Yoon Jo Kim

Thermal Characterization of Interlayer Microfluidic Cooling of Three-Dimensional Integrated Circuits With Nonuniform Heat Flux

It is now widely recognized that the three-dimensional (3D) system integration is a key enabling technology to achieve the performance needs of future microprocessor integrated circuits (ICs). To provide modular thermal management in 3D-stacked ICs, the interlayer microfluidic cooling scheme is adopted and analyzed in this study focusing on a single cooling layer performance. The effects of cooling mode (single-phase versus phase-change) and stack/layer geometry on thermal management performance are quantitatively analyzed, and implications on the through-silicon-via scaling and electrical interconnect congestion are discussed. Also, the thermal and hydraulic performance of several two-phase refrigerants is discussed in comparison with single-phase cooling. The results show that the large internal pressure and the pumping pressure drop are significant limiting factors, along with significant mass flow rate maldistribution due to the presence of hot-spots. Nevertheless, two-phase cooling using R123 and R245ca refrigerants yields superior performance to single-phase cooling for the hot-spot fluxes approaching ∼300 W/cm 2 . In general, a hybrid cooling scheme with a dedicated approach to the hot-spot thermal management should greatly improve the two-phase cooling system performance and reliability by enabling a cooling-load-matched thermal design and by suppressing the mass flow rate maldistribution within the cooling layer.

Absorption Heat Pump/Refrigeration System Utilizing Ionic Liquid and Hydrofluorocarbon Refrigerants

The ionic liquid butylmethylimidazolium hexafluorophosphate (bmim)(PF6) and five different hydrofluorocarbon refrigerants were investigated as the working fluid pairs for a waste-heat driven absorption heat pump system for possible applications in electronics thermal management. A significant amount of the energy consumed in large electronic systems is used for cooling, resulting in low grade waste heat, which can be used to drive an absorption refrigeration system if a suitable working fluids can be identified. The Redlich–Kwong-type equation of state was used to model the thermodynamic conditions and the binary mixture properties at the corresponding states. The effects of desorber and absorber temperatures, waste-heat quality, and system design on the heat pump performance were investigated. Supporting experiments using R134a/(bmim)(PF6) as the working fluid pair were performed. Desorber and absorber outlet temperatures were varied by adjusting the desorber supply power and the coolant temperature at the evaporator inlet, respectively. For an evaporator temperature of 41 C, which is relevant to electronics cooling applications, the maximum cooling-to-total-energy input was 0.35 with the evaporator cooling capability of 36 W and the desorber outlet temperature in the range of 50 to 110 C. [DOI: 10.1115/1.4007111]

Co-design of signal, power, and thermal distribution networks for 3D ICs

Heat removal and power delivery are two major reliability concerns in the 3D stacked IC technology. Liquid cooling based on micro-fluidic channels is proposed as a viable solution to dramatically reduce the operating temperature of 3D ICs. In addition, designers use a highly complex hierarchical power distribution network in conjunction with decoupling capacitors to deliver currents to all parts of the 3D IC while suppressing the power supply noise to an acceptable level. These so called silicon ancillary technologies, however, pose major challenges to routing completion and congestion. These thermal and power/ground interconnects together with those used for signal delivery compete with one another for routing resources including various types of Through-Silicon-Vias (TSVs). This paper presents the work on routing with these interconnects in 3D: signal, power, and thermal networks. We demonstrate how to consider various physical, electrical, and thermo-mechnical requirements of these interconnects to successfully complete routing while addressing various reliability concerns.

