Review
Thermal management of microelectronics with electrostatic fluid accelerators

https://doi.org/10.1016/j.applthermaleng.2012.08.068Get rights and content

Abstract

Optimal thermal management is critical in modern consumer electronics. Typically, a thermal management scheme for an electronic system involves several physical principles. In many cases, it is highly desirable to enhance heat transfer at the solid-air interface while maintaining small size of the thermal management solution. The enhancement of heat transfer at the solid-air interface can be achieved by several physical principles. One principle that is getting increased attention of thermal management design engineers is electrostatic fluid acceleration. This paper discusses recent breakthroughs in state-of-the-art of electrostatic fluid accelerators (EFAs). The paper compares and contrasts EFAs’ design and performance metrics to those of other airside cooling technologies used in small form factor applications. Since the energy efficiency, flow rate, and acoustic emissions are highly influenced by the scale of the airside cooling devices, the paper also presents the analysis of fundamental effect of scaling laws on heat transfer performance. The presented review and analysis helps drawing conclusions regarding achievable comparative performance and practicality of using different design approaches and physical principles for different applications.

Highlights

► Discuss breakthrough in state-of-the-art of electrostatic fluid accelerators (EFA). ► Compare EFAs' performance metrics to those of other airside cooling technologies. ► Show analysis of fundamental effect of scaling laws on heat transfer performance.

Introduction

For years, the majority of electronic devices relied on conventional rotary fans and heat pipes as a method of thermal management. Although these cooling devices were successful in the past, their application to advanced microelectronics faces several critical challenges due to rapid technology advancement. These technological breakthroughs have resulted in enormous component density and heat flux generation, which limit the amount of available thermal management technologies that thermal engineers can use.

Power delivery components currently occupy up to 30% of the computer’s printed circuit board (PCB) area. As thermal engineers continue to reduce the size of the electronic devices, power delivery components will cover more than 30% of the PCB area. The increased component density creates higher heat flux and requires more efficient thermal module for proper functionality. Moreover, it is expected that the anticipated transistor density will be more than 1010 per die [1] by 2015. As the CMOS manufacturing processes progress and switch from 90 nm to 16 nm, the predicted power density will grow exponentially by approximately 13 times [1], leading to a dramatic increase in heat flux. This heat flux needs to be transferred elsewhere to maintain lifetime of future microelectronics.

Additionally, skin temperature of consumer electronics becomes critical, especially for smaller form factor consumer electronics, such as smartphones, tablets, and laptops. The skin temperature is the temperature on the external surface of the electronics where we touch or hold them. In general, if the skin temperature on consumer electronics is higher than 45°C, long term use of the consumer electronics on human laps or hands will cause increasing skin damage. The skin damage will become worse for small form factor consumer electronics if there is no any proper thermal management solution aiming on this issue.

Furthermore, computer market analysis shows that the global production of small form factor computers has grown approximately 10 times in the past two years [2]. As a result, there is an urgent need for efficient compact thermal modules that ensure functionality of small form factor electronic devices in terms of size, reliability, acoustic level, and heat transport capability; however, the current technology addressing these terms is still not providing needed performance characteristics.

Currently, different kinds of heat transfer technologies can be classified into four categories: airside, heat transporter, active solid-state heat spreader, and passive thermal interface material (TIM), as listed in Table 1.

For airside cooling technologies, the primary conventional thermal management solution for most electronic devices uses both rotary fans and heat sinks. Although heat sinks can passively dissipate lower heat flux for components like low-end graphic processing units (GPU) or dynamic random access memory (DRAM), the heat sink’s most significant application is at the heat transfer interface between the heat source and the last step cooling components (rotary fans). In addition, many small form factor electronic devices, such as laptops [3], [4], [55] and cell phones [4], successfully demonstrate the use of emerging airside cooling technologies like electrostatic fluid accelerators (EFAs), piezoelectric fans, and synthetic jets. EFAs are also known as electrohydrodynamics (EHD) air mover, ionic wind pump, ionic wind engine, corona wind pump, and plasma fan.

