James G. Maveety

Closed-loop electroosmotic microchannel cooling system for VLSI circuits

The increasing heat generation rates in VLSI circuits motivate research on compact cooling technologies with low thermal resistance. This paper develops a closed-loop two-phase microchannel cooling system using electroosmotic pumping for the working fluid. The design, fabrication, and open-loop performance of the heat exchanger and pump are summarized. The silicon heat exchanger, which attaches to the test chip (1 cm/sup 2/), achieves junction-fluid resistance near 0.1 K/W using 40 plasma-etched channels with hydraulic diameter of 100 /spl mu/m. The electroosmotic pump, made of an ultrafine porous glass frit with working volume of 1.4 cm/sup 3/, achieves maximum backpressure and flowrate of 160 kPa and 7 ml/min, respectively, using 1 mM buffered de-ionized water as working fluid. The closed-loop system removes 38 W with pump power of 2 W and junction-ambient thermal resistance near 2.5 K/W. Further research is expected to strongly reduce the thermal resistance for a given heating power by optimizing the saturation temperature, increasing the pump flowrate, eliminating the thermal grease, and optimizing the heat exchanger dimensions.

Micromachined jets for liquid impingement cooling of VLSI chips

Two-phase microjet impingement cooling is a potential solution for removing heat from high-power VLSI chips. Arrays of microjets promise to achieve more uniform chip temperatures and very high heat transfer coefficients. This paper presents the design and fabrication of single-jets and multijet arrays with circular orifice diameters ranging from 40 to 76 /spl mu/m, as well as integrated heater and temperature sensor test devices. The performance of the microjet heat sinks is studied using the integrated heater device as well as an industry standard 1 cm/sup 2/ thermal test chip. For single-phase, the silicon temperature distribution data are consistent with a model accounting for silicon conduction and fluid advection using convection coefficients in the range from 0.072 to 4.4 W/cm/sup 2/K. For two-phase, the experimental results show a heat removal of up to 90 W on a 1 cm/sup 2/ heated area using a four-jet array with 76 /spl mu/m diameter orifices at a flowrate of 8 ml/min with a temperature rise of 100/spl deg/C. The data indicate convection coefficients are not significantly different from coefficients for pool boiling, which motivates future work on optimizing flowrates and flow regimes. These microjet heat sinks are intended for eventual integration into a closed-loop electroosmotically pumped cooling system.

Characterization of solder interfaces using laser flash metrology

Abstract Thermal conductivity and normalized thermal resistance measurements on eight bulk solders and three tri-layer samples were performed using the laser flash technique. In this study, the laser flash technique is demonstrated to be capable of measuring thermal conductivity of 0.7-mm-thick solders ranged from 20 to 90 W/mK with an average uncertainty of 11%. Laser flash was then used to measure the intrinsic thermal resistance of Cu/Solder/Cu, tri-layer sandwich structures. Analysis shows that the total intrinsic thermal resistance measurement is confounded by sample voiding. C-SAM scanning ultrasonic microscope metrology was shown to be useful in providing information regarding solder voiding and the quality of the solder/copper interfaces. The laser flash method also proved to be useful in assessing voiding effects resulting from different assembly processes and different contacting surfaces on package thermal performance.

Two-phase microchannel heat sinks for an electrokinetic VLSI chip cooling system

The trend towards higher speed and greater integration of modern ICs requires improved cooling technology. This paper describes the design and characterization of a two-phase microchannel heat sink in an electrokinetic VLSI chip cooling system. The heat sink achieves a thermal resistance of 1 K/W for a 1.2 cm/spl times/1.2 cm silicon thermal test chip under open-loop operation with a water flow-rate of 5 ml/min. Preliminary tests show that a closed-loop EK-pumped system running at 1.2 ml/min and 12 psi removes 17.3 W, with heat rejection at an aluminum fin array. Further optimization of the microchannel dimensions and the working fluid operating pressure are expected to lower the resistance below 0.25 K/W.

A Novel Carbon Nano Tube based Wick Structure for Heat Pipes/Vapor Chambers

The paper introduces the novel concept of using carbon nano tube (CNTs) based wick structures for high performance heat pipes and vapor chambers. This ongoing research aims to replace the copper wick structures with high conductive CNT wick structures. Individual carbon nanotubes possess extremely high thermal conductivities of the order of 2000-3000 W/m-K. With such a material as the wick in a heat pipe, the effective thermal conductivity of the fluid saturated wick will be significantly higher that a copper-based wick.