Electronics Cooling
INTRODUCTION:
Recent development in semiconductor and other mini- and micro-scale electronic technologies and continued miniaturization have led to very high increase in power density for high-performance chips. Although impressive progress has been made during the past decades, there remain serious technical challenges in thermal management and control of electronics devices or microprocessors. The two main challenges are: adequate removal of ever-increasing heat flux and highly non-uniform power dissipation. According to a report of the International Electronics Manufacturing Initiative (INEMI) Technology Roadmap, the maximum projected power dissipation from high-performance microprocessor chips will reach about 360 W by 2020. In fact, the micro- and power-electronics industries are facing the challenge of removing very high heat flux of around 300 W/cm2 while maintaining the temperature below 85°C. Furthermore, due to increasing integration of devices, the power dissipation on the chip or device is getting highly non-uniform, as a peak chip heat flux can be several times that of the surrounding area.
Conventional cooling approaches are increasingly falling apart to deal with the high cooling demand and thermal management challenges of emerging electronic devices. Thus, high performance chips or devices need innovative techniques, mechanisms, and coolants with high heat transfer capability to enhance the heat removal rate to maintain their normal operating temperature. Unless they are cooled properly, their normal performance and longevity can deteriorate faster than expected. In addition, the failure rate of electronic equipment increases with increasing operating temperature. Reviews and analyses on research and advancement of conventional and emerging cooling technologies reveal that microchannel-based forced convection and phase-change cooling (liquid) are among the most promising techniques that can achieve very high heat removal rates.
CLASSIFICATION OF COOLING TECHNIQUES:
In general, thermal management is categorized into active cooling techniques and passive cooling techniques. Mechanically assisted cooling subsystems provide active cooling. Active cooling technique offer high cooling capacity. They allow temperature control that can cool below ambient temperatures. In most cases active cooling techniques eliminate the use of cooling fans or they require less cooling. The passive cooling subsystems are not assisted by mechanical equipment’s. The conventional passive cooling techniques include applying effective heat spreaders and heat sinks to the electronic package.
All the methods are classified into four broad categories in order of increasing heat transfer effectiveness, for the temperature difference between the surfaces and the ambient is 80 °C and compared the methods as shown in fig. 2:
– radiation and natural convection (155–1550 W/m2),
– forced air-cooling (800–16000 W/m2),
– forced liquid cooling (11000–930000 W/m2), and
– liquid evaporation (15500–1400000 W/m2).
COOLING METHODS:
1) Air cooling
Air cooling is the simplest and principal method of thermal control most widely used for variety of electronic systems ranging form portable electronics to large business systems. The advantages of air cooling are its ready availability and ease of application. Before 1964, all IBM computers were cooled solely by forced air. In many cases air moving devices are in stalled at the bottom or top of a column of boards to provide sufficient cooling. For high heat flux, a push-pull air flow arrangement with air moving devices at both the bottom and top of the column of boards was used to provide high pressure drop capability. Low-power electronic systems are conveniently cooled by natural convection and radiation. When natural convection is not adequate, the forced convection is adopted by a fan or blower to blow the air through the enclosure that houses the electronic components.
A. Natural convection and radiation
Natural convection and radiation cooling is desirable because of its simplicity. Circuit boards that dissipate up to about 5 W of power can be cooled effectively by natural convection. It is familiar in consumer electronics like TV, VCD, etc. by providing enough vents on the case to enable the cooled air to enter and the heated air to leave the case freely.
B. Forced convection
When natural convection cooling is not adequate, forced convection is provided by external means such as a fan, a pump, a jet of air, etc. In electronic systems cooling, fan is a popular means of circulating air over hot sur faces. For forced convection the hot surfaces are characterized by their extended surfaces such as fins in heat sinks. The use of micro jet of air to cool hot spots is more attractive.
C. Enhancement with heat sinks
In many instances, thermal enhancement techniques such as heat sinks is required to cool high density microelectronic packages found in modern circuit boards. It increases the effective surface area for heat transfer and lower thermal resistance between source and sink. Heat sinks can be operated under free or forced convective modes depending on the cooling load requirement. Due to their inherent simplicity, reliability and low long term costs, natural convection heat sinks have proven to be instrumental in cooling single or multiple chip circuit boards.
LIQUID COOLING
Because of high heat transfer coefficients with liquids than gases, liquid cooling is far more effective than gas cooling for high power electronic collections. The potential problems such as leakage corrosion, extra weight and condensation makes liquid cooling reserved for applications involving power densities that are too high for safe dissipation by air cooling. The electronic components are indirect contact with the liquid, therefore the heat generated in the components is transferred directly to the liquid. The electronic components are usually completely immersed in the dielectric fluid. Liquid cooling is classified as direct and indirect cooling.