Traditional heat transporter cooling solutions, such as heat pipes, liquid, and spray cooling technologies, are designed for higher heat flux applications. Among them, heat pipes are the most common heat transporter cooling solution; they are always used with rotary fans for thermal management of laptops and computer servers. Likewise, vapor chambers and phase change cold plates are derivatives of heat pipes, which provide heat dissipation for even higher heat flux electronic devices.

Emerging heat transporter cooling technologies like microchannels [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], micropumps [16], [19], [20], [21], and droplet manipulation [22] are focused on local high heat flux dissipation and local hot spot cooling applications. Although the industry has used electrospray technology for different applications for couple of decades, the performance and applications for thermal management in electronics is still limited [23], [24], [25], [26], [27], [28], [29], [30].

For several decades, thermoelectric (Peltier effect), thermionic, and thermotunneling effects have been used for heat transfer cooling. These devices can be categorized as active solid-state heat spreaders. Heat transfer performance enhancement of these devices has been investigated through various approaches, such as material combination, synthesis processes and figure-of-merit [31], [32]. Theoretically, cooling devices based on thermotunneling effect have highest coefficient of performance among these effects; its thermal efficiency is approximately 70% [32].

Passive thermal interface materials (TIM) for heat transfer cooling include greases, phase change materials, gels, adhesives, and graphene. The thermal resistance of a TIM is much lower than that of air, commercial silver epoxy, and metal system [33]. The historical perspective, current status, future development, reliability, and performance degradation of the thermal interface materials have been discussed at length by Prasher [34].

Improving the thermal resistance of current thermal management stacks is critical to enhance heat transfer performance for consumer electronics. Current thermal management stacks for consumer electronics consist of thermal grease, heat pipes, heat sinks, and conventional rotary fans. The analysis shows that the thermal resistance of conventional rotary fans and heat sinks dominates the overall heat transfer performance of current thermal management stacks. In the past, a lot of progress have emphasized on improving heat pipes’ and thermal grease’s thermal resistance, while the improvement in thermal resistance of conventional rotary fans and heat sinks is limited, because their geometry and structure remains the same.

Additionally, although thermal management solutions have been emphasized on the distinct demands and requirements of the applicable environment, airside cooling technologies remain the preferred choice for thermal management of most consumer electronics, especially for applications on small form factor electronics where thermal management performance of conventional rotary fans degrade as their dimensions are miniaturized (see Section 4.1).

Electrostatic fluid accelerators (EFAs) have the most potential to become critical elements in airside thermal management solutions for advanced microelectronics because they have no mechanical moving parts, have ultra thin and small form factor structure, and can fit in small physical rooms where other mechanical technologies cannot make it. EFAs are highly scalable electrohydrodynamics (EHD)-based solid-state devices that offer silent operation and dynamic enhanced airflow profiles in the boundary layer [35], [36]. EHD-based air movement can accelerate bulk airflow [37], [38], [39], [40] or disturb the thermal boundary layer close to the solid-fluid interface for heat transfer enhancement [7], [41]. Despite the significant developments in the field of EHD over the last half-century [42], [43], [44], [45], it has only been in the last decade that research has investigated the direction of micro- and meso-scale EFA cooling applications. Correspondingly, many technological hurdles remain, such as electrode degradation [46], [47], [48], [49] and prevention of ozone generation [41], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63].

This paper reviews potential state-of-the-art airside thermal management technologies, especially EFAs, for small form factor consumer electronics (i.e., notebooks), and compares the heat transfer performance of EFAs to other airside cooling technologies.

According to the worldwide PC market sales report [64], computer sales in the United States and worldwide will reach approximately 84.2 and 368.4 million, respectively, by 2013. Furthermore, as shown in Fig. 1 and Fig. 2, worldwide mobile computer sales will dominate over 55% of total computer sales by 2013, leading to almost 6 times the market growth when compared to sales in 1990. This change indicates large future demand for small form factor electronics [65].