Thermal Characterization of Interlayer Microfluidic Cooling of Three-Dimensional IC With Non-Uniform Heat Flux

It is now widely recognized that three-dimensional (3D) system integration is a key enabling technology to achieve the processing speeds and performance needs of future microprocessor integrated circuits (ICs). To provide modular thermal management in 3D stacked ICs, interlayer microfluidic cooling scheme is adopted and analyzed in this study. The effects of cooling scheme and essential geometry variations on the routing completion and congestion of electrical interconnect are quantitatively analyzed. Also, the thermal and hydraulic performance of several two-phase refrigerants is discussed in comparison with single-phase cooling. The results show that refrigerants in two-phase flow are thermally preferred due to the higher heat transfer coefficients, and relatively constant fluid temperature throughout the microchannel. However, the large internal pressure and pressure drop act as significant limiting factors in realizing the merits of two-phase cooling. It is also concluded that integration of high performance hot-spot thermal management is a key to addressing a challenge of mass flow rate mal-distribution.Copyright

Two-phase flow and heat transfer in pin-fin enhanced micro-gaps

Thermal management of integrated circuits (IC) has emerged as one of the key challenges for continued performance enhancement of modern microprocessors. Cooling schemes utilizing two-phase, microfluidic technologies are some of the more promising modular thermal management solutions for next generation devices. In this study, the flow and heat transfer in pin-fin enhanced micro-gaps are experimentally investigated. It has been known that pin-fin structures inside micro-gaps can increase convective heat transfer coefficients in single phase flow conditions. However, two-phase microfluidic cooling is becoming an increasingly popular method in thermal control of electronics, and this cooling strategy has not been well characterized for pin-fin enhanced micro-gaps. Pin-fin, micro-gap structures studied had a pin diameter, height and pitch of 150μm, 200μm and 225μm, respectively, providing an aspect ratio of 1.33. Both the overall micro-gap width and length are 1cm. The working fluid used was R245fa. The structure contained a transparent cover which allowed for visualization of flow through the micro-gap. A high speed camera allowed for image capture and characterization of various two-phase flow regimes. The thermal performances of the heat sink were experimentally evaluated using pressure drop and temperature measurements.

Compact Modeling of 3D Stacked Die Inter-Tier Microfluidic Cooling Under Non-Uniform Heat Flux

Three-dimensional (3D) stacking is an emerging trend for future high performance microsystems. The stacking of chips increases the power density significantly, with associated thermal concerns. For interior tiers in a 3D stack, the surface area for the heat removal is further reduced. Also, realistic power maps on active tiers produce highly non-uniform patterns, with maximum heat fluxes 5–10 times the average values. Conventional air-cooled heat sinks cannot meet these thermal requirements. In this paper, a compact thermal model of a multi-layer chip stack subjected to the realistic power map was developed. Interlayer pin fins were modeled on the back of the chips, with water as the coolant. It was found that the compact model ran ten times faster than the full computational fluid dynamics/heat transfer (CFD/HT) model, with errors within 5%. The compact model was then used to analyze the thermal characteristics of 4 layer stacked dual core Penryn microprocessors, with a total power of 172.4 W. The non-uniform power map produced maximum heat flux of 300 W/cm2 in the hot spots. The results show that at a pressure drop of 20 kPa and inlet water temperature of 20 °C, the temperature of the bottom tier is higher than the other three tiers and the maximum temperature of the bottom layer is 56 °C. This is because the bottom layer thermal management only depends on one-sided pin fin cooling, while the other layers have double-sided cooling. A dual pass channel configuration reduced the maximum temperature to 50 °C. Overall, the interlayer pin fin cooling was found to be a very effective thermal management method for 3D stacked die packaging.Copyright

Heat transfer mechanisms in pulsating heat-pipes with nanofluid

In this study, the effect of silver nanofluid on a pulsating heat-pipe (PHP) thermal performance was experimentally investigated to figure out how nanofluid works with PHP. A closed loop PHP was built with 3 mm diameter tubes. Thermocouples and pressure transducers were installed for fluid and surface temperature and pressure measurements. The operating temperature of the PHP varied from 30–100 °C, with power rates of 61 W and 119 W. The fill ratio of 30%, 50%, and 70% were tested. The results showed that the evaporator heat transfer performance was degraded by the addition of nanoparticles due to increased viscosity at high power rate, while the positive effects of high thermal conductivity and enhanced nucleate boiling worked better at low power rate. In the condenser section, owing to the relatively high liquid content, nanofluid more effectively improved the heat transfer performance. However, since the PHP performance was dominantly affected by evaporator heat transfer performance, the overall benefi...

