Electronic Cooling System - Engineering Seminar Report

electronic cooling system

          Electronic equipment has made its way in to practically every aspect of modern life, from toys to high power computer. Electronic components depend on the passage of electric current and they become potential sites for excessive heating since the current flow through a resistance is accompanied by heat generation. Continued miniaturization of electronic system has resulted in a dramatic increase in the heat generated per unit volume. Unless properly designed and controlled; high rates heat generation result in high operating temperatures for electronic equipment, which jeopardizes its safety and reliability.

Electronic equipment has made its way in to practically every aspect of modern life, from toys to high power computer. Electronic components depend on the passage of electric current and they become potential sites for excessive heating since the current flow through a resistance is accompanied by heat generation. Continued miniaturization of electronic system has resulted in a dramatic increase in the heat generated per unit volume. Unless properly designed and controlled; high rates heat generation result in high operating temperatures for electronic equipment, which jeopardizes its safety and reliability.  Thermal management is a   challenging field that the developments and improvements in thermal management hardware have assisted the electronics packaging community by enabling higher heat fluxes in ever-smaller packaging volumes The growth of commercial electronics coupled with the need for better cooling, has enabled some thermal management technologies to move from expensive aerospace technologies to costs that are viable for commercial packaging.

The story of packaging in the past decade is tightly coupled to the continuous and rapid development of integrated circuit technology as exemplified by Moore's Law (i.e., the number of transistors in an IC doubles every 18 months).

Once the integrated circuit was invented, it was an inevitable trend that more and more of the functionality of a given electronic system would be integrated into fewer and fewer chips. The increased functionality led to requirements for greater I/O (input-output) connections and greater power. Packaging, as always, had to satisfy the role of an enabling technology to allow the full performance potential of a semiconductor to be realized in a given application. Package technology experienced dramatic changes to keep pace with these rapid changes in IC functionality.

          Thermal interface material (TIM) is essential when two or more solid surfaces are in the heat path. Standard machined surfaces are rough and wavy, leading to relatively few actual contact points between surfaces. The insulating air gaps created by multiple voids of "contacting" hard surfaces are simply too large a thermal barrier for even modest power applications. The first tactic in overcoming this barrier is to fill the voids and eliminate air by introducing a third material to the heat path that is fluidic and wets the surfaces. As shown in Figure 1, a thermal interface material essentially changes the thermal path between rough-surfaced solids from conduction through point contacts and air to conduction entirely through solids             

An important property of any TIM is its thermal conductivity. In addition to thermal performance, TIM’S are selected on several other critical criteria as well. Ease of use in assembly and rework are important in high-end applications, as is long-term stability (reliability). Elastomeric pads were developed as an alternative to early grease solutions, largely for the manufacturing advantages they offered. Phase change materials emerged as a technology that captured the thermal performance advantage of grease and combined it with the assembly ease of a solid pad.

          Heat sinks are needed when the heat transfer to the ambient air directly from the top of the package or via the system board is not sufficient to keep the semiconductor device within the allowable temperature range.

          The increase in power across all applications involving ICs led to the more frequent need for heat sinks It gave more flexibility in managing heat loads far beyond the original design intent of a system.

          An example is on a graphics processor in an expansion card in a PC. The mainstay in the heat sink industry: the extruded aluminum heat sink was supplanted in higher power applications. For a given heat sink form factor, its thermal performance can be enhanced, in general, by the following two approaches:

1.      Increasing the thermal conductance of components comprising the heat sink.
2.      Increasing the fin area exposed to the airflow.

          Developments in fan technology closely paralleled developments in heat sinks. Fans have been subject to the same cost sensitivity and form factor demands as heat sinks. For example, miniature fans are now being used in laptop computers. Since fans are active devices, noise and long-term reliability are further considerations.
          The heat removal capability of a heat sink is directly proportional to the mass flow of air through its fin structure. A simple way of getting higher thermal performance from a heat sink is by pushing more air through it. However, in life, nothing is free, so using a fan at a higher pressure drop leads to greater power consumption in moving the air and greater noise. This is the inevitable struggle in fan usage.                                                                                                     