According to the thermal market sales report [2], the global sale of product subsegments for thermal modules, which include broad-defined fans and heat pipes, will be 8,589.2 million units by 2013. Fans, blowers, and fan-heat sinks will continue to dominate over 50% of the thermal module market, whereas heat pipes and cold plates will occupy approximately 48% of the thermal market by 2013. The market share for thermoelectric coolers (TECs), classified as “Others” in Fig. 3, will be less than 2%. Therefore, in 2013, it is likely that airside cooling devices will continue to be mainstream for the thermal management of most state-of-the-art and future electronic devices, especially in the fast growing market of small form factor electronics.

Improved airside thermal management solutions are necessary for next generation small form factor consumer electronics. Currently, thermal management solutions for most consumer electronics rely on operation of conventional rotary fans and their derivatives, such as fan-heat sink combinations and fan-heat pipe combinations. In part, this is due to low unit prices and ease of manufacturing and installing of conventional rotary fans. Although conventional rotary fans can work efficiently in large form factor electronics, their major disadvantages are the noise level, accumulation of dust on the entire structure after long periods of operation. Performance degradation in heat transfer rate is also another disadvantage of rotary fans and their derivative when they are used in small form factor consumer electronics due to their limited inner physical space. Additionally, the current growing market is geared toward smaller form factor electronic devices. Conventional rotary fans and heat sinks are no longer functioning well for thinner and smaller consumer electronics.

Efficient airside thermal management solutions are important for small form factor consumer electrics. This following scenario demonstrates the necessity of airside cooling solutions to ensure proper functioning of laptops. An Intel core i5 CPU laptop is used as an example. The assumptions are (1) the laptop’s surface (aluminum, 1 mm in thickness) area of 20 cm × 25 cm; (2) using an ultra thin diamond (3 mm in thickness) as thermal conductor attached on top of the CPU to transfer heat from the CPU to the laptop’s skin; (3) the surrounding air temperature of 25°C; (4) natural convection heat transfer coefficient of 15 W/m2K; and (5) steady state heat transfer condition. The CPU’s power dissipation under normal load is 26 W. Under the assumption, natural convection heat transfer alone will result in a laptop skin temperature of approximately 60°C. This temperature far exceeds skin temperatures (45°C) that are safe for prolonged exposure to bare skin.

Furthermore, although elaborate cooling technologies have been developed, such as liquid cooling and refrigeration, airside cooling technologies remain the most promising cooling solution for consumer electronics market.. This is not only because of their simplicity, but also because the last step of heat exchange with the ambient environment remains necessary. Additionally, to transfer approximately 1.4 W of heat at a temperature difference of approximately 40°C (Table 2), the required device volume for the rotary fan and the AFM-cantilever EFA are 720 mm3 and 38 mm3 [66], respectively. This indicates not only a large reduction in the dimensions of the device, but also that higher transduction efficiency can be achieved by using EFA-based cooling devices. Therefore, EFAs have the potential to become the critical performance-enhanced airside thermal management solution in small form factor electronic devices.

As shown in Table 1, in addition to EFAs, both piezoelectric fans [3], [4], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77] and synthetic jets [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94] have been investigated as potential airside cooling technologies; their background is briefly reviewed next.

The synthetic jet, unlike the conventional air jet, is well-known for being a zero-net-mass jet because the system requires no additional mass. The synthetic jet is a device consisting of a flexible membrane, an air cavity, and an orifice nozzle, as displayed in Fig. 4(a). When the membrane moves upward, shown in Fig. 4(b), the surrounding air is sucked into the cavity. Then, when the membrane moves downward, air inside the cavity is forced out of the cavity and creates a vortex ring that moves outward, as shown in Fig. 4(c). By operating this device, induced airflow can be utilized to control surface temperature of integrated circuit components in electronic device directly under or near the airflow jet. The synthetic jet lands itself well for local hot spot cooling of electronics in an open environment, and is a competing and promising technology for cooling of light emitting diodes (LEDs) [95] for lighting applications.

