Development of liquid cooling techniques for flip chip ball grid array packages with High Heat flux dissipations

In this paper, development of single-phase liquid cooling techniques for flip chip ball grid array packages (FBGAs) with high flux heat dissipations is reported. Two thermal test chips with different footprints, 12 mm/spl times/ 12 mm and 10 mm /spl times/10 mm, respectively, were used for high heat flux characterizations. A liquid-cooled aluminum heat sink with an area of 15 mm (L) /spl times/12.2 mm (W) populated by microchannels was designed and fabricated. The microchannel heat sink was assembled onto the chip, using a thermal interface material to reduce the contact thermal resistance at the interface. A variable speed pump was used to provide the pressure head for the liquid cooling loop. The measured thermal resistance results ranged from 0.44 to 0.32/spl deg/C/W for the 12-mm chip case and from 0.59 to 0.44/spl deg/C/W for the 10-mm chip case, both under flowrates ranging from 1.67/spl times/10/sup -6/ m/sup 3//s to 1.67/spl times/10/sup -5/ m/sup 3//s. An analytical model of the flow and heat transfer in microchannel heat sinks is also presented. Computational predictions agree with the measurements for pressure drop within 15% and thermal resistances within 6%. The analytical results indicate that thermal interface resistance becomes a key limitation to maximizing heat removal rate from electronic packages.     

Chip-level spray cooling of an LD-MOSFET RF power Amplifier

We report here the application of water spray cooling directly to the top surface of a lateral diffused metal oxide semiconductor field effect transistor (LD-MOSFET) in a 500-MHz RF power amplifier. With the amplifier running in Class A, spray cooling at a flow of 0.14 l/min increases the output power from 66 W to 84 W, and the power-added efficiency increases from 26% to 34%, all at 34 W input. This improvement is attributed to a large spray-induced reduction in junction temperature and total package thermal resistance. At the point of highest measured RF output and DC power dissipation, the reduction in junction temperature and total thermal resistance were estimated to be from /spl ap/214/spl deg/C to /spl ap/115/spl deg/C and from /spl ap/1.5/spl deg/C/W to /spl ap/0.6/spl deg/C/W, respectively, and the maximum spray-induced heat flux was /spl ap/162W/cm/sup 2/. In Class AB, the increase in output power and power-added efficiency are less, /spl ap/8%, but the amplifier can be driven harder before failure occurs. The maximum output in class AB is 79 W compared to 70 W without spray cooling.     

Nanoscale Thermal Transport and Microrefrigerators on a Chip

In this paper we review recent advances in nanoscale thermal and thermoelectric transport with an emphasis on the impact on integrated circuit (IC) thermal management. We will first review thermal conductivity of low-dimensional solids. Experimental results have shown that phonon surface and interface scattering can lower thermal conductivity of silicon thin films and nanowires in the sub-100-nm range by a factor of two to five. Carbon nanotubes are promising candidates as thermal vias and thermal interface materials due to their inherently high thermal conductivities of thousands of W/mK and high mechanical strength. We then concentrate on the fundamental interaction between heat and electricity, i.e., thermoelectric effects, and how nanostructures are used to modify this interaction. We will review recent experimental and theoretical results on superlattice and quantum dot thermoelectrics as well as solid-state thermionic thin-film devices with embedded metallic nanoparticles. Heat and current spreading in the three-dimensional electrode configuration, allow removal of high-power hot spots in IC chips. Several III-V and silicon heterostructure integrated thermionic (HIT) microcoolers have been fabricated and characterized. They have achieved cooling up to 7 /spl deg/C at 100 /spl deg/C ambient temperature with devices on the order of 50 /spl mu/m in diameter. The cooling power density was also characterized using integrated thin-film heaters; values ranging from 100 to 680 W/cm/sup 2/ were measured. Response time on the order of 20-40 ms has been demonstrated. Calculations show that with an improvement in material properties, hot spots tens of micrometers in diameter with heat fluxes in excess of 1000 W/cm/sup 2/ could be cooled down by 20 /spl deg/C-30 /spl deg/C. Finally we will review some of the more exotic techniques such as thermotunneling and analyze their potential application to chip cooling.     