A. Direct liquid immersion cooling
The electronic components are completely immersed in the dielectric fluid. Such cooling involves the pool boiling of a working fluid on a heated surface, which is an example of a two-phase cooling technology used in microelectronic applications. It is a highly effective cooling strategy for the following reasons:
a. The phase change, liquid to vapor greatly increases the heat flux from heated surface, and;
b. The high thermal conductivity of the liquid medium enhances the accompanying convection.
A prominent cooling scheme for microelectronic devices is immersion cooling with dielectric fluids. The dielectric fluids used for immersion cooling are a refrigerant-type fluid that has a moderated boiling point, such as R-113. R-113 is used for power electronic devices; however it is not compatible for computer because of probable long term corrosion.
B. Enhancement techniques for boiling heat transfer
Realizing the importance of enhancement of boiling heat transfer from electronic components many have investigated the use of surface micro structures that were fabricated directly on a silicon chip or a simulated chip. These include sintered or flame sprayed porous coatings , laser drilled cavities , a sand blasted and KOH treated surface ,a dendritic heat surfaces , hexagonal dimples fabricated by photo-etching ,re-entrant cavities, porous surfaces fabricated by alumina particle spraying and painting of silver flashes , diamond particles and aluminium and cop per particles and micro pin fins produced by dry etching . Heat sink studs with drilled holes, micro fins, multi layered micro channels and pores, and pin fins with and with out micro porous coating have been developed and tested. These includes a vapor blasted surface, drilled cavities, micro fins, micro studs and microgrooves ,multi layered microchannels and pores, pin fins, pyramid and square studs with microgrooves, cylindrical pin fins and single cylindrical stud with low pro file micro structures and square pin fins with and with out painted aluminium particles.
C. Jet Impingement
One of the successful methods to remove high heat flux dissipated by the electronic components is jet impingement. A jet of liquid with high heat transfer coefficients is directed at the heat source to cool the sur face, and large heat transfer rates will occur at or near the stagnation point and drop off further away. Depending on the surface temperature of the component and working fluid, the jet impingement can be single phase or two phase heat transfer. The jet is also classified into free surface and submerged. The jet flows within the same fluid in the same state (i. e., gas into gas or liquid to liquid) means sub merged and free sur face, which means that the liquid jet is injected into a gaseous environment.
D. Spray Cooling
Spray cooling can be implemented by means of liquid jets or liquid drop lets. In spray cooling, the cooling agent is injected through nozzles or orifice onto the electronic module as shown in fig.12. The pressure drop across the nozzle or orifice forms the spray that impinges on the surface and forms a thin liquid film. The heat dissipated from the equipment initiates boiling, which leads to evaporation of the cooling agent. The constant impingement by the spray forces convection of the cooling agent and contributes there by the cooling of equipment. The hot liquid and vapor cools in the container returns to reservoir through a drain to repeat the cycle.
Spray cooling is attractive for the following reasons:
1. direct spraying on the heat source eliminates the thermal resistance present in the bonding layer used for attaching heat source to the heat spreader, and the ratio between power spent for cooling process and the heat removed decreases faster for spray cooling than channel cooling.
2. The spray cooling is one of the most promising high heat flux method.
3. The key requirements for the liquid used for spray cooling of electronic components is that it must be non-conducting or a dielectric liquid.
4. Water is employed frequently as a cooling agent.
5. A thin protective layer is coated onto the electronic components to protect against short circuits because water has a very low dielectric strength.
This limitation results for an alternate candidate to provide more effective cooling.
CONCLUSION:
Advances in electronics and semiconductor technologies have led to a dramatic increase in heat flux density for high-performance chips and components, whereas conventional cooling techniques and coolants are increasingly falling short in meeting the ever-increasing cooling need of such high heat-generating electronic devices or microprocessors. Despite good progress been made during the past decades, there remain some serious technical challenges in thermal management and cooling of these electronics. High-performance chips and devices need innovative mechanisms, techniques, and coolants with high heat transfer capability to enhance the cooling rate for their normal performance and longevity. With superior thermal properties and cooling features, nanofluids offer great promises to be used as coolants for high-tech electronic devices and industries. The emerging techniques like microchannels with these new fluids can be the next-generation cooling technologies.
REFERENCES:
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CONTRIBUTERS:
Shraddha Paithanpagare, Rucha Pansare, Maitreya Patankar, Gaurav Pathak, Kaustubh Patil.
