Electronics Protection - June/July 2011 - (Page 12)

Feature Military’s Demands Drive Innovative Thermal Management Solutions Dave Turner, Sr. Mechanical Engineer, Parvus Corp. Jared Francom, Lead Project Engineer, Parvus Corp. As military programs continue to push the limits of computing system requirements, rugged computer systems must continue to evolve to endure the toughest conditions. To meet these stringent requirements, rugged subsystems suppliers must produce innovative thermal management technologies that can withstand situations that may threaten a computer’s durability. Thanks to ever-increasing processor speeds, advances in thermal management will continue to rank as one of the most important trends in rugged computing design. This article will discuss existing thermal management techniques that are helping military electronics sustain rugged conditions and innovative solutions employed by engineers to meet the military’s stringent requirements. blocking air movement. This build up of air effectively reduces the useful dissipative surface area of a heat sink. The turbulence of the air affects the thickness of the boundary layer and therefore the rate of heat transfer. For example, the more air molecules that contact the surface of the heat sink, the greater the dissipative effect. The boundary layers of rugged devices need to be carefully evaluated as it is not uncommon for embedded systems to be installed in a stagnant air environment. As such, boundary layers can greatly influence the effectiveness of heat sinks and other convection cooling methods. Derating Ensures Optimal Performance Rugged Design Relies on Thermal Management With heat issues often credited as the largest contributor to system failures, designing rugged systems to meet these thermal challenges is critical. The key to successful thermal design is getting heat into contact with ambient air for convection to the external environment as quickly as possible. By implementing a variety of thermal management techniques, heat can be dissipated fast enough to prevent thermal runaway, an increase in temperature that changes the conditions in such a way that causes exponential temperature increases, ultimately leading to destruction. Rugged system designs often must specify extremely rugged components that have warranties guaranteeing performance up to 85°C. To ensure optimal performance for these components, derating can be employed. A technique where electronic devices are operated at less than their rated maximum power dissipation, derating takes into account the case/body temperature, the ambient temperature and the type of cooling mechanism used. Derating increases the margin of safety between part design limits and applied stresses, thereby providing extra protection for the part. By applying derating for an electrical or electronic component, its degradation rate is reduced and the reliability and life expectancy are improved. Managing Cold Temperatures Conduction Cooling is Key to Thermal Management Relying on convection cooling inside a system has its limits as heat is dissipated rather slowly (high thermal resistance) to the surface of the chassis where it can be dissipated off the surface of the chassis. However, advancements in conduction cooling have had a tremendous impact on rugged system design. For example, custom-designed clamshell heat sinks can be fitted to encapsulate each printed circuit card assembly, using wedge locks as the contact point with the card stack to dissipate heat more quickly (lower thermal resistance) to the chassis. The clamshells, which essentially work as heat spreaders, and the thermally conductive gap pad significantly reduce thermal issues. Heat spreaders are also being designed to accommodate a number of thermal options, such as top-mounted heat sinks, fan heat sinks and heat pipes to effectively cool microprocessors. Innovative heat pipe/heat spreader combinations are proving especially effective in the thermal management of standalone rugged boxes (Figure 1). Figure 1. Aluminum heat spreader plates When complete encapsulate boards in this tactical switch convection cooling is router to optimize conduction cooling to the chassis. not possible, or when designing the cooling properties of system enclosures, thermal designs must consider the thermal boundary layer. The thermal boundary layer is a layer of warm stagnant air that builds up between the fins of heat sinks 12 Although thermal management mainly surrounds the task of moving heat to the outer chassis, when designing for rugged conditions, managing cold temperatures must also be considered. Cold temperatures can cause a variety of problems with electronic devices, including problems with voltage sags and clock frequencies, creating timing difficulties. Descending to cold temperatures also causes rapid contractions, which can cause problems with powering on a device. To help mitigate these problems, screening each electrical component to ensure it is rated to operate from -40°C to 85°C is imperative. Parvus also takes the additional step to screen each CPU board before it is installed. Thermal Management Solution for Electronic Warfare Aircraft For over a decade, Parvus has supplied the US military with rugged cockpit computer subsystems for an electronic warfare aircraft program. In its latest follow-on project, Parvus developed a multi-mission computer to support both electronic warfare and laser targeting. To provide enhanced thermal ruggedization for the computer, three thermal envelopes had to be managed. This included the stagnant air inside the main subsystem chassis, the air between the main chassis and the Air Transport Rack (ATR) aluminum chassis and the ambient air. To ensure heat could rapidly transition these envelopes, Parvus engineers carefully managed the selection of rugged components, implemented conduction cooling where possible, plus provided components with the most heat direct conduction links to ambient air. For example, the main CPU has a direct heat pipe link to a heat sink exposed to ambient air. The main thermal management goal in this deployment was to keep the thermal resistance of each heat source to ambient air as low as possible and practical. June/July 2011 www.ElectronicsProtectionMagazine.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - June/July 2011

New Material Adapts to Strain
XPAL Power Debuts PowerSkin Battery-Boosting Smartphone Cases
Ruggedizing Enclosures: From Military to Seismic to Mobile Applications
The Benefits of Using Thermal Simulation Software in Electronics Design
Designers Face a Potentially Confusing Assortment of Surge Protection Device Choices
Military’s Demands Drive Innovative Thermal Management Solutions
Integra Enclosures Releases new Product to Genesis Line of Enclosures
New LCR COTS Filters Perform in Extended Temperature Range for Added Reliability
Thermacore Introduces VME64x/VPX Cold Plates for Military Cooling Applications
Laird Technologies Expands Thermal Gap Filler Line
ASI New Line of 13mm MCB’s offer UL 1077 Supplemental Protection
New Deutsch HDP Series Electrical Connector
Industry News
Calendar of Events

Electronics Protection - June/July 2011