The theaters of operation for marine, air and land vehicles continue to redefine the term “rugged environment.” No longer bound by on-board human operators, autonomous and unmanned military vehicles are now free to operate within unparalleled temperature, vibration and shock regimes. The limitation on survivability of such vehicles is defined primarily by the robustness of their internal electronics. To meet these challenges, suppliers of hardened electronic systems must employ various ruggedization techniques.
To appreciate ruggedization innovations moving forward, it is important to note some prior advances. In the early 90s, the IEEE organization standardized the conduction-cooled Eurocard, which was then applied to the VME-bus standard. The concept of the mechanically-interchangeable conduction-cooled card continued to be utilized and applied to newer backplane bus standards including CompactPCI (cPCI) and VPX. But the mechanical interchangeability standard did more than ensure proper fit of COTS circuit cards within a chassis; it provided a valuable boundary separation between the chassis and the plug-in circuit card. This eventually led to the establishment of ruggedization levels — boundary conditions that allow the decoupling of ruggedization efforts required for the chassis and the circuit cards. Given the card-to-chassis boundary conditions, the enclosure designer is given the minimum performance that the chassis must deliver for the circuit cards, and the circuit card designer is given the minimum environment within which the circuit card must survive.
Given that the boundary between the chassis and circuit cards is well defined, from a vibration ruggedization perspective it is necessary to maintain that separation with respect to mutual interaction of the enclosure and the circuit cards housed within it. With respect to shock and vibration ruggedization, this ultimately means that the chassis should not amplify the input shock and vibration levels at frequencies critical to the enclosed circuit card assemblies.
The solution to this is what is often referred to as the 1-octave rule, which defines that, for the chassis, the natural frequency of the mode with greatest mass participation be no closer than one octave from the natural frequency of any internal circuit card. Considering that a typical ruggedized 6U VME circuit card assembly resonates between 200Hz and 450Hz, and a 3U card can resonate at frequencies greater than 1000Hz, the stiffening required of the chassis assembly to satisfy this rule is substantial.
Stiffening ribs and other stiffening geometries are typically machined or cast into the walls of the chassis to reduce deflection while reducing weight, and ultimately increase the enclosure’s resonant frequency. For enclosures that are assembled from separate machined walls, dip brazing can be used to join the chassis walls in an almost seamless fashion. However, in cases where intricate chassis wall geometry does not permit the use of dip brazing because of the possibility of salt entrapment and potential latent corrosion, conductive epoxy is used at the wall-to-wall interfaces to provide a sturdy, permanent assembly. Vacuum brazing can also be utilized to create laminated honeycomb-like wall assemblies, which are then bonded together using a variety of readily available adhesives.
In addition to stiffening geometries and construction methods, the use of high strength, lightweight materials is necessary to reduce the inertial response of the chassis walls to the vibration imparted on the enclosure from the vehicle. This is also beneficial in reducing overall enclosure weight as part of a larger supplier initiative to maintain size, weight and power (SWaP).
However, weight reduction must not ultimately affect the chassis’ ability to transfer and discard heat from the circuit cards. Regardless of the overall heat rejection mechanism used to expel heat from the enclosure, the chassis must rely on thermal conduction to move heat from the circuit cards.
In general, maximizing conduction requires added material volume or optimization of thermal conductivity through careful selection of materials. For this reason, aluminum alloys are generally regarded as the material of choice for ruggedized enclosures, having a weight three times lighter than copper and a thermal conductivity 20 times that of titanium alloys. This allows for increased wall thicknesses and improved thermal conductivity without significant weight increases. Additional advances in blended composites are being developed to provide enhanced thermal and structural properties within a single amalgamated raw material that will ultimately provide thermal conductivity greater than copper with a weight less than that of aluminum.
Chassis Heat Rejection Methods
Added to the careful juggling act of managing weight, thermal conductivity and stiffness is the need to ultimately expel heat from the chassis, either to the ambient environment or to some other supplied cooling media. Natural convection is always a desirable choice for the vehicle integrator as it provides passive cooling with no additional cooling systems needed from the vehicle. To allow for natural convection, cooling fins or pin fin arrangements are added to the external machined or cast surfaces of the chassis walls. These added fins increase the exposed surface area of the chassis which allows for a lower temperature rise from ambient air to the chassis for a given amount of heat. Careful placement and orientation of the fins is required to allow for greater thermal convection in areas of high heat concentration.