          To cool electronic components, one can use air and liquid coolers as well as coolers constructed on the principle of the phase change heat transfer in closed space; i.e., immersion, thermosyphon, and heat pipe coolers. Each of these methods has its merits and draw-backs because, in the choice of appropriate cooling, one must take into consideration not only the thermal parameters of the cooler but also design and stability of the system, durability, technology, price, application, etc. they are most effective and have great potential as power levels and volume requirements increase. For these reasons heat pipes have been applied up to now mainly in applications with special working conditions and requirements, such as in space thermal control, in aircraft devices, in traction drives in audio amplifiers, in cooling of closed cabinets in harsh environmental conditions, etc. Heat pipes, because of their high thermal conductivity, provide an essential isothermal environment with very small temperature gradients between the individual components. The high heat transfer characteristics, the ability to maintain constant evaporator temperatures under different heat flux levels, and the diversity and variability of evaporator and condenser sizes make the heat pipe an effective device for the thermal control of electronic components

Heat Pipe Operation
          Heat pipes are sealed vacuum vessels that are partially filled with a working fluid, typically water in electronic cooling, which serves as the heat transfer medium. The heat pipe envelope is made of copper in a myriad of shapes including cylindrical, rectangular, or any other enclosed

geometry. The wall of the envelope is lined with a wick structure, which provides surface area for the evaporation/condensation cycle and capillary capability. Since the heat pipe is evacuated and then charged with the working fluid prior to being sealed, the internal pressure is set by the vapor pressure of the working fluid.

          As heat is applied to the surface of the heat pipe, the working fluid is vaporized (Figure 1). The vapor at the evaporator section is at a slightly higher temperature and pressure than other areas. This creates a pressure gradient that forces the vapor to flow to the cooler regions of the heat pipe. As the vapor condenses on the heat pipe walls, the latent heat of vaporization is transferred to the condenser. The capillary wick then transports the condensate back to the evaporator section. This closed loop process continues as long as heat is applied.

          For a heat pipe to function properly, the net capillary pressure difference between evaporator (heat source) and condenser (heat sink) must be greater than the sum of all pressures losses occurring throughout the liquid and vapor flow paths. This relationship, referred to as the capillary limitation, can be expressed mathematically as follows.

Pcmax is the maximum capillary pressure difference generated within the capillary wicking structure between the evaporator and condenser, Pl and Pv are the viscous pressure drops occurring in the liquid and vapor phases, respectively, and Pg represents the hydrostatic pressure drop.

          When the maximum capillary pressure is equal to or greater than the sum of these pressure drops, the capillary structure can return an adequate amount of working fluid (priming or repriming of the heat pipe) to prevent the evaporator wicking structure from drying out. When the sum of all pressure drops exceeds the maximum capillary pumping pressure, the working fluid is not supplied rapidly enough to the evaporator to compensate for the liquid loss through vaporization, and the wicking structure becomes starved of liquid and dries out (depriming of the heat pipe). This condition, referred to as capillary limitation, varies according to the wicking structure, working fluid, evaporator heat flux, operating temperature, and body forces .

          Although a plethora of designs of classical heat pipes are available, recent industry trends have frequently shown the limitations of these conventional designs. This has led to the evolution of novel concepts fitting the needs of present industry demands. A relatively new and emerging technology, Pulsating or Loop-type Heat Pipes (PHP), as proposed by Akachi represent one such field of investigation. This range of devices is projected to meet all present and possibly future specific requirements of the electronics cooling industry, owing to favorable operational characteristics coupled with relatively cheaper costs.
          Although grouped as a subclass of the overall family of heat pipes, the subtle complexity of internal thermo-fluidic transport phenomena is quite unique, justifying the need for a completely different research outlook. A comprehensive theory of operation and a reliable database or tools for the design of PHPs according to a given microelectronics-cooling requirement still remain Unrealized

          There was a demand for increased packaging density and performance reasserted itself and heat flux is again increasing dramatically. This has prompted many to ask what the practical limits of air-cooling are and when is it necessary resort to some form of liquid cooling?

          With standard fans a maximum heat transfer coefficient of maybe 150 W/m2K can be reached with acceptable noise levels, which is about 1 W/cm2 for a 60°C temperature difference

          Indirect liquid cooling schemes are denoted by the presence of distinct physical barriers between the chip and the liquid. Several possible implementations are illustrated in Figure 4. The most conservative implementation is to attach a separable cold plate to a lidded module (Figure 4a). When greater thermal performance is desired, the interface between the cold plate and the module can be eliminated by integrating the cold plate with the module (Figure 4b). Taking the concept to the limit, chip-scale high performance cold plates of the type discussed in Reference can be attached directly to the chips (Figure 4c).