A Microjet Array Cooling System for Thermal Management of High-Brightness LEDs

In this paper, a microjet-based cooling system is proposed for the thermal management of high-power light-emitting diodes (LEDs). Preliminary experimental investigation and numerical simulation on such an active cooling system are conducted. In the experiment investigation, thermocouples are packaged with LED chips to measure the temperature and evaluate the cooling performance of the proposed system. The experimental results demonstrate that the microjet-based cooling system works well. For a 2 times 2 LED chip array, when the input power is 5.6 W and the environment temperature is 28degC, without any active cooling techniques, the temperature of 2 times 2 LED chip array substrate reaches 72degC within 2 min and will continue to increase sharply. However, by using the proposed cooling system, when the flow rate of micropump is 9.7 mL/s, the maximum LED substrate temperature measured by the thermocouples will remain stable at about 36.7degC. As for the numerical optimization, the comparison between the simulation and experimental results is presented to confirm the feasibility of the simulation model. By using the simulation model, the effects of microjet diameter, top cavity height, micropump flow rate, and jet device material on system performance are numerically studied. According to the preliminary test and numerical optimization, an optimized microjet cooling system is fabricated and applied in thermal management of a 220-W LED lamp. The temperature test demonstrates that the cooling system has good performance.     

Miniature loop heat pipes-a promising means for cooling electronics

Loop heat pipes (LHPs) are highly efficient heat-transfer devices, which have considerable advantages over conventional heat pipes. Currently, miniature LHPs (MLHPs) with masses ranging from 10-20 g and ammonia and water as working fluids have been developed and tested. The MLHPs are capable of transferring heat loads of 100-200 W for distances up to 300 mm in the temperature range 50-100/spl deg/C at any orientation in 1-g conditions. The thermal resistance for these conditions are in the range from 0.1 to 0.2 K/W. The devices possess mechanical flexibility and are adaptable to different conditions of location and operation. Such characteristics of MLHPs open numerous prospects for use in cooling systems of electronics and computer systems.     

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.     

Superlattice microrefrigerators fusion bonded with optoelectronic devices

A three-dimensional (3-D) 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 (SL) microrefrigerators for various device sizes, ranging from 40/spl times/40 to 120/spl times/120/spl mu/m /sup 2/. We discussed both the maximum cooling and cooling power densities (CPDs) for experimental devices, analyzed their nonidealities, and proposed an optimized structure. The simulation results demonstrated that the experimental devices with an optimized structure can achieve a maximum cooling of 3/spl deg/C, or equivalently, a CPD over 300W/cm/sup 2/. Furthermore, we found it was possible to achieve a maximum cooling of over 10/spl deg/C; equivalently, a CPD over 900W/cm/sup 2/, when the figure of merit (ZT) of InGaAs/InAlAs SL was enhanced five times with nonconserved lateral momentum structures. Besides monolithic growth, we also proposed a fusion bonding scheme to simply bond the microrefrigerator chip on the back of the hot spots, defined as two-chip integration model in this paper. The cooling effect of this model was analyzed using ANSYS simulations.     

Thermal analysis of high power LED package with heat pipe heat sink

The goal of this study is to improve the thermal characteristics of high power LED (light-emitting diode) package using a flat heat pipe (FHP). The heat-release characteristics of high power LED package are analyzed and a novel flat heat pipe (FHP) cooling device for high power LED is developed. The thermal capabilities, including startup performance, temperature uniformity and thermal resistance of high power LED package with flat heat pipe heat sink have been investigated experimentally. The obtained results indicate that the junction temperature of LED is about 52 °C for the input power of 3 W, and correspondingly the total thermal resistance of LED system is 8.8 K/W. The impact of the different filling rates and inclination angles of the heat pipe to the heat transfer performance of the heat pipe should be evaluated before such a structure of heat pipe cooling system is used to cool high power LED system.     

