Showing papers by "Avram Bar-Cohen published in 2015"

Nanothermal Interface Materials: Technology Review and Recent Results

Abstract: Thermal interface materials (TIMs) play a critical role in conventionally packaged electronic systems and often represent the highest thermal resistance and/or least reliable element in the heat flow path from the chip to the external ambient. In defense applications, the need to accommodate large differences in the coefficients of thermal expansion (CTE) among the packaging materials, provide for in-field reworkability, and assure physical integrity as well as long-term reliability further exacerbates this situation. Epoxy-based thermoplastic TIMs are compliant and reworkable at low temperature, but their low thermal conductivities pose a significant barrier to the thermal packaging of high-power devices. Alternatively, while solder TIMs offer low thermal interface resistances, their mechanical stiffness and high melting points make them inappropriate for many of these applications. Consequently, Defense Advanced Research Projects Agency (DARPA) initiated a series of studies exploring the potential of nanomaterials and nanostructures to create TIMs with solderlike thermal resistance and thermoplasticlike compliance and reworkability. This paper describes the nano-TIM approaches taken and results obtained by four teams responding to the DARPA challenge of pursuing the development of low thermal resistance of 1 mm2 K/W and high compliance and reliability TIMs. These approaches include the use of metal nanosprings (GE), laminated solder and flexible graphite films (Teledyne), multiwalled carbon nanotubes (CNTs) with layered metallic bonding materials (Raytheon), and open-ended CNTs (Georgia Tech (GT)). Following a detailed description of the specific nano-TIM approaches taken and of the metrology developed and used to measure the very low thermal resistivities, the thermal performance achieved by these nano-TIMs, with constant thermal load, as well as under temperature cycling and in extended life testing (aging), will be presented. It has been found that the nano-TIMs developed by all four teams can provide thermal interface resistivities well below 10 mm2 K/W and that GE's copper nanospring TIMs can consistently achieve thermal interface resistances in the range of 1 mm2 K/W. This paper also introduces efforts undertaken for next generation TIMs to reach thermal interface resistance of just 0.1 mm2 K/W.

Two-Phase Thermal Ground Planes: Technology Development and Parametric Results

Abstract: Defense Advanced Research Project Agency's (DARPA's) thermal ground plane (TGP) effort was aimed at combining the advantages of vapor chambers or two-dimensional (2D) heat pipes and solid conductors by building thin, high effective thermal conductivity, flat heat pipes out of materials with thermal expansion coefficients that match current electronic devices. In addition to the temperature uniformity and minimal load-driven temperature variations associated with such two phase systems, in their defined parametric space, flat heat pipes are particularly attractive for Department of Defense and commercial systems because they offer a passive, reliable, light-weight, and low-cost path for transferring heat away from high power dissipative components. However, the difference in thermal expansion coefficients between silicon or ceramic microelectronic components and metallic vapor chambers, as well as the need for a planar, chip-size attachment surface for these devices, has limited the use of commercial of the shelf flat heat pipes in this role. The primary TGP goal was to achieve extreme lateral thermal conductivity, in the range of 10 kW/mK–20 kW/mK or approximately 25–50 times higher than copper and 10 times higher than synthetic diamond, with a thickness of 1 mm or less.

Near-junction microfluidic thermal management of RF power amplifiers

Abstract: While gallium nitride (GaN) is attracting broad attention as the wide bandgap material of choice for both industrial and defense applications, thermal impediments present a significant barrier to full exploitation of its inherently high electron sheet charge density and electrical breakdown voltage. For the last four years, the Defense Advanced Research Projects Agency (DARPA) has pursued research focused on reduction of near-junction thermal resistance through use of diamond substrates and convective and evaporative microfluidics. The options, challenges, and techniques associated with the development of this embedded thermal management technology are described, with emphasis on the accomplishments and status of efforts related to GaN power amplifiers.

Microcontact-Enhanced Thermoelectric Cooling of Ultrahigh Heat Flux Hotspots

Abstract: The dissipated power of insulated gate bipolar transistor and high electron mobility transistor amplifiers is typically nonuniform, resulting in areas of elevated temperature, or hotspots, which can have very large heat fluxes, on the order of 1000 W/cm2. While various bulk cooling systems are being researched to remove large amounts of heat, they uniformly reduce the chip temperature, leaving the temperature nonuniformity. Therefore, advanced hotspot cooling techniques, which provide localized cooling, are also required to unlock the full potential of cutting edge power devices. Thermoelectric coolers have previously been demonstrated as an effective method of producing on-demand cooling for the removal of localized hotspots. However, the heat flux of the hotspots that can be cooled is limited by the maximum cooling flux of thermoelectric devices. This paper demonstrates the thermal and reliability performance of a microcontact-enhanced thermoelectric cooling configuration, which uses a contact structure etched directly out of the electronic substrate to concentrate the cooling produced by a commercially available thermoelectric module. The 22 K of cooling, resulting in a hotspot temperature rise of <6 K for a heat flux of 2.5 kW/cm2, was experimentally demonstrated using a Laird HV37 thin-film thermoelectric module with a maximum device level cooling flux of 66 W/cm2. A numerical model was created, and it is predicted that when the chip and microcontact geometry is optimized, hotspots with heat fluxes in excess of 3 kW/cm2 can be cooled by nearly 40 K, reducing the hotspot temperature rise to 0 K.

