Showing papers by "Avram Bar-Cohen published in 2004"

Optimal numerical design of forced convection heat sinks

Abstract: The objective of this paper is to describe the development of a computationally efficient computer-aided design (CAD) method, which uses a finite element numerical model (FEM) coupled with empirical correlations, to create an optimum heat sink design, subject to multiple constraints. A thermal optimization "challenge" problem, representative of anticipated heat sink requirements in the near future, is solved to demonstrate the proposed methodology. Particular emphasis is placed upon micro-processor central processing unit (CPU) chip cooling applications where, in addition to thermal requirements, the heat sink design specification includes constraints upon size, total mass, and air coolant pressure drop across the heat sink.

Thermal management of portable electronic equipment using thermoelectric energy conversion

Abstract: It has been well established that thermal management of electronic equipment is critical for the continued success of the microelectronics industry. Portable electronic devices, such as notebook computers and cellular telephones, require that the thermal solution be small, light, and energy efficient. Small-scale thermoelectric (TE) technology used to generate electricity from the waste heat of the microprocessor provides an opportunity to de-couple the thermal solution of electronic equipment from battery power. Suski proposed the concept of using a TE module to generate electricity from the waste heat of a microprocessor in a patent. The configuration shown in the patent, referred to here as the 'direct attach' configuration, does not lend itself to higher power microprocessors, as the thermal resistance for state-of-the-art TE modules is on the order of 15 K/W. This paper proposes an alternate configuration, referred to as the 'shunt attach' configuration, as a viable solution for using TE generation in the thermal management of portable electronic equipment. The new concept uses an alternate heat path to shunt excess heat away from the TE module. By managing the heat flow paths, sufficient electricity can be generated by using today's off-the-shelf TE technology to drive a cooling fan.

Use of superlattice thermionic emission for "hot spot" reduction in a convectively-cooled chip

Abstract: The ITRS roadmap indicates the thermal solution for high performance and cost performance computers must improve by a factor of two over the next decade to keep pace with the development of electronic equipment. However, the roadmap does not address the spatially non-uniform heating issues caused by densely routed circuitry. The localized high heat fluxes that occur due to non-uniform heating require an even more aggressive thermal solution to ensure the circuit temperature stays below the specified value. An approach to mitigating the effect of localized "hot spots" is the spread some of the heat from the high heat flux areas to areas of lower heat flux on the chip and/or substrate. Heat spreading at the die level may be accomplished through high thermal conductivity coatings on the backside of the silicon and/or the use of micro heat pipes. Alternatively, solid-state-energy-conversion devices, such as those employing thermionic emission, can be bonded to the back of the silicon chip. The solid state coolers absorb heat at the localized "hot spot" region and transport it via electrons to a region of lower heat flux. Si/SiGe superlattice micro-coolers have demonstrated a cooling power density exceeding 500 W/cm/sup 2/ in an isolated configuration. In this study we investigate the integration and packaging of such micro-coolers with IC chips. Preliminary results suggest that a significant reduction in temperature can be achieved for 70 micron localized hot spots dissipating 100's of W/cm/sup 2/.

Thermal modeling and performance of high heat flux SOP packages

Abstract: This paper explores the thermal challenges in advanced system-on-package (SOP) electronic structures, as well as candidate thermal solutions for these highly demanding cooling needs. The heat fluxes on the active surfaces are expected to approach 100 W/cm/sup 2/. The impact of this high flux is exacerbated by the relatively low thermal conductivity of the organic materials in SOP packaging. Detailed three-dimensional (3-D) finite element simulations were used to study the temperature distributions in a typical SOP package, and to provide guidance for the development and implementation of "compact thermal models". These models were used to evaluate and compare the performance of various thermal technologies and to establish the most promising thermal management alternatives. The use of direct liquid cooling, by immersion of the components in inert, nontoxic, high dielectric strength perfluorocarbon liquids was seen to provide effective cooling over a range of anticipated SOP power dissipations.

Experimental demonstration of thermal management using thermoelectric generation

Abstract: It has been well established that thermal management of electronic equipment is a critical need for the continued success of the microelectronics industry. Portable electronic devices, such as notebook computers and cellular telephones, require that the thermal solution be small, light, and energy efficient. Small-scale thermoelectric (TE) modules can be used to generate electricity from the waste heat of the microprocessor. The electricity can then be used to power the thermal solution. The concept of using a TE module to generate electricity from the waste heat of a microprocessor in a notebook computer class device is discussed by Solbrekken, et. al. In that study, the 'shunt attach' design configuration is introduced and parametrically explored. Using the models developed in that study, a prototype is designed such that a 25 W processor is kept below 85/spl deg/C in a 25/spl deg/C ambient using 'heat-driven cooling'. The prototype built using off-the-shelf TE modules verifies that a 25 W microprocessor can be kept below 85/spl deg/C in a 25/spl deg/C ambient, cooled solely by a fan driven with TE generated electricity.

