size. There is a need to extend the current air cooling using passive (non-moving) devices. For significant improvements we have found out that this can be done in only one way: by using thermoelectric technology. The thermoelectric device is a compact electric cooler which can pump heat from a cold side to hot side with no moving parts by using the Peltier effect. So far different authors also have been proposing a cooling solution based on the module [3], [4]. This solution includes a phase change spreading device (i.e. heat pipe, vapor chamber) used for uniform heat spreading to the cold side of. Several modules are used to introduce a negative resistance into the integration chain and thus can decrease the operating temperature of the spreading device attached to the heat source [3]. The fact that a solution with has not yet became common is because of some misconceptions in the industry such as that requires large power consumption or they are un-reliable [5]. This paper will try to ease this fear of thermoelectric and eliminate the misconceptions. We will firstly show the integrated heat sink performance enhancement by applying, using several heat sink configurations. By taking these results into consideration, we did optimize by introducing the hybrid solution which is using two parallel heat paths to dissipate the CPU heat: a path and conventional non- path. Two fin structures are working separately. The first fin structure is attached to the hot side of the. The second fin structure is attached to the heat pipe condenser. The heat pipes are sandwiched between the cold side of the and the CPU, thus some of the CPU heat by-passes the path. This concept reduces required power consumption and acoustic noise within current heat sink volume. In addition, some reliability test results are also reported. 2. Basis of Thermoelectric cooler integrated heat sink As figure 1 shows, consists of two types of semiconductors, p-type and n-type, and these semiconductors are connected in series by copper interconnects. These components are sandwiched by an electrical insulator such as a ceramic substrate. It can be seen that when DC current is supplied to the, pumping heat from heat source, temperature difference between one side and the other side is generated. This is the so called Peltier effect. Although heat normally flows from a hot to cold body side, can transfer heat from a lower to higher temperature body. integrated heat sinks utilize this advantage /6/$2. 26 IEEE 144

2 Figure 2 shows the schematic of a conventional integrated heat sink structure and explains the possible enhancements due to the use of s. The heat sink consists of heat spreader attached on to the CPU,, and a heat sink subject to forced convection. The spreading device is used for spreading heat from the heat source and thus heat can be input uniformly to. And is sandwiched between the heat spreader and convective heat sink by using thermal interface material. The graph to the right compares the temperature distribution between integrated heat sink and heat sink without. The red line displays the temperature distribution of heat sink without from heat source to air through the heat sink. The temperature of heat source is determined by the combined thermal transfer characteristics of Heat sink and Heat spreader. However, when is applied to the system, temperature distribution for integrated heat sink is depicted by the blue-line with effect, and the temperature can decrease from T case to T case. In this figure, each arrow represents heat flow and Heat sink needs to dissipate heat from heat source and the power applied to. Copper Interconnects 3.1. Prototype and test setup Figure 3 shows prototype of integrated heat sink. The prototype consisted of vapor chamber having 11 x 55 mm in foot print and 5 mm thickness, 2 pieces of 5 x 55 mm module and radiation heat sink having 52 pieces of 55 x 52.5 mm fin plates. The test setup is shown in figure 4. In this test, DC power sources were used to supply power to the heat source and. Voltage and current meters were used to calculate heat source and power consumption. To provide the cooling air, two 6 x 6 mm DC fans were employed. Driving the cooling fans, we measured the sink temperature and inlet air temperature to calculate sink to ambient resistance. We examined heat sink performance, varying COP which is determined by; QHeat Source COP = (1) P where Q Heat