Thermofluid characteristics of high-quality flowin chip-scale microgap channels

Abstract: Two-phase flow in chip-scale microgap channels offers highly potent thermal management capability and is the foundation for the emerging ”embedded cooling” paradigm of electronic cooling. While heat transfer and pressure drop in such flows are intimately tied to distinct forms of vapor-liquid aggregation, insufficient attention has been paid to characterizing the behavior of high-quality, short channel, microgap flow. This paper focuses on the results of simultaneous photographic and infrared visualization of such two-phase flows, under diabatic conditions, in a single 10 mm × 12 mm× 200 μm microgap channel cooled by low mass fluxes of FC-72. Churn flow, displaying pulsatile flow of confined vapor bubbles and slugs, is found to be the dominant flow regime for this short microgap channel configuration, yielding heat transfer coefficients that are an order-of-magnitude higher than all-liquid flow but are relatively insensitive to flow quality and heat flux. With increasing heat flux, local dryout and regions of elevated wall temperature grow and, periodically, expand to cover a majority of the microgap channel area. The average heat transfer coefficient is found to be in approximate agreement with the Shah correlation for low qualities and the Chen correlation for midlevel flow qualities, but both correlations are found to considerably overestimate the heat transfer coefficient at higher qualities, especially when local dryout becomes more frequent.

Thermal Characteristics of a Chip-Scale Two-Phase Microgap Cooler

Abstract: Two-phase heat transfer and pressure drop results for a chip-scale, uniformly heated, microgap channel, with nominal gap heights of 100, 200, and 500 μm and using HFE-7100 and FC-87 as the working fluids, are reported. Average heat transfer coefficients in the range of 5 to 30 kW/m2-K were observed, for exit qualities up to 60%. Local heat transfer coefficients, obtained through an inverse computational technique, are found to vary strongly with thermodynamic quality and to fall within ±30% of the predictions of the venerable Chen correlation.

Modeling and Simulation Challenges in Embedded Two Phase Cooling: DARPA’s ICECool Program

Abstract: Modeling and simulation of two-phase phenomena, as well as their impact on electrical performance and physical integrity are critical to the success of embedded cooling strategies. In DARPA’s Intrachip/Interchip Embedded Cooling (ICECool) program, thermal/electrical/mechanical co-simulation and modeling tools are being applied to the analysis and design of RF GaN MMIC (Monolithic Microwave Integrated Circuit) Power Amplifiers (PA) and digital ICs, with the ultimate goal of achieving greater than 3X electronic performance improvement. This paper addresses various simulation strategies and numerical techniques adopted by the DARPA ICECool performers, with attention devoted to co-simulation through coupled iterations of thermal, mechanical and electrical behavior for capturing device characteristics and predicting reliability and “best in class” simulations that can provide an understanding of device behavior during rugged operating conditions impacted by multi-physics environments. The effect of CTE (Coefficient of Thermal Expansion) mismatch on bond and structural integrity, the impact of cooling fluid choice on performance, the factors affecting erosion/corrosion in the microchannels, as well as electro-migration limits and joule heating effects, will also be addressed. A separate discussion of various two-phase issues, including interface tracking, system pressure drops, conjugate heat transfer, estimating near wall heat transfer coefficients, and predicting CHF (Critical Heat Flux) and dryout is also provided.Copyright © 2015 by ASME

Review of Most Recent Progress on Development of Polymer Heat Exchangers for Thermal Management Applications

Abstract: Polymeric materials have several favorable properties for heat transfer systems, including low weight, low manufacturing cost, antifouling, and anticorrosion. Additionally, polymers are typically electrical insulators, making them favorable for applications in which electrical conductivity is a concern. Examples of utilizing these favorable properties are discussed. The drawbacks to raw polymer materials include low thermal conductivity, low structural strength, and poor stability at elevated temperatures. Methods of mitigating these unfavorable properties, including loading the polymer with other materials and developing new polymers, are discussed. Enhanced geometric designs enabled by additive manufacturing can also improve thermal performance of polymer heat exchangers. Results of a research study utilizing additive manufacturing toward developing high-performance and cost-effective polymer heat exchangers for an air-to-liquid application are reviewed and discussed. Finally, needs for further research on enhancing polymer thermal performance are discussed.Copyright © 2015 by ASME

