Development of a Compact High Efficiency Cooling Device for the PA8000 Processor* Guy R. Wagner Hewlett-Packard Co., Fort Collins Systems Laboratory 3404 E. Harmony Road Fort Collins, Colorado 80525 Phone: 970-898-3282 Fax: 970-898-2607 e-mail: Guy_Wagner@hp.com *Best Paper of ISPS 1997 Conference. Abstract When the new Hewlett-Packard PA8000 processor was first fabricated, it became clear that, although it represented a new level of processor performance, it would also present a new level of challenge in system cooling with its power of over 80 watts. Extruded heat sinks with a fan mounted on top (fansinks) had been adequate for cooling the previous generation of processor chips. However, due to space restrictions, the cooling device had to fit in an existing desktop engineering workstation chassis with less than 50 mm of clearance above the processor. A new cooling device, called the TurboCooler, was developed to meet the temperature, noise, and space restrictions imposed by the system specifications. The TurboCooler, with a heat sink-to-air thermal resistance as low as 0.2 C/watt, allows the engineering workstation to meet its goal of 4.5 bels of acoustic noise output in the office environment. Key words: TurboCooler, Fansink, Heat Sink, Processor Cooling, and Impingement Cooling. 1. Introduction Processor power levels have been increasing steadily over the years. Large, passive, extruded heat sinks or extruded heat sinks with a top mounted fan (fansinks) had been adequate for cooling the previous generation of processor chips. When the new PA8000 processor was first fabricated, it became clear that, although it represented a new level of processor performance, it would also present a new level of challenge in system cooling with its power of over 80 watts. Since the processor was destined for use in a desktop engineering workstation, the system fan speed could not be simply turned up without the acoustic noise from the system becoming intoler able. Intitial calculations indicated that a new type of cooling device was needed with less than half the thermal resistance of the best fan assisted heat sinks (fansinks) available. To further complicate the cooling problem, the cooling device had to fit into an existing desktop engineering workstation chassis with less than 50 mm of clearance between the processor and the overhead power supply. To keep system noise in check, the main DC fans in the system had to operate at a low voltage resulting in linear air velocities through the system of less than 0.3 m/s. These velocities were dictated by the objective to keep the system cooling noise almost inaudible in a typical office environment. 2. Available Cooling Devices All existing cooling technologies were investigated from simple extruded heat sinks to liquid cooling with the result that none of the existing cooling devices could handle the power output of the PA8000 processor chip within the design, noise, and reliability constraints 127
Intl. Journal of Microcircuits and Electronic Packaging imposed by the system. The volume required to cool the processor with a finned extruded heat sink 1 was calculated at 1400 cm 3 which is far more than the 500 cm 3 available in the system. Large fansinks 2 had a thermal spreading resistance that was too great and did not fit within the system height constraints. Finned heat pipes with blowers also consumed too much volume and produced too much acoustic noise. 3. TurboCooler Development After several attempts at designing a fan assisted heat sink, it became clear that the fan housing added a lot of extra height to the cooling device without adding any additional cooling capacity. It was decided to try a heat sink design where the fan housing was actually part of the cooling structure. The results of the first attempt at designing an integrated fan housing/heat sink were encouraging. However, the device was difficult to manufacture since it consisted of a finned aluminum base structure with an aluminum venturi ring attached (Figure 1) where the plastic fan venturi was previously located. By making good thermal contact between the base and the ring, it was hoped that the ring structure would also transfer the heat to the air resulting in a greater cooling capacity. To the designer s surprise, adding the aluminum ring to the fan actually decreased the performance of the fansink. After analyzing the problem, it was found that the ring decreased the airflow at a greater rate than the increase in the heat transfer surface area. Taking into account the pressure distribution and the airflow directions around the fan blades, a new device shown in Figure 2 was fabricated that met the dimensional and acoustic constraints of the existing chassis as well as the cooling load of the new processor. By making the base and the ring structure out of one cylinder of aluminum, the slots cut through the ring and into the base allowed additional air to flow in from the side of the device. The TurboCooler cross section in Figure 3 shows the center cavity that houses the fan. Figure 1. Early TurboCooler with an aluminum fan venturi ring. Figure 2. 