Electronics Protection - Summer 2014 - (Page 14)

Feature How Thermal Ground Plane and Compact Air-Cooled Heat Sinks are Revolutionizing Thermal Management Nelson J. Gernert, Vice President, Engineering and Technology Thermacore Electronic devices used in military and aerospace equipment are subject to temperatures ranging from -40°C to 100°C and in radar applications, even as high as 150°C. These temperatures tend to drift higher as designers increase device density by putting more processing power inside a smaller package. As a result, device designers are looking for advanced, cost-effective thermal management and heat rejection technologies that can improve performance while reducing size and weight. Two recently developed technologies, a planar, two-phase heat pipe known as a Thermal Ground Plane (TGP) and compact, high performance air-cooled heat sink, that can be combined in a compact assembly to overcome the constraints encountered with conventional thermal management methods. Designing New Devices for More Power and Control Increases in power and control have revolutionized the level of performance obtainable in semiconductor devices such as amplifiers, lasers, microprocessors, graphics processors, thyristors, insulated gate bipolar transistors (IGBTs), symmetric gate commutated turnoff thyristors, silicon controlled rectifiers (SCRs) and intelligent power modules (IPMs). But the most demanding arena for designers in modern electronics is the world of military applications, where electronics must not only operate flawlessly in harsh environments, but also meet demanding size, weight and power (SWaP) constraints in current and future generation equipment. For example, naval onboard power conversion systems and aerospace power electronics offer a variety of harsh thermal and environmental challenges: rugged environments involving exposure to salt water, heavy weather, extreme temperatures and vibration; extremely high power loads (sometimes in excess of 3,000 W) and high heat fluxes due to concentrations of electronic components. The technical challenge, however, is that increases in power and decreases in size also require designers to deal with tremendous increases in heat generation and heat fluxes, all within the context of smaller and smaller areas for thermal solutions. Thus, the combination of smaller footprint (thanks to increased miniaturization) and greater power makes life much more difficult for thermal engineers who need to develop solutions that protect the life and performance of these high-performance devices. Of course, the challenge of increased heat loads doesn't involve just the amount of space available for thermal solutions. The configuration and the location of powerful electronics systems also are factors that make solutions more difficult. As a result, design engineers need to consider the following thermal solution constraints: * Limited space availability due to packaging and other space competing electronic components nearby * Operation in oddly shaped or situated spaces * Near maintenance-free operation * Thermal solutions that operate independently of gravity * Resistance to shock & vibration and environmental corrosion * Quiet operation * Avoiding drawbacks associated with liquid-based cooling (pipes, storage of chemicals, short circuit potential, ongoing or continuous maintenance is required, high cost, etc.) 14 Summer 2014 * www.ElectronicsProtectionMagazine.com Conventional Materials Face Thermal Management Challenges Cooling of electronics has traditionally been performed using air-cooled (forced or natural convection) metallic conductors in the form of extruded heat sinks and heat spreaders. Typically, aluminum or copper extrusions are bonded to the electronics case and air is blown over the extrusion using a fan or blower. Passive two-phase heat transfer devices such as heat pipes are another thermal management technology often used in military and aerospace applications because of their thermal efficiency, simplicity and reliability. Heat pipes employ three components: a vacuum-tight containment shell or vessel, a working fluid, and a capillary sintered-powder metal wick structure. Without moving parts, the wick generates a capillary action that circulates a heated liquid from the evaporator side to the condenser side of the heat pipe where heat is ejected. Although heat pipes provide effective heat removal over long distances and are bendable, flexible, and withstand high-g conditions, shock/vibration and freeze/thaw cycles, conventional heat pipes also have thermal transport and geometric limitations. Another challenge for conventional heat pipes is harmonizing the CTE between the heat source material and the heat sink material. If there are disparities in CTE between materials, then temperature changes will cause the devices to expand and contract, which create mechanical stresses on the interface bond that will eventually induce thermal failure. Using a solid conductor at the interface, like diamond, can improve thermal conductivity and CTE matching, but it is very expensive. Another cooling challenge for heat sinks is achieving adequate air side heat transfer directly at the electronic component level. Factors that limit the effectiveness of air as a cooling medium include: lack of available volume for the heat sink, required surface area is too high and the pressure drop is too large. The alternative to air cooling is a pumped-liquid or refrigerantbased cooling system. This solution creates its own set of challenges by adding complexity, weight and the potential for failure, a major issue where heat sink maintenance is difficult or impossible. Adding components such as pumps, compressors, nozzles and other components can also significantly increase cost. TGP and Compact Air-Cooled Heat Sinks: Extending The Benefits of Passive Thermal Technology To overcome these challenges, a new form of heat pipe, known as a Thermal Ground Plane (TGP), was developed employing a planar geometry. The heat-pipe TGP, uses the same reliable components and sintered powder metal wick struc- Figure 1. Thermal Ground Plane A planar heat pipe in 30 by 30 by ture of a regular heat pipe. It is a 3 mm thick dimensions. passive heat transfer device using a two-phase cooling approach. But instead of a cylindrical shape, the form is a thin, planar structure known as a vapor chamber, sometimes referred to as a flat heat pipe, that makes an ideal substrate for mounting electronic de- http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Summer 2014

Editor's Choice
New High Efficiency LED Technology Benefits From Best Practice Thermal Management Design
Passive Thermal Management of Lithium-ion Batteries Using Phase Change Materials
Considerations for Powering Military Applications
How Thermal Ground Plane and Compact Air-Cooled Heat Sinks are Revolutionizing Thermal Management
Access Control Solutions for Railway Infrastructure
20 Data Center Downtime Study Puts Focus on Maximum Protection
Cooling and Shielding in the Right-Sized Enclosure
Enclosures
Thermal
Power
Hardware
EMI/EMC/ESD
Industry News
Calendar of Events

Electronics Protection - Summer 2014

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