Waste-Heat Driven Miniature Absorption Refrigeration System Using Ionic-Liquid as a Working Fluid

An ionic-liquid (IL) is a salt in a liquid state usually with an organic cation and inorganic anion. ILs provide an alternative to the normally toxic working fluids in absorption systems, such as the ammonia/water system. They also eliminate the problems of poor temperature match, crystallization and metal-compatibility problems of the water/LiBr system. In the present study, an IL is explored the working fluid of a miniature absorption refrigeration system so as to utilize waste-heat within the system for low-cost, high-power electronics cooling. To determine performance benchmarks for the refrigerant/IL (e.g. [bmim][PF6 ]) pairs, system-level simulations have been carried out. An NRTL model was built and used to predict the solubility of the mixture as well as the mixture properties such as enthalpy and entropy. The properties of the refrigerants were determined using REFPROP 6.0. Saturation temperatures at the evaporator and condenser were 25°C and 50°C, respectively. Chip power was fixed at 100 W with the operating temperature set at 85°C. R32 gave the highest operating efficiency with the maximum coefficient of performance (COP) of ca. 0.55 while R134a and R152a showed comparable performance with the maximum COP of ca. 0.4 at the desorber outlet temperature of 80°C. When waste-heat is available for the system operation, R134a and R152a COPs were comparable or better than that of R32.Copyright

TWO-PHASE FLOW AND HEAT TRANSFER IN PIN-FIN ENHANCED MICRO-GAPS WITH NON-UNIFORM HEATING

In modern microprocessors, thermal management has become one of the main hurdles in continued performance enhancement. Cooling schemes utilizing single phase microfluidics have been investigated extensively for enhanced heat dissipation from microprocessors. However, two-phase fluidic cooling devices are becoming a promising approach, and are less understood. This study aims to examine two-phase flow and heat transfer within a pin-fin enhanced micro-gap. The pin-fin array covered an area of 1cm × 1cm and had a pin diameter, height and pitch of 150μm, 200μm and 225μm, respectively, (aspect ratio of 1.33). Heating from two upstream heaters was considered. The working fluid used was R245fa. The average heat transfer coefficient was evaluated for a range of heat fluxes and flow rates. Flow regime visualization was performed using high-speed imaging. Results indicate a sharp transition to convective flow boiling mechanism. Unique, conically-shaped two-phase wakes are recorded, demonstrating 2D spreading capability of the device. Surface roughness features are also discussed.Copyright

Design of an Absorption Based Miniature Heat Pump System for Cooling of High Power Microprocessors

An absorption based miniature heat pump system, which can be driven by the low quality waste heat, is designed for chip cooling applications. Miniaturization is achieved, as the chemically-driven absorption/desorption process permits pressurization of the working fluid in liquid phase, requiring much smaller displacement volume than in vapor compression systems. The goal of this work is to design a system that keeps the chip junction temperature near room temperature, while removing 100 W of heat load. Water/LiBr pair is used as a working fluid. A dual micro-channel array evaporator is used to reduce both the mass flux through each micro-channel and the channel length, thus minimizing the pressure drop. Microchannel arrays for the desorber and condenser are placed in intimate communication with each other using a hydrophobic membrane, which provides a common chemically-selective interface between the desorber and condenser to separate the water vapor from LiBr solution. The separated water vapor is immediately cooled and converted into a liquid phase at the condenser side. For direct air cooling of the condenser and absorber, offset strip fin arrays are used. The performance of the components and the entire system is numerically evaluated and discussed.Copyright