Noting that natural convection is driven by air density differential and thus inherently gravity-dependent, fins must be oriented such that convective air flows between the fin patterns in a vertical direction. For this reason it is essential to know beforehand the intended orientation of the ruggedized electronic system within the vehicle. Ultimately, the fin length and fin population densities are driven by the allowable temperature rise from ambient air to the chassis, as well as the ambient temperature that the electronic system will be subjected to, and the proximity of other obstructions mounted around the electronic system within the vehicle.
Forced-air convection is another cooling method utilized. Unlike natural convection, forced-air convection is not dependent on the orientation of the electronic system within the vehicle since the cooling airflow direction is driven by fans. Moreover, forced-air convection is inherently more efficient since the resulting convection is greatly increased by fan speed and hence the volumetric flow rate of air. In addition, the chassis design often incorporates an elaborate maze of fin patterns and cooling channels within the chassis walls to direct air to areas of the chassis where cooling is more critically needed.
Like forced-air convection, liquid cooling utilizes a labyrinth of cooling channels integrated into the chassis walls. The liquid-cooled channels are constructed using a laminate approach with machined fins or fin-inserts sandwiched between sheets of aluminum. The laminate construction is completed using aluminum vacuum brazing to form rigid wall assemblies, which are then bonded together to form an enclosure. The overall convective properties of the cooling channels can be controlled through the designed intricacy of the internal fin patterns, or through the use of various styles of fin-insert material which provide vary- ing degrees of thermal conductivity and directly proportional degrees of pressure drop.
Further innovations in chassis-to-circuit card heat transfer, including liquid flow-through circuit card assemblies, make use of available liquid coolant by directing cooling fluid through channels within an integral heatsink mounted on the circuit card assembly. This allows for greater circuit card power dissipations, greater component population densities and ultimately greater circuit card functionality. Overall, liquid cooling provides a very efficient means of exhausting heat from the electronic system but requires significant support from the vehicle in terms of the required electronic cooling system to supply coolant at a sufficient pressure, temperature and flow rate to overcome the overall pressure drop from the chassis cooling channels.
Circuit Card Ruggedization
Opposite the chassis along the inter-mechanical separation boundary is the plug-in circuit card which ultimately provides the electronic system functionality and is thus the internal heat source. Requirements for ruggedizing these plug-in cards are not unlike the requirements for ruggedization of the chassis, but the implementation of those ruggedization methods is significantly different.
Ruggedization of a plug-in circuit card assembly begins with careful and meticulous component placement. This is done for thermal management as well as shock and vibration hardening. For a typical VME, cPCI, or VPX circuit card assembly, the center zone of the circuit card represents the location of maximum board temperature and maximum board deflection under vibration and shock. During the component placement design activity, components of high wattage, large mass, and large footprint are identified. Since a plug-in circuit card assembly conducts its heat to the chassis through the circuit card thermal interface — the edges of the card where the circuit card assembly directly contacts the chassis — components with greater power dissipation and lower allowable junction temperatures must be placed closer to the card’s thermal interfaces.
At the same time, because the center of the circuit card represents the center of deflection for the printed circuit board under shock and vibration, it is essential to locate heavy components away from the center of the board in order to reduce inertial response to shock and vibration, and hence reduce overall board deflection. Likewise, since components with large footprints experience greater lead stresses when placed central to the circuit card — a result of overall board curvature during cyclical vibration response — those components ideally must be placed along the peripheral edges of the circuit card.
This creates a position-jockeying situation among a variety of components, which ideally must all be placed along the card edge closest to the circuit card thermal interface. The need to specifically locate high-wattage components along the card edge is then mitigated through the use of integral aluminum heat sinks, thermal pads, copper slugs, heat pipes, graphite heat spreaders, and even internal copper planes within the circuit board, all of which transfer heat from high wattage components to the circuit card thermal interface. In doing so, the location of high wattage components becomes less critical and can therefore be removed from the list of components vying for position along the card edge.
Stiffening frames secured to the center of the circuit card are also used to greatly reduce overall board deflection at the center of the circuit card. By doing so, larger and more-massive components can be centrally located on the circuit board without compromising component lead integrity under vibration and shock. Typically, the circuit card stiffening frame geometry is integrated with the needed heatsink material to form a single cover-like aluminum structure that provides both deflection resistance and heatsinking capabilities.
Overall, ruggedization of electronic systems for military use focuses on thermal management, reducing deflection of internal components, and minimizing resonance coupling between circuit cards and the enclosure assembly. Particular attention is placed on reducing component temperature rise above ambient so as to allow for greater functionality with higher power densities. Using these ruggedization techniques, electronic systems suppliers are able to improve overall system SWaP by packaging denser, more complex circuitries within smaller volumes while maintaining components within nominal working parameters, despite the severity of the environment in which they operate.