Heat transfer from square pin-fin heat sinks using air impingement cooling

Experimental and numerical results are presented for heat transfer from a C4 mounted organic land grid array (OLGA) thermal test chip cooled by air impingement. Five heat sink geometries were investigated for Reynolds numbers ranging from 9,000 to 26,000. The dimensionless nozzle-to-heat sink vertical spacing z/D was varied between 2 and 12. In this study, we investigate the interactions between heat sink geometry, flow conditions and nozzle setting and how they affect the convective heat transfer and overall cooling of the test chip as measured by total thermal resistance /spl theta//sub ja/. Optimizing fin arrays by minimizing the overall heat sink thermal resistance instead of focusing solely on maximizing the heat transfer from the fins is shown to be a better design criterion. We also provide results that show cooling performance gains can be obtained by inserting a deflector plate above the heat sink.     

高功率微波装置散热热沉传热流动特性

高功率微波装置在运行时面临的高热流密度散热是当前热控必须解决的难题。微小通道热沉散热结构简单,换热能力突出,在一定程度上能够解决高热流密度散热的问题。但使用微小通道热沉散热时,散热面温度在沿工质流动方向不断升高,这对器件稳定运行不利。而射流冲击技术中流体垂直于热源喷射,温度边界层薄,温度梯度大,换热效果强。将射流冲击技术与微通道热沉相结合,不仅能提高换热系数,增大换热量,而且能实现良好的温度均匀性。对高热流密度下射流冲击微小通道热沉进行数值模拟,分析不同射流孔径对其传热和流动特性的影响。结果表明,增大远离出口处的射流孔径,有利于提高传热效率和减小流动阻力。优化后的射流微通道热沉,在质量流量为14 g/s时,换热系数接近39 000 W/(m2·K)。     

Thermal budget limits of quarter-micrometer foundry CMOS for post-processing MEMS devices

Thermal budget limits for low stand-by power ( LSP), 0.25 /spl mu/m foundry CMOS devices have been investigated, in order to assess the impact of post-processing microelectromechanical devices devices. Resistance increases for vias (metal-to-metal contacts) rather than transistor-performance shifts limits the post-processing thermal budget. An empirical relation is found to predict the via resistance increase for various annealing conditions, based on third-order reactions of vacancies supplied by surface diffusion of metal atoms. The resistance increase is strongly dependent on annealing time and temperature. With a criterion of 10% increase, 6 h at 425/spl deg/C, 1 h at 450/spl deg/C, and 0.5 h at 475/spl deg/C are the maximum allowable thermal budgets, respectively. Electromigration (EM) of via chain structures was also evaluated, and after annealing for 6h at 425/spl deg/C showed only a 33% decrease in the EM lifetime.     

Al/sub 0.10/Ga/sub 0.90/As-GaAs microcoolers

GaAs-based microcoolers were fabricated and tested. An Al/sub 0.10/Ga/sub 0.90/As layer grown on GaAs, having a lower thermal conductivity and comparable electrical conductivity to that of the substrate, was employed in the microcooler structure to reduce the heat conduction back from the heat sink. Maximum cooling temperatures of 0.87 /spl deg/C and 2.3 /spl deg/C were obtained at ambient temperatures of 25 /spl deg/C and 100 /spl deg/C, respectively, from 60 /spl times/ 60 /spl mu/m microcoolers.     

Heat dissipation in high-power GaN electronics on thermally resistive substrates

The heat dissipation in GaN devices grown on low thermal conductivity lithium gallate (LGO) substrates was investigated. The thermal conductivity of single-crystal LGO was measured utilizing the 3/spl omega/ technique for temperatures ranging from 100 K-500 K. For the GaN layer, the thermal conductivity was estimated using a phonon transport model which included dislocation density and temperature dependence. These data were then used in a finite element program to determine the thermal behavior of a heterojunction field-effect transistor. Based on a maximum junction temperature of 500 K, it was found that devices with a power dissipation of 1 W/mm were possible if the primary heat dissipation path was through the low thermal conductivity substrate. However, in using a front side cooling scheme, results suggest that it may be possible to develop devices with power dissipation in the range of 10 W/mm.     