Source and P describe power consumption of heat source and, respectively, and COP is abbreviation for Coefficient Of Performance. Cold Side P-type Semiconductor HEAT Position Hot Side - + DC Source Electrons moving heat to hot side N-type Semiconductor Temp. Figure 3: Prototype of integrated heat sink, vapor chamber configuration Figure 1: Schematic of Thermoelectric Cooler CPU + Heat sink TIMB CPU TIMA Heat Spreading Device TIM Heat Source Position w/ T amb T case T case effect w/o Temp. Figure 2: Schematic of enhancement heat sink and concept explanation 3. integrated vapor chamber heat sink In this section, advantage of using on enhancement of heat sink thermal performance and fan noise reduction effect will be discussed. DC-FAN Duct Heat Source V A Heat sink Vapor chamber A Figure 4: Schematic of Test setup 3.2 Enhancement of thermal performance with To check enhancement of heat sink performance with, we did thermal test in accordance with test condition listed in table 1. Ambient temperature 45 C Fan driving voltage 12 V Heat input from heat source 92 W Heat source 32 x 32 mm COP 1 5 Table 1: Thermal test condition V

3 Test result is shown in figure 5. In the graph, blue dots show thermal resistance of integrated heat sink with varying COP and red line shows thermal resistance of heat sink without, which consisted of same vapor chamber and same radiation heat sink for comparison. Reviewing the result of integrated heat sink, it is confirmed that thermal resistance became lower with decreasing COP, which means increasing applying power to, and we could achieve thermal resistance less than.1 K/W under 3 of COP. Next, comparing thermal resistance between heat sink with and without, it can be found out that applying could significantly improve thermal performance of heat sink in a same form factor. We can also confirm that improvement became larger as decreasing COP. We could confirm.2 K/W improvement of thermal resistance at the maximum at COP=1 in this test Vapor chamber +, Sink to ambient Effect with COP without Figure 5: Enhancement of heat sink performance by applying, in keeping same form factor 3.3 Noise reduction Next, we discuss about an effect of heat sink on fan acoustic noise reduction. Test was conducted in accordance with test condition listed in table 2, installing the heat sink in a tower type desk top PC chassis. During the test, to exhaust warmed air in chassis effectively, heat sink was ducted to grill on back panel of chassis as figure 6 shows. And fan noise was measured at 1m distance from PC back panel. Environment temperature Room temp. Fan driving voltage 6-12 V Heat input from heat source 96 W Heat source 25 x 25 mm COP 2-5 Table 2: Noise reduction test condition Figure 7 shows the test results. The vertical axis of this graph represents noise reduction based on the noise measured with 12 V of fan driving voltage, which means fan noise is equal to zero when fan voltage is 12 V, and horizontal axis represents thermal resistance determined by sink temperature and air temperature in chassis. In this graph, green dot shows thermal resistance of heat sink without by driving the fan at 12 V, and yellow, blue and red dots represent thermal resistance with various fan voltages at COP=5, 3 and 2, respectively. 1 m Microphone HOT AIR Back side grill Heat sink Exhaust duct PC chassis Figure 6: Noise Reduction test setup Noise Reduction db COOL AIR Noise reduction vs. Sink to Ambient Resistance 4.5 db COP=2 COP=3 COP=5 w/o 12 db Front side grill Figure 7: Noise reduction by applying Reviewing the graph,.32 K/W is thermal resistance of heat sink without at 12 V fan, but one can see that applying could reduce fan noise by 12 db, keeping the same thermal resistance. And comparing among performance with integrated heat sink, thermal resistance.2 K/W was measured with a 12 V driving fan at COP=5, but 3. db, 4.5 db noise reduction could be obtained by changing COP to 3 or 2 respectively, in keeping same resistance.2 K/W. Taking these results into consideration, we can mention that fan acoustic noise could be reduced by applying and optimizing COP, while maintaining the same thermal resistance. 4. integrated heat pipe remote type heat sink We designed other configuration of integrated heat sink which has heat removing component apart from heat source with heat pipe. In this section, especially, we focused on heat sink performance with varying heat source powers.

