Embedded Cooling of Electronic Devices

The following sections are included:

• Tribute
• About the Editors
• Contents

Emerging cooling solutions, such as single-phase liquid cooling, two-phase cooling, and thermoelectric coolers, have been shown to achieve better cooling performance and efficiency than traditional cooling mechanisms, especially for extreme hotspots and large thermal gradients in chips. Maximizing computing performance under permissible temperature limits in future systems can be achieved with the help of co-design and optimization of processors together with these emerging cooling methods. To facilitate design space exploration and optimization with new cooling technologies, there is a need for fast and accurate thermal models for emerging cooling techniques. This chapter presents steady-state compact thermal modeling methodologies for the aforementioned emerging cooling solutions. The chapter also provides cooling evaluation and validation results for these emerging cooling solutions using their corresponding compact thermal models.

To improve the capacities and capabilities of microelectronic devices, 3D packaging is an attractive solution which can provide increased functional density at a decreased cost per function. However, classical cooling approaches, such as the use of a heat sink or cold plate on the topmost component, are gradually becoming incapable of maintaining a temperature below the manufacturer-prescribed maximum junction temperature in a 3D-packaged system since the heat is compounded between different die layers. Using micro- and nanofluidic systems to expose the die directly to a dielectric coolant, known as embedded cooling, can provide far lower junction-to-ambient thermal resistances. Further, employing two-phase heat transfer methods can facilitate ultrahigh heat removal to tackle die-level hotspots reaching 1 kW/cm² on each high-power tier in a 3D microelectronic device. This chapter presents a brief overview of microscale evaporative cooling technologies that can be implemented in embedded cooling methods capable of removing ultrahigh heat fluxes from 3D chips. The following two major topics are covered in this chapter: (1) device fabrication and integration, and (2) the characterization of thermal and operational performances through temperature measurement and flow visualization. Additionally, we emphasize research findings related to the use of hollow micropillars to create asymmetric evaporating droplet arrays. These droplet arrays can provide an ultrahigh cooling performance that exceeds the maximum heat flux reported earlier in the literature and reaches up to 14.70 kW/cm², with a heat transfer coefficient of 7 × 106 W/m²K.

The two-phase cooling of microelectronics is studied by adapting a mechanistic phase change model that can be used with commercial computational fluid dynamics and heat transfer (CFD–HT) codes. Silicon microdevices with variable density of pin fins and hotspots are simulated, and results are validated with in-house experimental data generated for the purpose of studying the two-phase flow regimes and their thermal/hydraulic implications. The CFD–HT modeling approach is found to constitute a valuable tool in the design and analysis of heterogeneous microfluidic cooling devices under two-phase operation.

This chapter describes the experimental and modeling studies of two-phase chip-embedded cooling. The experimental investigation included chip-embedded cooling of a thermal test vehicle (TTV) and an embedded-cooled microprocessor (ECM). The modeling study included the development of both full- and reduced-physics two-phase flow boiling models, which were validated against the experimental data. The TTV was designed with chip-embedded radially expanding microchannels and a power map to mimic an eight-core microprocessor. The TTV enabled an experimental investigation of the limits of two-phase cooling, demonstrating cooling capability well beyond that of current microprocessor heat dissipation. Thermal modeling of the TTV chip-embedded cooling showed good agreement with the experimental data. The ECM module provided a test vehicle to directly compare chip-embedded cooling to a commercial air-cooled system. The air-cooled microprocessors in an IBM server were fully characterized in an environmental chamber and then modified by creating chip-embedded microchannels on the backside of the microprocessor die. A set of workloads was used to compare the performance of air- and embedded-cooled microprocessors. The ECM showed a significant reduction in chip junction temperature compared to air cooling, with the model in good agreement with experiments. The results showed that two-phase dielectric flow boiling provided highly effective cooling in microchannels with dimensions compatible with 3D chip stack cooling.

High-power radio-frequency (RF) amplifiers (HPAs) are widespread throughout commercial and military systems, from personal cell phones to large-scale communications networks, to the most advanced military radar and electronic warfare platforms. Ever-increasing performance demands, along with the desire for smaller sizes, have increased local heat fluxes beyond levels which can be effectively managed using conventional “remote” cooling paradigms, and the situation only worsens as developments in electronics fabrication outpace those in thermal management. This either leads to high operating temperatures, degraded system performance, and reduced life or to sub-optimal performance by “throttling back” the system to maintain suitable thermal control.

A very promising thermal management approach for reducing thermal resistance and restoring full system electrical potential is embedded cooling, in which the coolant is brought close to or in contact with the active heat source, effectively removing as many of the offending thermal interfaces as possible. This chapter reviews the embedded cooling concept and illustrates its benefits by comparing measured HPA thermal and RF performance using remote and embedded cooling approaches. Computational fluid dynamics and conjugate heat transfer simulations are shown to compare very favorably with the measured data, allowing accurate performance predictions to be made without the need for costly hardware iterations. Finally, some practical considerations for implementing embedded cooling in actual systems, such as the potential for leakage, clogging, and erosion, are discussed, and some recommendations are provided to mitigate such issues.

