Keith Reynolds, Technical/Marketing Manager for Schroff UK, explores the mechanical and thermal design of the latest enclosure systems for rugged applications
Developed by VITA as a successor to VMEbus, the VPX specification features a particularly robust high-speed connector, together with an extremely solid guide unit between the plug-in board and backplane. VPX systems are therefore well suited to the task of protecting sensitive electronics from shock and vibration and have been adopted by users in the military, aerospace and railway sectors.
However, for systems aimed at the toughest environments, issues such as thermal management, structural integrity and two-level maintenance compatibility still needed to be addressed and, to this end, the VPX REDI (Rugged Enhanced Design Implementation) specification was developed.
In addition to the VPX REDI base specification (VITA 48.0), there are also several ‘dot’ specifications. VITA 48.2, for example, defines the mechanical specifications of conduction-cooled VPX REDI microcomputers.
For Schroff this was the starting point for developing a new series of VPX REDI systems, which consist of a modular case with a suitable backplane, an upgradable cooling solution, power supply and other accessories such as clamshells and PCB retainers.
The modular design allows users not only to configure a system to meet their exact needs but also to subsequently expand or upgrade the system if those needs change.
As light weight and robustness are often prerequisites of equipment for aerospace and defence applications, the case of this system is formed from machined aluminium-alloy parts.
The base, top cover, side, front and rear elements are all bolted together, and a variety of finishes can be applied, including black anodised, nickel-plated and yellow chromated.
Pre-drilled holes in the side panels, top cover and base allow other optional parts such as mounting -brackets to be attached.
The front and rear elements are ¬symmetrically designed, with identical hole positions and bolt-on dimensions. Basic configurations of the case contain no gasketing, but where requirements dictate, either a pure IP seal (to protect against the ingress of moisture and dust) or a combined IP and EMC seal can be fitted in a groove provided for this purpose.
The VPX specification allows for both 3U and 6U implementations, using the familiar single and double Eurocard formats of 100 x 160mm and 233 x 160mm.
Guides for the PCBs are milled into the base and top covers of the case, and here VPX defines three possible pitch increments of 0.80″, 0.85″ or 1.00″ for the separation between boards.
Thermal management
In a basic system, heat is drawn from the PCBs via a simple board frame or a closed clamshell to the surface of the case, where it is cooled by convection alone. To improve the system’s thermal performance, heat sinks can be attached to one or more sides of the case exterior. The ribs of the heat sink serve to increase the overall surface area of the system and therefore boost the heat dissipation from it.
Another option is to place a sheet-metal cover over the heat sinks and install a fan with suction chamber at the rear of the case.
The cover creates an air channel, with air being sucked by the fan through the heat sinks and drawn to the rear. This forced air cooling further increases the heat dissipation capacity of the system.
Even better thermal performance can be achieved by replacing the heat sinks with liquid-cooled plates.
The cooling liquid draws the heat from the case surface and is pumped to, for example, a chiller unit.
All the enhanced cooling elements – heat sinks, cover and liquid-cooled plates – are attached to the case using the same fixing points.
Particularly during design and development phases, the precise cooling requirement may not yet be known. However, with this modular approach to thermal management, the cooling capacity of the system can be upgraded or adapted as required ¬without difficulty.
An important aspect of the system design is the use of accessories such as clamshells (pictured) and PCB retainers (e.g. Wedge-Loks or Card-Loks).
These devices help to protect boards from the effects of shock and vibration, as well as drawing away the heat ¬generated on the boards.
Clamshells are available with standard covers and can also be custom machined to match the contours of the board topography, allowing heat to be removed from particular hot spots with the aid of thermal gap filling materials.
Fitted with an insertion/extractor handle and PCB retainers at the top and bottom, clamshells can be specified in 3U or 6U heights and in widths of 0.80″, 0.85″ or 1″ corresponding to the guides in the case.
Another requirement of the VITA REDI specifications is compatibility with the two-level maintenance system, which is employed, for example, by the US military. Under this system, it must be possible for the operator to replace failed PCBs in the field, under difficult conditions, ¬without the risk of damaging anything.
This procedure is greatly facilitated by the use of torque-limiting PCB retainers such as the Series 223 and 224
Card-Loks from Schroff’s sister company Calmark.
These devices allow boards to be fitted into the chassis with a ¬prescribed clamping force, and without the need for a torque wrench. The devices are designed such that no damage can be done to the board or to the Card-Lok itself by excessive ¬tightening.
