Getting Heat Out
By Darryl McKenney, VP, Engineering Services, Mercury Systems, Inc.
Mercury Systems is a publicly listed company based in Chelmsford, MA, USA and is a leading supplier of commercially developed, open sensor and Big Data processing systems for critical commercial, defense and intelligence applications. We design and build end-to-end, open-sensor processing subsystems. Our product set spans the entire ISR (Intelligence, Surveillance, and Reconnaissance) sensor processing chain, from acquisition to dissemination, helping customers address a broad range of sensor processing. Mercury Systems have worked on over 300 programs, including Aegis, Patriot, SEWIP, Gorgon Stare and Predator/Reaper.
If we examine typical military electronics CPU Modules and Mezzanines over the last few decades (Figure 1), what is very clear is that their Power levels have increased dramatically. The VPX, formerly known as VITA 46, is an ANSI standard (ANSI/VITA 46.0-2007) that provides VMEbus-based (Versa Module Europa bus) systems for CPUs with support for switched fabrics over a new high speed connector. It was defined by the VITA (VME International Trade Association) working group, that includes Mercury Systems, and it has been designed specifically with defense applications in mind, with an enhanced module standard that enables applications and platforms with superior performance. Basically, all CPU boxes in ISR applications must comply with this standard and its successor VITA 48.
We are finding that devices such as microprocessors and FPGAs (field-programmable gate arrays) have been running ever faster while their size has been constantly shrinking, which obviously has increased heat densities and threatened product reliability. But after nearly a decade of honing our Design for Reliability (DfR) thrust we have produced new design processes and implemented new procedures such that we have reduced the number of engineered change orders by over an order of magnitude. This demand for higher and higher functionalities in defense electronics has led to conflicting demands for more heat management, more sensitive signals, shorter design cycles, higher test coverage and all within ever tighter defense budgets. To add to all this, our products have to be highly reliable with years of operational run time in a wide range of harsh environments. You can imagine the challenges this poses for test engineers, signal-integrity engineers and mechanical engineers when it comes to designing new PCBs and enclosures. Many of today’s high powered modules cannot be cooled using legacy cooling approaches. The bottomline is that in our business heat is the primary enemy of module reliability.
At Mercury we offer three types of products to our customers, Air-cooled (A/C) Modules, Conduction-cooled (C/C) Modules and what we call Air Flow-By™ (AFB) Modules. In all cases we go through detailed design and testing processes to design the units for our customers. Our evaluation of each technique’s cooling efficiency is highlighted in Figure 2. For our thermal CFD simulation needs we use Mentor Graphics’ FloTHERM product which helps expedite our design process.
Air-cooling provides easy access to module debug connectors, front panel I/O and mezzanine modules. This combination simplifies system development and configurability while the system is in its greatest state of flux and requirements are not all identified. A major drawback is that air-cooled modules are not typically designed to be deployed in rugged environments. Conduction-cooling has been the preferred method of cooling for deployed systems for many years.
The modules are designed to handle the rugged shock and vibration levels, while the systems seal the modules away from harmful elements. A major challenge with conduction cooling is that it is heavier than air-cooled and thermally challenged with higher power modules. Air Flow-By – a new cooling technology designed by Mercury – delivers the best of both worlds. It provides the efficient point source cooling of an air-cooled module with the rugged deployment capabilities of conduction-cooling.
To give a simple example of how we apply FloTHERM to one of our XMC-Air-Cooled products (Figure 3), we employed a standards based approach to bring heat from the mezzanine modules to the carrier module’s heatsink. We discovered via CFD simulation (Figure 4) that we could do this by adding “hooks” for a thermal bridge between the carrier module heatsink and the mezzanine module heatsink. The net effect was a thermal solution that was compliant to standards and allowed for a wide range of mezzanine modules to be placed on a host while limiting any potential changes to a single component. We discovered with FloTHERM that we could get a 5°C Processor thermal reduction - half an Order of Magnitude. This leads to significant impact on mean time between failures (MTBF) too.
In summary, our new thermal-management solutions are capable of dissipating tremendous amounts of thermal energy, while still meeting the same or smaller size, weight and power requirements for the overall solution. By understanding the thermal profile for each specific component that makes up a system using FloTHERM, we created innovations in the mass transfer of thermal energy that work at the individual component, module and subsystem level.
- “We Need More Power Scotty! Getting the Heat Out: Innovations for Cooling the Next Generation of Embedded Computing Electronics” by Dan Coolidge & Darryl McKenney, Embedded Technology Trends, Long Beach, CA, January 21-23, 2013
- December 2012, “Mercury Computer Systems Announces Cold Plate Technology for Embedded and OpenVPX Technologies”: http://www.coolingzone.com/index.php?read=162&onmag=true&type=press#sthash.wssfI7Hp.dpuf
- Test & Measurement World, Nov 2008, “Quality by Negotiation”, pp32-35
Air Flow-By is a trademark of Mercury Systems, Inc.