4 4.1 Prototype and test setup The prototype is shown in figure 8. This heat sink consisted of remote spreader with 5 heat pipes, 6 pieces of 4 x 4 mm, and radiation heat sink having 12 x 8 mm in foot print and 39 mm of height. Radiation heat sink consisted of aluminum base plate and 9 pieces of.4 mm thickness copper fin plates. Figure 8: Heat pipe remote type integrated heat sink In performance test, we used wind tunnel, fan to provide cooling air and flow meter to measure air flow rate as figure 9 shows. And we check thermal performance with varying COP and heat source power. Test condition is listed in table 3. Here, focusing on required COP to keep same thermal resistance.5 K/W at each heat input, it is found out that COP=5 or 18 W for power consumption- was required at heat input 88 W, but COP=2 or power 75 W- was needed at heat input 15 W. This means that as higher heat input become, required COP to maintain same thermal performance must be decreased in other words higher power would be required to cool high wattage CPU Heat pipe +, Sink to ambient Higher power Q=88 W Q=12 W Q=15 W COP Wind Tunnel Heat Sink No Bypass Heat Spreader Figure 1: Thermal performance of heat pipe type integrated heat sink Fan & Flow meter Heat Source Air Flow Figure 9: Schematic of test setup V A Ambient temperature 45 C Air flow rate 45 CFM Heat input from heat source 88-15W Heat source 4 x 4 mm COP 2-5 Table 3: Thermal test condition 4.2 Thermal test result Figure 1 shows test result at each heat source power. In this graph, green, red and blue dots represent heat sink thermal resistance at heat input 15, 12 and 88 W with varying COP, respectively. And orange dashed line represents thermal resistance of heat sink without. One can confirm that applying could improve thermal performance of this configuration as same as the case of vapor chamber configuration. In addition, this result tells us that thermal resistance became higher as increasing heat source power, instead of same COP. We could obtain.5 K/W red line- with 3 of COP and at heat input 12 W. V A 5. Optimizing heat sink structure Taking the above into consideration, high performance or higher power consumption is considered to be required to meet a demand of suitable thermal solution to cool near-future CPU having higher wattage. But this outlook is easily assumed not to be appropriate, considering a demand of reducing heat sink cost and system running cost. So in this section, we study optimization of heat sink design to reduce power for cooling high wattage CPU. 5.1 Hybrid Heat Sink concept Before starting optimization, we reviewed the result of heat pipe remote type with arranging performance by COP, as shown in figure 11. Generally, it is theoretically said that generated temperature difference of can be decided by COP [6], as red line shows in figure 11. And plotting our measured temperature difference, we can confirm that measured difference was constant at same COP even if heat input was changed, and also that the measured difference was approximately corresponding with theoretical curve. Based on these studies, in order to reduce required power, a method to reduce heat passing through was considered. And we came up with an idea of separating heat pass in two ways, and designed hybrid structure of integrated heat sink and heat pipe remote heat sink as figure 12 shows. Using this figure, a detail of this concept is explained as follows. At first, heat from CPU is separated to two ways, as red and blue arrows shows. One of separated heat is transferred to

5 remote heat sink through heat pipe and removed here directly as red arrow shows, and another separated heat is pumped by and removed from radiation heat sink attached on hot side as blue arrow shows. At this point, by optimizing a ratio of passing heat between remote heat sink part and integrated heat sink part so that heat passing can be much smaller than a pumping capacity of, we considered that power can be reduced with improving heat sink performance. Temperature difference K Generatead Temp. Difference Q= 88 W Q=12 W Q=15 W Theoritical COP Figure 11: Generated temperature difference between cold side and hot side In right side of the figure, thermal resistance network of this Hybrid heat sink is represented. Nomenclature is as below: R HP : Heat pipe thermal resistance R FIN1 : Remote heat sink resistance R BASE : Base plate spreading resistance R : Thermal resistance of R FIN2 : Radiation heat sink thermal resistance R RMT : Heat pipe remote heat sink resistance (=R HP +R FIN1 ) R HS : integrated part thermal resistance (=R BASE -R +R FIN2 ) Reviewing the chain, it can be understood that heat pipe remote heat sink part and integrated heat sink part is connected thermally in parallel, so thermal resistance of this solution R SA can become smaller than individuals, which means that heat sink performance can be improved. 