Evaporation-Enhanced, Dynamically Adaptive Air (Gas)-Cooled Heat Sink for Thermal Management of High Heat Dissipation Devices

To address the thermal management challenges associated with high power dissipation devices, we describe a novel hybrid thermal management device that enables significant enhancement of conventional air-cooled heat sinks using on-demand and spatially controlled droplet/jet impingement evaporative cooling. The device architecture modifies an air (gas)-cooled heat sink by adding a multiplexed, planar microelectromechanical system (MEMS)-based droplet ejector array as a capping surface of the finned structure of a conventional heat sink. Such a minimal modification of the heat sink allows one to exploit high heat flux evaporative cooling by virtue of delivering streams of liquid droplets or jets to the highly thermally conducting heat-spreading surface of the heat sink fins. The phase change associated with liquid droplet evaporation results in significant $(sim 50%)$ enhancement of the dissipated thermal load, beyond what could be achieved by using air (gas) cooling alone. Finally, among the additional key attractive features of the described technology is its ease of implementation (i.e., modification of commercially available heat sinks), paving the way to power-efficient, low-cost thermal management of high power dissipation devices.      

Al/SiC carriers for microwave integrated circuits by a new technique of pressureless infiltration

Materials with high thermal conductivity and thermal expansion coefficient matching with that of Si or GaAs are being used for packaging high density microcircuits due to their ability of faster heat dissipation. Al/SiC is gaining wide acceptance as electronic packaging material due to the fact that its thermal expansion coefficient can be tailored to match with that of Si or GaAs by varying the Al:SiC ratio while maintaining the thermal conductivity more or less the same. In the present work, Al/SiC microwave integrated circuit (MIC) carriers have been fabricated by pressureless infiltration of Al-alloy into porous SiC preforms in air. This new technique provides a cheaper alternative to pressure infiltration or pressureless infiltration in nitrogen in producing Al/SiC composites for electronic packaging applications. Al-alloy/65vol% SiC composite exhibited a coefficient of thermal expansion of 7/spl times/10/sup -6/ K/sup -1/ (25/spl deg/C-100/spl deg/C) and a thermal conductivity of 147 Wm/sup -1/K/sup -1/ at 30/spl deg/C. The hysteresis observed in thermal expansion coefficient of the composite in the temperature range 100/spl deg/C-400/spl deg/C has been attributed to the presence of thermal residual stresses in the composite. Thermal diffusivity of the composite measured over the temperature range from 30/spl deg/C to 400/spl deg/C showed a 55% decrease in thermal diffusivity with temperature. Such a large decrease in thermal diffusivity with temperature could be due to the presence of micropores, microcracks, and decohesion of the Al/SiC interfaces in the microstructure (all formed during cooling from the processing temperature). The carrier showed satisfactory performance after integrating it into a MIC.     

Experimental investigation on the performance of CPU coolers: Effect of heat pipe inclination angle and the use of nanofluids

An experimental study of heat transfer performance of a CPU cooling heat pipe, examining the effects of inclination angle and nanofluids, has been conducted. It is shown that inclination angle of the unit has a significant effect on the cooling process, since it directly influences the operation of the evaporator. The effect is mainly due to the capillary effect and boiling limits of the heat pipe. The results demonstrate that for a given CPU temperature, there is a threshold angle at which the thermal resistance of the heat pipe increases dramatically. It is observed that as the CPU temperature increases, the threshold angle decreases from 60° to 30°. Introduction of 0.5 wt% Al₂O₃ nanoparticles to the water coolant of heat pipe has led to a decrease in thermal resistance. It is shown that at 10 W, the presence of nanofluid has reduced the thermal resistance by 15%, while at 25 W, the thermal resistance has dropped by 22%.     