73 mm diameter TurboCooler without the Internal fan. Figure 3. Cross section of a TurboCooler. 4. TurboCooler Airflow Figure 4 illustrates how a portion of the incoming air is pulled into the top of the TurboCooler while the remaining portion is pulled in across the upper fins by the low pressure created around the fan blades. With the tight clearance between the inside wall of the upper fins and the outside edge of the fan blades, the air is scavenged off the inner surface of the wall creating a high heat transfer rate in this area. All the air is then exhausted out the bottom through the lower fins of the device at about 3.5 m/s in a 10 mm thick airflow layer. The high air velocity impinging on the lower fins creates a high heat transfer rate in the exhaust area. The exhaust air is used to cool additional components around the processor. The angle of the fins on the TurboCooler was matched to the exhaust airflow angle 3 from the fan to minimize the pressure drop across the exhaust fins of the TurboCooler as shown in Figure 5. As the clearance above the TurboCooler is decreased, the ratio of air entering the top decreases while the air entering from the side increases. The cooling capacity decreases by only 9% as the clearance above the TurboCooler is reduced from infinity to 2 mm. This double-pass airflow design adds to the high efficiency of the device by using the cooling air twice. The 50 mm tall, 90 mm diameter Tur bocooler achieves a sink-to-air thermal resistance of 0.21 C/ 128
TurboCooler. Figure 7 shows the oblique angle between the side of the fan blade and the side fins of the TurboCooler. By paying close attention to the blade angles, the TurboCooler produces very high cooling at low noise levels. Figure 8 shows the acoustic noise output of the TurboCooler as a function of the fan voltage. Figure 4. Airflow through a TurboCooler. Figure 6. Oblique angle formed between the trailing edge of the blade and the exhaust fins of the TurboCooler. Figure 5. Exhaust fin angle matched to the exhaust airflow angle from the fan blade. watt. This represents more than a two-fold increase in performance over the best existing fansink designs in this size range. Analysis of the TurboCooler airflow patterns is beyond the capability of commercial electronic cooling computational fluid dynamics programs. Previous attempts at analysis have assumed uniform air velocity at the entrance to the heat sink and isothermal fins 5. Due to the close proximity of the fan blades to the heat sink, the airflow velocity profile is nonuniform over fins with a varying temperature profile. Figure 7. Oblique angle formed between the side of the blade and the side fins of the TurboCooler. 5. Designing for Low Acoustic Noise The fin geometries have been optimized for low noise operation by machining them with angles that are oblique to the blade angles of the fan. These oblique angles are maintained on both the sides and the bottom of the TurboCooler. By maintaining oblique angles between the fan blade and the heat sink, the blade passage tones from the device are minimized. Figure 6 shows the oblique angle Figure 8. Sound power output from a 90 mm diameter between the trailing edge of the fan blade and the exhaust fins of the TurboCooler. 129
Intl. Journal of Microcircuits and Electronic Packaging 6. Performance Several TurboCooler designs have been evaluated to find the optimum design parameters. The TurboCooler parameters that have been optimized are material, diameter, number of slots, slot width, slot depth and depth of the base. All the performance values are taken by measuring the temperature of the base of the TurboCooler with respect to the temperature of the incoming air. The thermocouple in the base is placed on the centerline of the cylinder approximately 1 mm above the bottom surface. A high thermal conductivity 6063-T5 aluminum 4 was used as the material of choice for the TurboCooler. Due to the massive conduction volume of the base and the high thermal conductivity of the aluminum (209 W/m K), there is little variation in thermal performance between a 50 mm square heat source and a 25 mm square heat source. 