Many new technologies have been developed for flat panel displays, and the technical achievements have brought high-performance displays to the market. The demand for higher brightness and image resolutions, however, requires proper thermal management technology, and an inherent increase in heat dissipation from a display requires proper thermal management. Currently, liquid crystal displays (LCDs) dominate the flat panel display market, whereas micro-light-emitting diodes (Micro-LEDs) and organic light-emitting diodes (OLEDs) have emerged as alternative technologies. These displays implement different technologies to reproduce an image, and their thermal phenomena show different characteristics as well. The objective of thermal management in a flat panel display is to ensure picture quality based on thermal reliability. This chapter presents thermal phenomena and characteristics in various types of displays. In addition, the effects of thermal design parameters on a display at the maximum temperature and temperature uniformity are investigated, and thermal management strategies for the various state-of-the-art displays are discussed.

Philosophically, embedded-cooling strategies seek to remove as many barriers as possible between the primary thermal management solution and the component(s) of interest. However, in many cases, such as three-dimensional chip stacks and flexible electronics, conduction processes can still limit primary or secondary heat transfer pathways. Thus, materials development and an enhanced understanding of conduction physics represent opportunities for improving the cooling performance of the overall solution. Here, recent advances in bulk cubic crystals, two-dimensional layered materials, and advances in understanding conduction physics are briefly discussed, with a critical outlook on their potential for improving embedded cooling of electronic devices.

With an increase in microelectronic system density, using conventional technologies for cooling continues to become more challenging and is often a limiter of performance and efficiency. One key challenge is to address high background heat fluxes generated across entire chips and packages while mitigating localized hotspots with even higher heat fluxes. In this chapter, cooling of integrated circuits with non-uniform power maps using non-uniform micropin-fin heat sinks is investigated. Two heterogeneous micropinfin samples were fabricated, and single-phase experiments with deionized water were performed to investigate the effectiveness of local micropinfin clustering for hotspot cooling. Large background heat fluxes (250 W/cm²) were cooled in conjunction with even higher hotspot heat fluxes (500 W/cm²), with the lowest achieved average junction-to-inlet thermal resistance (R_th) of 0.092^°C/W. Subsequently, to demonstrate the benefit of microfluidic cooling on active silicon, a micropinfin heat sink was etched into the bulk of an Altera Stratix-V field-programmable gate array (FPGA) built using a 28 nm CMOS process. With a benchmark pulse compression algorithm running on the FPGA, thermal and electrical measurements were made. Deionized water was used as a coolant, with flow rates ranging from 0.15 to 3.0 mL/s and inlet temperatures ranging from 21^°C to 50^°C. An average junction-to-inlet thermal resistance of 0.07^°C/W or a heat transfer coefficient of 1.59 × 10⁴ W/m² °C was achieved.

Advances in the packaging approaches and semiconductor materials used for high-power electronic devices will usher in a paradigm shift in the thermal management strategies employed to dissipate extremely high heat fluxes at reasonable operating temperatures. Traditional “remote cooling” systems that have been commonly used to manage thermal loads generated by these devices suffer from parasitic interfacial, conduction, and spreading resistances, which lead to large temperature gradients. The next generation of “embedded cooling” systems will bring coolant very close to the heat source, eliminating these thermal resistances but requiring a solution that can directly manage high local heat fluxes without an intermediate heat spreader. This work highlights recent advances in the experimental characterization and modeling of two-phase embedded-cooling systems. Specifically, it summarizes the development of two-phase hierarchical manifold microchannel heat sinks, a promising high-performance embedded-cooling solution, as well as experimental and numerical studies investigating the underlying microchannel flow boiling phenomena. Static and dynamic flow boiling instabilities among parallel channels, along with the potential for coupling between transient heating conditions and these two-phase flow dynamics, offer unique implementation challenges that may arise in embedded-cooling systems. With continued progress in the understanding of these phenomena and predictive high-fidelity numerical modeling of microchannel flow boiling, embedded cooling systems will alleviate the thermal challenges that currently limit the power and performance of many electronic devices, enabling technological advancements across many industries.

Many packaging experts believe Moore’s law will hold for another decade due to recent technological efforts to merge multiple integrated circuit dies horizontally and vertically into a single package, commonly referred to as heterogeneous integration. With simultaneous shrinkage in chip size and larger heat dissipation, the flux values have increased exponentially. To tackle the skyrocketing thermal management challenges, the electronics industry and academic research community have examined various thermal management solutions. Embedded cooling eliminates most of the sequential conduction resistances from the chip to ambient, unlike the separable cold plates/heat sinks. Though there is a high heat transfer potential by embedding the cooling solution onto an electronic chip, there are still technological risks associated with the implementation of these technologies and uncertainty about which technologies should be adopted. This chapter discusses the thermal spreading limits, fluid selection considerations, as well as conventional and additive manufacturing-based embedded-cooling technologies.

Enhanced cooling, coupled with novel designs and packaging of semiconductors, has revolutionized communications, computing, lighting, and electric power conversion. It is time for a similar revolution that will unleash the potential of electrified propulsion technologies to drive improvements in fuel-to-propulsion efficiency, emission reduction, and increased power and torque densities, which are essential for the decarbonization and efficiency enhancement of aviation and other sectors. To improve the cooling of emerging electrical machines and realize performance and cost metrics of next-generation electric motors, it is crucial to enable electromagnetic and thermo-mechanical co-design through innovative design topologies, materials, and manufacturing techniques. This chapter focuses on the most recent progress in thermal management of electric motors, with particular focus on the future electrification of aviation propulsion, where ultrahigh-efficiency and high-specific-power (kW/kg) electric motors and power electronics play a key role. The chapter also includes a co-design-based simulation case study that analyzes the comparative impacts of selected technological areas, which, among others, can enable cost-effective efficiency enhancements and weight reductions and address the limitations of existing materials and cooling technologies.

The following section is included:

• Index