5.2 Proto-type of hybrid structure Figure 13 shows a proto-type of hybrid structure. Heat sink dimension is 86 x 8 mm in foot print and 78 mm of height. Remote heat sink part consisted of 4 heat pipes and 43 pieces of 8 x 4 mm aluminum fin plates. And integrated heat sink part consisted of 2 pieces of 4 x 4 mm off-the-shelf modules and heat sink having 53 pieces of 46 x 19 mm fin plates. For performance test, we employed a 8 x 8 mm DC fan to provide cooling air and measured thermal and acoustic performance. And in order to check validity of proposed concept, for comparison, we also made conventional structure which had same volume as this hybrid structure and consisted of copper solid heat spreader, 2 pieces of 4 x 4 mm and aluminum radiation heat sink with 6 pieces of 6 x 8mm fin plates. Test condition is listed in table 4. For acoustic performance test, installing heat sink in acoustic chamber, we measure fan noise with microphone put 1 m distance from the heat sink. Remote HS T amb R FIN1 R FIN2 HS R RMT R R HS R HP R BASE T sink CPU T case Transferred by HP and dissipated Pumped by and dissipated power consumption 1 R SA = 1 R RMT + R 1 HS Figure 12: Hybrid heat sink concept

6 Figure 13. Hybrid concept integrated heat sink Ambient temperature 25 C Fan driving voltage 6 12 V Heat input from heat source 8-13 W Heat source 25 x 25 mm COP 1-2 Table 4: Performance test condition 5.3 Comparison of thermal performance Figure 14 compares thermal resistance at heat input 13 W between proposed hybrid and conventional structures with 12 V driving fan. In this graph, horizontal axis represents COP which is represented by heat source power dividing by power consumption and, red and blue dots show thermal resistance of hybrid and conventional structure, respectively. This result shows that thermal resistance of hybrid structure was lower than conventional one when COP was the same. And maximum difference.15 K/W was confirmed at COP=2 between these structures. In addition, reviewing this result, we can find that conventional structure needed less than 3 of COP when the same thermal resistance as Hybrid structure at COP=11 would be required Comparison of Sink to ambient, Q=13W, 8x8 Fan 12V Conventional Hybrid COP Figure 14: Comparison of thermal resistance, at heat source power 13 W with varying COP And next figure 15 compares thermal resistance at COP=3, 5 and 1 between these structures, with varying heat input. In this graph, solid dots represent thermal resistance of hybrid structure for each COP, and outlined circles show performance of conventional structure. As you can see, despite that thermal resistance of conventional structure became higher as higher power of heat sources, Hybrid structure stayed almost constant thermal resistance for each COP. So we can consider hybrid structure more suitable solution for cooling high wattage CPU than conventional structure. Taking these results into consideration, we can also mention that hybrid structure could reduce require power with improving thermal performance in keeping same volume Comparison of Sink to ambient, 8x8 Fan 12V HB, COP=3 HB, COP=5 HB, COP=1 Conv, COP=3 Conv, COP=5 Conv, COP= Heat Source Power W Figure 15: Comparison of thermal resistance, with varying heat source power 5.4 Comparison of acoustic noise Figure 16 compares fan acoustic noise between two structures at heat source power 13 W, taking thermal resistance in horizontal axis. In this graph, red dots represent performance of hybrid structure with varying fan driving voltage at constant COP=1.8 and blue dots represent performance of conventional structure. Reviewing the result, one can find out that hybrid structure could achieve thermal resistance.2 K/W at COP=1.8 and less than 4 db acoustic noise. But it is also found out that conventional structure could reach only.33 K/W at COP=2 at the same noise level, and even if fan noise was increased to 56 db i.e. increasing fan speed-, thermal resistance could not become less than.2 K/W. And we could confirm 16.1 db noise reduction by taking Hybrid structure in this test. So we can learn from these results, that hybrid structure can reduce also fan noise with improving thermal performance of heat sink.