          Direct liquid (immersion) cooling brings the coolant in direct contact with the back of the chip. Historically, the coolant considered was a dielectric (e.g., fluorocarbon coolants or FCs) primarily because it would also be in direct contact with the chip interconnects and substrate top surface metallurgy (TSM). For this exercise, it is assumed that water can be brought in direct contact with the back of the chip. Obviously a seal or barrier preventing water from contacting the interconnects or TSM is needed. For now, it is assumed that such a seal/barrier is technically feasible.

          For the dielectric cooling cases, a few different options may be considered. For example, droplets of a dielectric coolant may be sprayed on the chip surface as shown in Figure 5. Based upon values reported in the literature spray cooling with a fluorocarbon liquid (i.e., FC-72) is capable

of supporting heat fluxes up to 50 to 60 W/cm2 while limiting chip junction temperatures to 85°C. Jet impingement of a dielectric liquid on the chip surface may also be considered (Figure 6). Researchers have reported chip heat fluxes approaching 120 W/cm2 and a chip to liquid temperature difference of 60°C while cooling a 6 x 6 mm silicon chip with a single jet of FC-72 liquid at a flow rate of 2.2 x 10-4 m3/s . It is expected that for the chip size under consideration here, the same heat flux and temperature difference could be achieved with four jets and a liquid flow rate per chip of 8.8 x 10-4 m3/s. Alternatively, the dielectric coolant flow could be directed through a micro channel heat sink attached directly to the chip as shown in Figure 7. As with indirect liquid cooling, single phase convection heat transfer is assumed. It is also assumed that a metal chip-to-heat sink interface of 2.5°C-mm2/W is the best achievable for the heat sink attach.
          Ultimately, given that an adequate chip to liquid seal can be provided, direct jet impingement of water on the back of the chip may be considered. For this case a recently published correlation for multiple jets is used to predict water jet impingement cooling performance.
          For these various options, the chip heat removal capability for a 60°C chip to liquid inlet temperature difference is estimated to be 60, 120, 300, and 460 W/cm2 for spray cooling, dielectric jet impingement cooling, forced convection dielectric cooling with a heat sink attached to the chip, and water jet impingement, respectively.

PIEZO FANS         
          Piezoelectric fans are low power, small, relatively low noise, solid-state devices that recently emerged as viable thermal management solutions for a variety of portable electronics applications including laptop computers and cellular phones. Piezoelectric fans utilize piezoceramic patches bonded onto thin, low frequency flexible blades to drive the fan at its resonance frequency. The resonating low frequency blade creates a streaming airflow directed at electronics components.

          The term 'micro' is applied to devices having hydraulic diameters of ten to several hundred micrometers, while 'mini' refers to diameters on the order of one to a few millimeters. In many practical cases, the small flow rate within micro-channels produces laminar flow resulting in a heat transfer coefficient inversely proportional to the hydraulic diameter. In other words, the smaller the channel, the higher the heat transfer coefficient. Unfortunately, the pressure drop increases with the inverse of the second power of the channel width, keeping the mass flow constant, and limiting ongoing miniaturization in practice.

          It is based on 'micro-scale ion-driven airflow' using very high electric fields created by nanotubes. As shown in Figure 5, the ionized air molecules are moved by another electric field, thereby inducing secondary airflow. Cooling a heat flux level of 40 W/cm2 has been reported.

          A comparison of the heat removal capability in terms of chip heat flux for the module cooling concepts.     
          A number of approaches show interesting industrial potential for the cooling of high-power electronics. For heat flux densities up to and maybe even beyond 50 W/cm2 air-cooling may remain the cooling option of choice. For heat fluxes over 100 W/cm2, some form of liquid-cooling appears to be the most viable option. Several papers have demonstrated solutions that may be industrially feasible for application in the range between 500 and 1000 W/cm2. Considering the range of efforts underway to extend conventional cooling technologies, as well as develop new ones, the future seems bright for accommodating high-heat flux applications.

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