Modeling Thermal Spreading Resistance in Via Arrays

Abstract: As thermal management techniques for 3D chip stacks and other high power density electronic packages continue to evolve, interest in the thermal pathways across substrates containing a multitude of conductive vias has increased. To facilitate the use of numerical models that can reduce computational costs and time in the thermal analysis of through-layer via (TXV) structures, much research to date has focused on defining effective anisotropic thermal properties for a pseudo-homogeneous TXV medium using isothermal boundary conditions. While such an approach eliminates the need to model heat flow through individual vias, the resulting properties can be shown to depend on the specific boundary conditions applied to a unit TXV cell. More specifically, effective properties based on isothermal boundary conditions fail to capture the local “micro-spreading” resistance associated with more realistic heat flux distributions and local hot spots on the surface of these substrates.This work assesses how the thermal spreading resistance present in arrays of vias in interposers, substrates, and other package components can be properly incorporated into the modeling of these arrays. We present the conditions under which spreading resistance plays a major role in determining the thermal characteristics of a via array and propose methods by which designers can both account for the effects of spreading resistance and mitigate its contribution to the overall thermal behavior of such substrate-via systems. Finite element modeling of TXV unit cells is performed using commercial simulation software (ANSYS).Compactly stated, micro-spreading contributes to the total resistance RT = R1d + (fu + fl)Rsp,max, where 0≤ f ≤ 1 are adjustment factors that depend on the conditions at the upper and lower surfaces of the via array layer and Rsp,max occurs under worst-case conditions.Copyright © 2015 by ASME

Characterization of Die-Attach Thermal Interface of High-Power Light-Emitting Diodes: An Inverse Approach

Abstract: An advanced inverse approach, based on the transient junction temperature behavior, is proposed and implemented to quantify the resistance of the die-attach thermal interface (DTI) in high-power light-emitting diodes (LEDs). After describing the unique transient behavior of high-power LEDs associated with the forward voltage method, a hybrid analytical/numerical model is used to determine an approximate transient junction temperature behavior, which is governed predominantly by the resistance of the DTI. Then, an accurate value of the resistance of the DTI is determined inversely from the experimental data over the predetermined transient time domain using numerical modeling. The proposed inverse approach is capable of determining the DTI to an accuracy of 0.01 K/W, which is sufficiently high to evaluate the die bonding manufacturing processes.

Non-contact method for characterization of small size thermoelectric modules

Abstract: Conventional techniques for characterization of thermoelectric performance require bringing measurement equipment into direct contact with the thermoelectric device, which is increasingly error prone as device size decreases. Therefore, the novel work presented here describes a non-contact technique, capable of accurately measuring the maximum ΔT and maximum heat pumping of mini to micro sized thin film thermoelectric coolers. The non-contact characterization method eliminates the measurement errors associated with using thermocouples and traditional heat flux sensors to test small samples and large heat fluxes. Using the non-contact approach, an infrared camera, rather than thermocouples, measures the temperature of the hot and cold sides of the device to determine the device ΔT and a laser is used to heat to the cold side of the thermoelectric module to characterize its heat pumping capacity. As a demonstration of the general applicability of the non-contact characterization technique, testing of a thin film thermoelectric module is presented and the results agree well with those published in the literature.

Structural Reliability of Novel 3-D Integrated Thermal Packaging for Power Electronics

Abstract: The continual increase of device power and package integration levels has driven the development of advanced power electronics packaging solutions. This study will focus on a numerical modeling approach to design analysis and material selection to improve solder joint reliability in one of these advanced solutions — a thermally integrated power electronics package that aims to dissipate hot-spot heat flux (5 kW/cm2) via mini-contact based thermo-electric (TE) cooling in addition to removing background heat flux (1 kW/cm2) by manifold-microchannel cooling. The methodology used for performing the structural reliability modeling is a non-linear finite element analysis (FEA) approach. Combined thermal and mechanical analyses were run to obtain stresses and strains in the solder joint used to integrate the TE cooler with the mini-contact and the mini-contact with the Silicon Carbide (SiC) chip. To predict the Mean Time to Failure (MTTF) of SAC305 at various levels of integration, a Physics of Failure (PoF) based methodology was applied using Engelmaier’s failure model.In this paper, we will discuss the results of analyses of tapered, t-shaped, and lofted shaped mini-contacts made out of SiC, copper and diamond. Both structural design and material selection affect hot-spot heat dissipation and solder joint reliability. SiC has a good thermal conductivity at room temperature (RT), however, with increase in temperature, its thermal conductivity drops, and this can adversely affect device performance in high temperature applications. On the other hand, one can take advantage of high conductivity materials like copper, diamond or silver-diamond composite to keep the device cool and thus, improve package life time. However, for such high conductivity materials, one will need to take into account the cost of manufacturing complex shapes without any compromise in package thermal or reliability performance.It was found that a ductile mini-contact material will share the thermal mismatch strain with the solder interconnection, while a brittle mini-contact material will shift the failure site inside the TE cooler. It was determined that a mini-contact structure tapered near its top base and lofted (constant cross-sectional area) near the chip (bottom base) would provide the best reliability results. Application of high conductivity composite material (silver-diamond composite) to enhance structural reliability is discussed.Copyright © 2015 by ASME