7 Acoustic Noise db Comparison of Noise, Q=13 W, 8x8 Fan COP=2.2 K/W 16.1 db COP= Sink to ambient K/W Conventional Hybrid, COP= K/W COP=2 Figure 16: Comparison of acoustic noise at heat source power 13 W Summarizing test results of hybrid structure, it can be concluded that hybrid structure can reduce required power and suppress fan acoustic noise with improving thermal performance for cooling high wattage heat source, in keeping same heat sink volume, compared with conventional structure. 6. Reliability of heat sink In this section, we are presenting intermittent results of reliability test, because reliability test is sill going on in our laboratory, using conventional structure with off-the-shelf module. 6.1 Thermal shock test Change ratio 5% 4% 3% 2% 1% % -1% -2% -3% -4% -5% No. 1 No. 2 Thermal Shock Test result Cycle number Figure 17: Thermal shock test result Firstly, we are studying thermal shock test result. This test is for checking influence of thermal stress on property. In actual test, test samples in a non-operational mode were held in thermal shock chamber at and 1 O C, and we checked electrical resistance every 2 cycles to 1 cycles and judged as pass if changing ratio was in a range of +/- 5 %. Figure 17 shows test result. In this graph, vertical axis represent change ratio of module resistance based on measured resistance before thermal shock test. Reviewing this result, since changing ratio was confirmed in +/- 1. % through 1 cycles, so change of property by thermal stress could not be confirmed. 6.2 High temperature storage test Secondary, we conducted high storage temperature test. In this test, the samples in a non-operational mode were held in a temperature controlled chamber set at 1 O C, and we checked electrical resistance every 12 hours and thermal resistance of heat sink every 24 hours. And we set fail criteria 5 % as same as thermal shock test criteria. Change ratio 5% 4% 3% 2% 1% % -1% -2% -3% -4% -5% High Temp Strage 1 o C No. 1 No Test hours Figure 18: High temperature storage test result electrical resistance change ratio Figure18 shows change ratio of electrical resistance. Since we can confirm that change ratio was in a range of +/-1 % from this graph, it is judged that there was not significant failure of in high temperature storage test. And in this test we measure also thermal performance of heat sink, inputting 65 W from heat source to heat sink. Figure 19 shows change ratio of thermal resistance at COP=3 through high temperature storage test. From this result, the change ratio can be considered smaller than criteria. Judging from these results, storage in high temperature environment is not considered having any influence on integrated heat sink. Now, reliability test is still going on and we are planning to continue reliability test including low storage temperature test, and so on.

8 Change ratio 5% 4% 3% 2% 1% % -1% -2% -3% -4% -5% High Temp Strage 1 C Test hours Figure 19: High temperature storage test result heat sink performance change ratio 7. Conclusions 1. Authors succeeded to develop a conventional compact and silent cooling device, which has.2 K/W of sink to ambient thermal resistance with less than 4 db fan acoustic noise and low power consumption 12 W, at heat input 13 W. 2. It was found out that combined with spreading devices could significantly extend air cooling performance within same form factor. 3. The possibility of significant noise reduction by was confirmed. 4. It was concluded that the new concept Hybrid structure, could reduce power consumption of and reduce the fan acoustic noise with improving thermal performance, and keeping same heat sink footprint. 5. As a result of limited thermal shock and high temperature storage tests the reliability data looks promising and no significant failures were observed References 1. Greg Chrysler, Next-Generation Thermal Management Materials & Systems, RTI Conference (Dallas, Oct. 22) 2. Takahiro shimura et. al, Heat pipe remote cooling for semiconductor devices, 13 th International Heat Piep Conference proceeding, pp Masami Ikeda et. al, Integration of Thermoelectric and Phase Change (liquid/vapor) devices with application to CPU cooling, International Microelectronics And Packaging Society 24 Advanced Technology Workshop on Thermal Management (Palo Alto, 24) 4. Ioan Sauciuc et. al, Thermal Performance and Key Challenges for Future CPU Cooling Technologies, International Electronic Packaging Technical Conference and Exhibition 25 (San Francisco, 25) 5. Dwight Johnson, High Watt Density Thermoelectric Coolers High Watt Density Thermoelectric Coolers Enhance Performance-Limited Heat Sinks, Coolcon 24 Advanced Cooling Technology Workshop (Scottsdale, 24) 6. Jim Bierchenk et.al, Extending the limits of air cooling with thermoelectrically enhanced heat inks, Intersociety Conference on Thermal Phenomena in Electronic Systems 24 proceeding, pp679