Thermal design and optimization of staggered polymer pin fin natural convection heat sinks

Abstract: The design and optimization of a thermally conductive PPS polymer pin fin heat sink, for an advanced natural convection cooled microprocessor application, are described. The geometric dependence of heat dissipation and the relationships between the pin fin length, pin diameter, horizontal spacing, and pin fin density for a fixed base area and excess temperature are discussed. The coefficient of thermal performance, COP/sub T/, that relates cooling capability to the energy invested in the formation of the heat sink, has been determined for such heat sinks and compared with conventional aluminum heat sinks. The thermal performance results obtained confirm that PPS (polyphenylene sulphide) polymer heat sinks provide a promising alternative to heat sinks fabricated of conventional materials.

Immersion cooling module for military COTS applications

Abstract: Thermal management needs of the electronics industry continue to be driven by the demand for increased functionality, high heat dissipation and density, component miniaturization, and the reliability benefits of junction temperature reduction and control. In military/aerospace applications, these challenges are exacerbated by harsh environment considerations (e.g. extreme temperatures, shock and vibration, humidity, corrosive surroundings, orientation variations), open standards form factors with defined physical envelopes, and increasing pressure to employ commercial-off-the-shelf (COTS) hardware. Clearly, there exists a need to explore and develop advanced cooling technologies that can meet the anticipated requirements of future military electronic systems. As a means to this goal, card-level passive immersion cooling experiments were performed with gas-saturated FC-72, FC-77, and FC-84 over a range of system pressures to characterize the heat dissipation performance of a VME-size laboratory prototype (233/spl times/160/spl times/18 mm). Chip heat fluxes in excess of 20 W/cm/sup 2/ were achieved while maintaining chip temperatures below 110/spl deg/C. Further, an overall module heat dissipation of 124 W was transferred with 80/spl deg/C module edge temperatures. The thermal performance of the module was limited by the presence of a vapor/air gap and was improved by increased system pressure. Results suggest that FC-84 at 1.5 atm may provide an acceptable compromise between operating pressure, vapor/air space growth, and boiling critical heat flux (CHF) limitations.

"Heat shield"-an enhancement device for an unshrouded, forced convection heat sink

Abstract: The inherent advantages of forced air cooling have led to the widespread use of fully- and partially-shrouded heat sinks for the thermal management of high power microprocessors. The superior thermal performance that is achievable in the fully-shrouded configuration is accompanied by a significant pressure drop penalty. The concept introduced in the current study, employs a thin sheet-metal "heat shield", placed around a partially-shrouded heat sink, to channel the flow directly into the heat sink. A combined numerical and experimental study has shown that the use of this "heat shield" can substantially enhance heat sink thermal performance, in a channel geometry and air flow range typical of commercial chip packages; making it comparable to that of a fully-shrouded heat sink, with a substantially lower pressure drop (/spl sim/50%). In addition, this thermal enhancement device can be easily retrofitted into existing systems; improving performance without major channel and/or fan modifications.

Superlattice microrefrigerators flip-chip bonded with optoelectronic devices

Abstract: A 3D electrothermal model was developed to study the InP-based thin film In/sub 0.53/Ga/sub 0.47/As/In/sub 0.52/Al/sub 0.48/As superlattice microrefrigerators for various device sizes, ranging from 40/spl times/40/spl mu/m/sup 2/ to 120/spl times/120/spl mu/m/sup 2/. We discussed maximum cooling and cooling power densities for current devices, analyzed the non-idealities of current devices and proposed an optimized structure. The simulation results demonstrated a maximum cooling of 30/spl deg/C with cooling power density over 300 W/cm/sup 2/ with an optimized structure based on the current device geometry. Furthermore, we also demonstrated that a maximum cooling, over 10/spl deg/C with power density over 900 W/cm/sup 2/, could be possible when the current figure of merit of InGaAs/InAlAs superlattice is enhanced five times with the non-conserved lateral momentum. Besides monolithic integration, we also propose a flip-chip bonded solution to integrate these microrefrigerator with the optoelectronic chips. Preliminary 3D electrothermal simulation will be present to analyze its cooling effects for this 2-chip integration model.