0.02 C/watt should be added to the curves if the heat source is reduced to a 25 mm square. Figure 9 shows the performance of various TurboCooler configurations with a 50 mm square heat source. A 56 mm diameter Panasonic fan was used for all tests. This fan uses the same motor, electronics and blades as a standard Panasonic FBA06A12H fan except for the base that has been retooled by Hewlett-Packard to fit into the TurboCooler. The fan is rated at 0.54 m 3 /min at 12 volts. In tests of the 90 mm diameter, 45 fin TurboCooler used in the C180 workstation, the fan has an operating point of 0.35 m 3 /min at 10 volts. There was concern expressed regarding the reliability of a fan embedded in a high temperature heat sink. To determine the life expectance of the fan bearings, a thermocouple was carefully inserted through a hole carved in the plastic base of the fan and attached to the lower fan bearing. Temperature measurements indicated that the plastic housing, due to its relatively low thermal conductivity, was not conducting a significant amount of heat to the fan bearing. The bearing temperature increase was measured at 14 C with 25 C incoming air and a 60 C heat sink. This yields a predicted L10 fan bearing life in excess of 100,000 hours. To protect the system from damage in the event of a fan failure, the rotation of the TurboCooler fan is monitored. If the system detects a locked rotor condition, a signal is sent to run a shutdown procedure before the processor overheats. 8. Cooling of the HP Visualize C180 Workstation The PA8000 CPU dissipating 82 watts of power is used in the HP Visualize C180 engineering workstation to provide a new level of computational power on the desktop. Without the use of the TurboCooler to cool the CPU, the system would have to be placed in a much larger enclosure. Air is drawn through the system by two 92 mm fans at the rear of the unit. One fan is dedicated to the processor and power supply side of the workstation. The other fan is used to cool the I/O boards and disk tray. By providing high performance cooling over the CPU with the TurboCooler and low air velocity through the rest of the enclosure, the acoustic noise of the system can be held to 4.5 bels at 30 C or lower ambient temperature. Through the use of temperature controlled DC fans, the system has the ability to operate at up to 40 C ambient air temperature at 3048 m altitude. Figure 10 shows the exploded view of the interior of the workstation. Figure 11 shows the HP Visualize C180 engineering workstation with a 48 cm (19 inch) monitor. Figure 9. Thermal resistance (sink to air) of various TurboCoolers versus fan voltage. 7. Fan Reliability Figure 10. Exploded view of the C180 internal components. 130
Components and Technology Conference, ECTC 94, pp. 442-449, May 1994. 3. D. G. Shepherd, Elements of Fluid Mechanics, Harcourt, Brace and World Inc., New York, Chapter 10, pp. 407-475, 1965. 4. Properties of Wrought Aluminum, Metals Handbook, American Society for Metals, 9th Edition, Volume 2, page 117, 1997. 5. C.R. Biber, Pressure Drop and Heat Transfer in an Isothermal Channel with Impinging Flow, Proceedings of the 13th Annual IEEE Semiconductor Thermal Measurement and Management Symposium, pp. 224-230, January 1997. Figure 11. Hewlett-Packard visualize C180 engineering workstation. 9. Summary The TurboCooler provides a 2X increase in performance over a traditional fansink and provides the level of cooling need for the new generation of high-powered CPUs. By transferring the heat as the air enters the TurboCooler and again as it exits, the performance achieved is much higher than in single pass fansinks. Due to the design of the TurboCooler, the required clearance is much less than that of a standard fansink with the fan housing mounted on top of the heat sink. By paying close attention to the geometry of the slots with respect to the fan blades, the acoustic noise generated by the TurboCooler is held to an almost inaudible level in the standard office environment. The TurboCooler packs the greatest amount of cooling into the smallest volume of any device tested within Hewlett-Packard to this date. The best configuration tested thus far has been able to achieve 0.2 C/watt with a 56 mm diameter fan. This is low enough to provide adequate cooling for both the current generation and the next generation of CPUs. Its volume with its internal fan in the HP Visualize C180 engineering workstation is 254 cm 3. A conventional extruded heat sink of the same cooling capacity would require 1400 cm 3. About the author Guy Wagner received his B.S.M.E. Degree from Iowa State University in 1970 and his M.S.M.E Degree from Iowa State University in 1972. He worked at Bell Laboratories from 1972 to 1981 in physical design of telephone switching systems. In 1981, he joined Hewlett- Packard Co. in Fort Collins, Colorado, where he developed cooling technology for the world s first 32 bit microprocessor. He is the inventor of the TurboCooler that is used to cool many of HP s PA8000 based computer systems. He is currently a Hewlett-Packard Technical Contributor in charge of the thermal and acoustic design of HPs engineering workstations. References 1. M. Moore, Determining the Thermal Performance of Extruded Heat Sinks, Thermalloy Technical Report, Number EIR 84-1012. 2. C. D. Patel, Backside Cooling Solution for High Power Flip Chip Multi-chip Modules, Proceedings of the 44th Electronic 131