Thermal Performance of Inkjet-assisted Spray Cooling in a Closed System

Gary Vondran1, Kostas Makris2, Dimosthenis Fragopoulos2, Constantin Papadas2, Niru Kumari1

1Hewlett Packard Co. 3000 Hanover St. Mail Stop 1601

Palo Alto, CA, USA, 94070 Phone: 1-650-857-2413

Email: gary.vondran@hp.com, niru.kumari@hp.com

2Integrated Systems Development Building B Atrina Center

32 Kifisias Ave. 15125 Maroussi, Greece Phone: +30 210 6895115

Email: kmakris@isd.gr, dfragop@isd.gr, papadas@isd.gr

ABSTRACT

As the number of processors per die increase, chip thermal hotspots become increasingly more concentrated within smaller and smaller areas. Furthermore, these hotspots can change as processors are dynamically throttled or taken in and out of sleep mode based upon load and overall thermal budgets. Current cooling solutions (e.g. heatsinks, heatpipes, and even liquid cooling solutions) extract heat from the chip level but cannot independently control temperature at the hotspot level. The presented solution utilizes InkJet heads to deliver precise coolant flow rate independently to each chip location to maintain very high heat transfer rate via sustained liquid-to-vapor phase change. The result is a 10-100x improvement in thermal extraction rates over existing cooling solutions, achieving heat transfer rate as high as 4.5kW/cm2. Additionally, because each hotspot is maintained independently eliminating any large temperature gradient over the entire chip surface area, the ability to operate chips at higher operating points is now possible. This paper presents a heat sink prototype based on the inkjet-assisted spray cooling technology. The heat sink utilizes an air-cooled vapor chamber to condense and recirculate the evaporated liquid to achieve a fully closed system within the vapor chamber enclosure. The design of the prototyped solution is presented

KEY WORDS: spray cooling, inkjet, liquid-vapor phase change, vapor chamber

INTRODUCTION

As the number of processor cores per die increase with semiconductor feature size reduction, thermal heat dissipation increases as a limitation on operating performance. Specifically hitting the thermal wall in the past decade has resulted in clock frequencies leveling off as a result of requirements to keep thermal budgets within existing cooling technologies. Further, as features shrink hotspots became more concentrated within even smaller areas and the thermal solutions can no longer simply address dissipation of the thermal average across the chip. Compounding this, multi-cores add two additional challenges: (1) More hotspots

distributed broadly across the chip, and (2) greater dynamics in each core’s workload. The latter not only includes cores being dynamically placed in sleep mode, but also systems that dynamically boost clock frequency of one core while throttling others to adjust for single thread performance needs.

Cooling technologies that rely on air cooling with physical contact have limitations when removing heat from non-uniform sources due to their inherent resistance to heat spreading [1]. Technologies like vapor-compression and solid state refrigeration have shown promise, but are limited in power density at heatsink operating temperatures above dew-point [2]. Non-refrigeration phase change techniques like thermosyphons with enhanced evaporation structures are reaching their limit close to 100 W/cm2 in passive configurations with uniform power density [3]. Two-phase cooling has the capability of removing high heat fluxes at low superheats with relatively small systems [3, 4]. Spray cooling has the potential to increase the heat flux rate, provide control [5, 6] and to address the problem of spot cooling [7]. Inkjet arrays can precisely dispense pico-liters of coolant at select locations with millisecond timing precision. Prior work experimentally demonstrated that a controlled inkjet based spray cooling can achieve up to 4.5kW/cm2 of thermal extraction [8, 9].

APPROACH

Presented is a prototype solution of an inkjet based spray cooling solution incorporated into a sealed vapor chamber within a microprocessor sized heatsink. Shown in Figure 1 is the basic structure of the developed solution. Specifically, coolant is dispensed to the hotspots at rates based upon measured temperature to maintain phase change at each location. Heat is absorbed by the phase change process into the vapor that is carried to the cool top surface (in this case cooled with fins and airflow, but other options can be used). The thermal energy is transferred to the fins and the vapor condenses back to liquid and recycled into the reservoir to feed the InkJet nozzles. One can view this as each chip location has its own independent coolant supply that is feed at rates to maintain the phase change at that location.

978-1-4244-9532-0/12/$31.00 ©2012 IEEE 1127 13th IEEE ITHERM Conference

Fig. 1 Prototype inkjet spray cooling structure

Figure 2 shows the inkjet spray cooling solution assembled

into a 10 cm x 13 cm x 1 cm vapor chamber. The first design target is a 15mm x 15mm chip dissipating 130W, with later chip version at 250W. This represents heat dissipation rate of 57 W/cm2 and 111 W/cm2, respectively. Both well within the previously published experimentally measured capabilities of this technology [8, 9]. Figure 3 shows the inside of the first experimental prototype orientated with the nozzles spraying up. Two inkjet heads each with 12 nozzles are used in this instance; however additional versions have been built incorporating 4 and 6 heads to cool the larger targeted chip area. The white material under the heads is the coolant reservoir for the heads. The wicking material on the condenser is utilized to re-circulate the condensed coolant to the coolant reservoir. Figure 3 shows the coolant can be manually dispensed before sealing to get the closed system.