Ion implanted AlGaN-GaN HEMTs with nonalloyed Ohmic contacts

In this letter, the incorporation of Si implantation into AlGaN-GaN high-electron mobility transistor (HEMT) processing has been demonstrated. An ultrahigh-temperature (1500/spl deg/C) rapid thermal annealing technique was developed for the activation of Si dopants implanted in the source and drain. In comparison to control devices processed by conventional fabrication, the implanted device with nonalloyed ohmic contact showed comparable device performance with a contact resistance of 0.4 /spl Omega//spl middot/mm, I/sub max/ of 730 mA/mm, f/sub t//f/sub max/ of 26/62 GHz, and a power of 3.4 W/mm on sapphire. These early results demonstrate the feasibility of implantation incorporation into GaN-based device processing as well as the potential to increase yield, reproducibility, and reliability in AlGaN-GaN HEMTs.     

Transient thermal characterization of AlGaN/GaN HEMTs grown on silicon

We studied a temperature increase and a heat transfer into a substrate in a pulsed operation of 0.5 length and 150 /spl mu/m gate width AlGaN/GaN HEMTs grown on silicon. A new transient electrical characterization method is described. In combination with an optical transient interferometric mapping technique and two-dimensional thermal modeling, these methods determine the device thermal resistance to be /spl sim/70 K/W after 400 ns from the start of a pulse. We also localized the high-electron mobility transistor heat source experimentally and we extracted a thermal boundary resistance at the silicon-nitride interface of about /spl sim/7/spl times/10/sup -8/ m/sup 2/K/W. Thermal coupling at this interface may substantially influence the device thermal resistance.     

Power transistors fabricated using isotopically purified silicon (/sup 28/Si)

It is well known that isotopic purification of group IV elements can lead to substantial increases in thermal conductivity due to reduced scattering of the phonons. The magnitude of the increase in thermal conductivity depends on the level of isotopic purification, the chemical purity, as well as the test temperature. For isotopically pure silicon (/sup 28/Si) thermal conductivity improvements as high as sixfold at 20 K and 10%-60% at room temperature have been reported. Device heating during operation results in degradation of performance and reliability (electromigration, gate oxide wearout, thermal runaway). In this letter, we discuss the thermal performance of packaged RF LDMOS power transistors fabricated using /sup 28/Si. A novel technique allows the cost effective deployment of this material in integrated circuit manufacturing. A clear reduction of about 5/spl deg/C-7/spl deg/C in transistor average temperature and a corresponding 5%-10% decrease in overall packaged device thermal resistance is consistently measured by infrared microscopy in devices fabricated using /sup 28/Si over natural silicon.     

Active cooling substrates for thermal management of microelectronics

Heat removal in printed wiring boards (PWB) is primarily accomplished through conduction. This work presents a microelectromechanical system (MEMS) device comprised of an active cooling substrate (ACS) designed and fabricated to add fluidic cooling functionality to the PWB. Thermal management is enhanced through the additional heat convection mode. Synthetic jet technology makes the compact, but easily integrated, MEMS cooling device possible. The fluid control unit, a synthetic jet, has been implemented in an epoxy-glass printed wiring board by multilayer lamination. An air reservoir is drilled through the core of printed wiring board. A flexible polymeric diaphragm and a low profile electromagnetic driver create an active pumping system to produce vibrating air jets downstream of microfluidic channels which transports heat generated by hot microelectronic components. Test heater chips have been directly die attached to the substrate. The integrated packaging system has been characterized mechanically, electrically, and thermally. Peak jet velocities of 14 m/s and average jet velocities of approximately 3 m/s have been achieved at actuator powers of 60 mW. This integrated active cooling substrate has the potential for broad applications in thermal management at the system packaging level.