Over the past few years, thermal design for cooling microprocessors has become increasingly challenging mainly because of an increase in both average power density and local power density, commonly referred to as “hot spots”. The current air cooling technologies present diminishing returns, thus it is strategically important for the microelectronics industry to establish the research and development focus for future non air-cooling technologies. This paper presents the thermal performance capability for enabling and package based cooling technologies using a range of “reasonable” boundary conditions. In the enabling area a few key main building blocks are considered: air cooling, high conductivity materials, liquid cooling (single and two-phase), thermoelectric modules integrated with heat pipes/vapor chambers, refrigeration based devices and the thermal interface materials performance. For package based technologies we present only the microchannel building block (cold plate in contact with the back-side of the die). It will be shown that as the hot spot density factor increases, package based cooling technologies should be considered for more significant cooling improvements. In addition to thermal performance, a summary of the key technical challenges are presented in the paper.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.

Presently, the microelectronics industry needs thermal solutions that are able to dissipate high heat fluxes at low thermal resistance. The majority of original equipment manufacturers (OEMs) within the microelectronics industry would like to achieve this by extending the application of air-cooling technologies since it implies minimal impact to the design of computer systems and is known to be a cost effective solution space. Spreading resistance through the base of the heat sink is one major component of the total thermal resistance from the silicon junction to the local ambient, especially if larger volume heat sinks are to be used. Until now, most of the research has focused on using phase change systems (i.e., vapor chambers) for reducing the spreading resistance of the heat sink base. Since no significant improvements have been achieved, there is a need to determine the envelope of the limitations for phase change-heat spreaders used in processor cooling, and to compare their performance against high thermal conductivity solid metals. Two simple models are presented to address the heat transfer limitations in phase change systems: the first one addresses wickless devices (i.e., thermo-siphons) and the second one deals with wick heat pipes. The first model predicts the heat transfer coefficients and thermal spreading resistance using Rohsenow's equation for nucleate pool boiling. The model results show that over the normal temperature ranges and heat fluxes encountered in cooling computer processors, the boiling heat transfer coefficient may achieve a maximum of 30 000 W/m 2 K for water. The second model proposed by Prasher et al.[3] is a conduction-based model, which can predict the thermal resistance of the wick heat pipes/vapor chambers. In this conduction model, the wick is assumed to be completely saturated with the fluid and it is assumed that only conduction heat transfer takes place through the wick. This model predicts performance limits based on the conduction resistance of the wick, whereas the first model predicts performance based on the boiling limits. Using these models, the ratio of phase change spreading resistance over solid metal spreading can be estimated. Contrary to common perceptions, this paper will show that there is a threshold envelope where a solid metal spreader (used as heat sink base) may have lower thermal spreading resistance than a copper/water-phase change system (i.e., vapor chambers or immersion boiling). It is also concluded that the phase change systems can be efficiently used only when they conduct heat to a remote heat exchanger.

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The cooling performance of piezoelectric actuators is evaluated for low-form-factor electronics in this work. A piezoelectric actuator is a cantilever made from metal or plastic with a piezoelectric material bonded to it. Under an alternating electrical current, the piezo actuator oscillates back and forth, generating airflow. Compared to conventional fans, these actuators have the advantages of low power consumption, low noise, and smaller dimensions. The parameters investigated in the experiments are actuator orientation, actuator-to-heat source distance, and actuator amplitude. For an actuator power consumption of 31 mW, the heat source temperature was lowered by more than 25°C compared to natural convection conditions (for a 2.45 W heater power dissipation). Performance comparisons against axial fans and natural convection heat sinks show that the piezo actuators perform significantly better in terms of power consumption and cooling volume.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.

Over the past few years, the air cooling technology improvements present diminishing returns for microprocessors cooling applications. Presently most of the proposed future cooling technologies (i.e. pumped liquid cooling or vapor compressor refrigeration) may need some fluid moving device and a large remote heat exchanger which requires additional volume. Due to the complexity, reliability issues and space requirements it is preferred to extend the air cooling within the current form factors and using passive devices. This paper will show that optimized thermoelectric modules combined with two-phase (liquid/vapor) passive devices can further improve the cooling capability compared to conventional air cooling technologies at reasonable thermoelectric cooler (TEC) power consumption. Current computational fluid dynamics programs are not yet well equipped to find out the most optimized TEC geometry (for a given COP and given thermal requirements) in a reasonable amount of computation time. Therefore, two modeling steps are proposed: find out the preliminary TEC geometry using an ID analytical program (based on uniform heat flux and a given COP) and use it as an input to CFD programs (i.e. Icepak®) for detailed predictions. Using this model, we confirmed that the conventional TEC technology must use some spreading device to dissipate the CPU heat to the TEC cold side. Different spreading devices are considered: solid metal, heat pipe, vapor chambers and single/two phase pumped cooling. Their individual performance integrated with TEC will be presented. In addition, we propose that the TEC performance to be controlled as a function of instantaneous CPU power consumption, ambient temperature and other parameters. This controller offers extra flexibility which can be used for either noise reduction or TEC power reduction. However, such power cycling of the TEC may affect the TEC reliability. Power cycling accelerated test data (>500,000 accelerated cycles) have been performed together with the life predictions will be presented in the paper.

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The thermal performance of piezoelectric actuators for cooling in low form factor applications is presented. A significant reduction in thermal resistance is achievable when compared to the baseline natural convection. Comparisons with fans and blowers of similar size result in comparable performance but at greatly reduced power consumption.

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