Fig. 2 Assembled sealed inkjet spray cooling vapor

chamber

Fig. 3 Internal view of inkjet spray cooling vapor chamber

Ink-jet head with 12

Coolant reservoir

Wicking material on condenser

Electrical wiring for actuating ink-jet head (going to control

Several details: - The solution is a fully sealed with no loss in coolant as it is

cycled through its phases from being sprayed, converted to vapor, condensed to liquid, and then feed back to the reservoir. Other than the inkjet nozzles, no other active devices are used in the cycle.

- The present coolant is water, the same base as used in inkjet inks. This enables the use of existing inkjet heads developed for printing without modifications.

- The current preferred chip temperature sensing approach is a mesh of thin film thermal sensors (diode or resistor) in the thermally conductive material sealing the base of the spray chamber and in contact with the chip being cooled. An alternate approach utilizes sensors embedded in the chip but that approach has dependency on the chip providers. The sensor mesh provides a general solution enabling the cooling device to be placed on any chip and automatically sense and respond to its hotspots.

- The condenser surface can be cooled by fins, heat pipe, liquid cooled or any other heat extraction structure.

- A small control circuit that can be mounted on the device takes the sensor array input and controls the nozzle firing rates and locations, creating a closed loop sensor-control system. The control board is designed to be powered by the 5V leads commonly provided on PC motherboards to power heatsinks with embedded fans. The goal was to enable the inkjet based solution to be a direct drop in replacement to existing heatsinks without requiring modifications to the motherboard.

Fig. 4 Test setup bottom view showing heat load Figures 4 and 5 show the test setup of the product

prototype. Specifically, Figure 4 is the underside of the test setup with an engineered thermal heat source attached to behave as chip to be cooled. Figure 5 is the sealed solution at reduced pressure to lower the water boiling point, as is typically done in vapor chamber and heat pipe solutions. The test setup has been run over an extended period of time and

showed the technology’s ability to manage temperatures at the chip surface, matching prior benchtop experiments [8, 9].

Fig. 5 Test setup

The initial product prototype has been assembled and is

presently demonstrating full functionality. A variety of experiments has been performed in order to explore the impact of critical setup parameter variations to the cooling performance. The goal is to gain understanding of the affects of the design parameters on performance, lifetime, reliability, and manufacturability of production devices. These included distance from inkjet nozzle to the bottom plate distance, bottom plate temperature, inkjet firing frequency, vacuum level and dissipated electrical power on the heat load. A critical operating concern is the inkjet head self-heating effect, which may limit the cooling performance. Stable operating point conditions have been achieved with a maximum firing frequency of 350Hz for both inkjet heads while maintaining the top plate temperature below 50ºC and the chamber absolute pressure at 0.3 bar (below atmospheric pressure).

Experimentation using the 2 inkjet head prototype showed the ability to dissipate 70W over the ¼ cm2 spray area (0.28 kW/cm2 based on spray area), maintain 50 degrees Celsius at the heatsink base. This is on target as the power dissipation rate should scale as the number of inkjet heads is increased to 6-8 to cover the targeted 15mm x 15mm chip area. The increased 6-8 heads is estimated to dissipate over 600 W over the typical chip area. It is noted that the observed heat dissipation rate in this case is lower than for the open system heat dissipation rate of 4.5 kW//cm2. The parametric studies are on-going to achieve higher heat dissipation rate, however, the current performance provides enough cooling for the state-of-art GPU and CPUs. One of ways to increase heat dissipation rate is to use a heat sink (as shown in Fig 1) at the condenser to increase the rate of condensing liquid. This can increase the firing frequency of the ink-jet heads and hence, increase the mass flow rate of the liquid.

Evaporator-side

Heat load

Electrical connections to control ink-jet heads

Finally, discussions have begun with the target lead adopters of this technology. Specifically, these were identified as those with greatest need for such a solution. These telecommunication rack systems, GPU, and high performance data center servers. Once proven in these applications and in production, it is seen cost optimizations can enable its use in broader markets.

Acknowledgments This work was developed with contributions from Sergio

Escobar-Vargas, Chandrakant Patel, Cullen Bash, Ratnesh Sharma, Will Allen, Matt Smith, Donna Van Zee, Ken Cannizzaro, Ariel Reich, and Ali Mira. In addition, appreciation goes out to HP Labs, ISD, Heatscape, and HP’s Intellectual Property Licensing management for their project support and funding.

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