A piezoelectric fan is a composite structure that converts electrical energy into mechanical energy by applying an AC voltage to the piezo element. The contraction and expansion of the piezo element vibrates the adjacent metal plate, inducing the movement of bulk air around the tip of the piezoelectric fan and dissipating heat generated from the hot surface. The most common configuration for piezoelectric fans is the combination of a piezo element and a metal plate (Fig. 5). The advantages of piezoelectric fans are simple structures, low acoustical noise, and efficient control of air speed and airflow. Furthermore, a reasonable lifetime could be longer than 25,000 hrs [77] and low device power consumption [77] are essential features over other competitive airside cooling technologies. Although piezoelectric fans are efficient when we use them to increase heat transfer rate of heat sinks [38], [39], piezoelectric fans still cannot generate significant airflow for thermal management of consumer electronics.

The detailed analysis and comparison of the heat transfer performance, and other figure-of-merits of airside thermal management technologies will be addressed in Section 3.4.

Traditional rotary fans have been successful in the thermal management market of electronics, but as the scale of the devices shrinks, their viability decreases as a thermal management solution in terms of acoustic level, reliability, and heat transfer rate. Furthermore, market analysis indicates that small form factor electronic devices will dominate future industrial uses and the consumer computer market. Performance-improved, airside thermal management devices for the application with small form factor electronic devices are necessary, and yet not commercially available. EFAs are the leading potential candidate for performance-enhanced, emerging airside cooling technology, due to their advantages in overcoming the technical hurdles that rotary fans encounter in small form factor applications. The successful demonstration of the EFAs’ thermal management capabilities in micro- and meso-scale in consumer laptops, suggests that EFAs can replace the thermal module market of rotary fans in small form factor consumer electronics.

Therefore, the purpose of this paper is to review and summarize the heat transfer characteristics of EFAs in terms of heat transfer rate, average thermal resistance, power consumption, and coefficient of performance (COP). The paper also offers a comparison of other figure-of-merits with other airside cooling technologies. Airside cooling technologies are analyzed in terms of the scaling law’s effect on the fundamental limits of energy efficiency, flow rate/heat removal capability, and acoustic level. In addition, this article describes the applications of emerging airside cooling technologies on laptops, cell phones, and integrated heat sinks.

Section snippets

Principle of electrostatic fluid accelerators (EFAs)

In this section, the following concepts are discussed in detail: the operating principles, the governing equations of the theoretical model, the modeling procedures, and the corresponding boundary conditions of a meso-scale EFA for thermal management.

Performance

The heat transfer performance of state-of-the-art EFAs are presented and analyzed in this section. In the first three sections, the heat transfer performance, including heat transfer rate, average thermal resistance, power consumption, and coefficient of performance (COP) of EFAs are discussed. Then, summaries according to the type of electrodes, flow field, spacing, etc., are given at the end of each section. Furthermore, the heat transfer performance of conventional rotary fans and the other

Fundamental limits of airside cooling technologies

As the scale of airside cooling devices shrinks to be applicable to small form factor electronics, the application of the traditional scaling law of each airside technology might not be appropriate. This section discusses the fundamental limitations involved when applying the traditional scaling law of each airside cooling technology on the respective scaled device.

The fundamental limits of airside cooling technologies depend heavily on scale. As the scale shrinks, a given technology effectives

Proof-of-concept applications of emerging airside cooling technologies

As discussed in this paper, emerging airside cooling technologies have been demonstrated in proof-of-concept small form factor electronic test vehicles (e.g., cell phones [4], laptops [3], [4], [55], [67], and printed wiring boards [91], [92]) to replace the conventional rotary fans for thermal management of these devices, and to demonstrate the heat transfer cooling ability. In this section, these proof-of-concept applications are described in-depth to understand each emerging airside cooling

Conclusion

Although many novel thermal management solutions have been aimed at higher heat flux applications to replace the current role of the convectional rotary fans, the airside cooling devices remain the main trend for the computer market according to sale and performance analysis. Furthermore, even though conventional rotary fans succeed in most power electronic devices in the computer market, as the scale for power electronic devices is reduced to the smaller form factor size, the heat transfer

Acknowledgements

Funding support from TESSERA Inc., Intel, and the University of Washington's Center for Commercialization is gratefully acknowledged.

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