Aerospace & Defence
High-tech packaging under extreme conditions
Electronics in aviation applications are subjected to extreme stresses and strains. A flying altitude of over 16,000m, speeds of more than 800km/h and extreme temperatures, put electronic packaging to the ultimate test. Not only absolute reliability, but also economically viable solutions in compliance with standards are called for. In this article Ahmet Tasseki and Maximilian Schober from the Polyrack Tech Group explain more
T
he ATR (air transportation rack) has become established as the standard for electronic equipment on board
aircraft. Officially known as ARINC 404, it defines the equipment dimensions used in avionics installations. Thus ATR cases offer, for example, standardised options for mounting the application and for integrating hardware and cooling. Thanks to their modular design in the basic structures ½, ¾, 1 and 1½ ATR, these can be cost effectively configured for many different applications. Depending on where they are used, the cases are exposed to harsh mechanical, climatic, chemical and electrical conditions. They need to be able to withstand dynamic stress and cope with shock and vibrations of up to 40g. Extreme temperatures, high altitudes, moisture, sand and dust present additional challenges.
Conduction/convection versus forced cooling
Factors such as working temperature range and operating temperature as well as maximum system performance play a decisive role in the deployment of an ATR
system in aerospace. They determine which type of cooling is used. As a general rule, ATR chassis are designed for operating temperatures within the range of -55°C to 85°C. The ambient temperature lies at around 50°C. If the power dissipation in the system is less than 120W, the use of a passively cooled system is appropriate. For optimised heat dissipation, manufacturers use conduction cooling. All heat sources i.e. the so-called hot spots, make contact with the clam shell. With this, the heat from the individual sources, such as the CPU, is transferred to the clam shell and then passed on to the case via the wedge locks. This way the heat is transported to the case surface, which is equipped with cooling fins, and conducted outwards. This is possible even when the case construction is completely airtight (IP67 and higher). This protects both the system boards and the power supply and other system components from environmental influences.
Heat dissipation in systems with a higher power rating is generally accomplished with additional forced cooling. With this type of cooling, the cooling fins are covered with an outer plate. Ventilation channels are thus formed between the outer plates and the actual case. The ventilation is generally installed at the rear of the ATR chassis. The relatively cold air is sucked in via the air inlet vents and transported through the ventilation channels. Here, cooling is accomplished by means of conduction and forced cooling. This method requires fans that are suitable for use at high altitudes, which deliver the required performance under special pressure conditions. At the same time, the ventilation device must be suited to the prevailing operating temperatures and
18 July/August 2015 Components in Electronics
atmospheric moisture. For applications in aircraft that operate in dry, dusty regions, it also makes sense to utilise a washable, electrostatic high performance air filter to deter dust and dirt.
Conduction cooling turns up again in the so-called ‘ruggedised’ cases. The aluminium conductor forms a stable mechanical reinforcement to which the board can be affixed at several points.
Protection against electromagnetic interference Aluminium constructions have an additional benefit - they offer inherent protection against electromagnetic radiation and permit an open grounding concept across the conductive surface. Electromagnetic compatibility (EMC) plays an important role for the system integrity in aircraft and must be incorporated into the design of the case right from component level. To ensure high EMC, the system should be constructed hermetically, e.g. by using dip soldering. Removable parts require an EMC/IP combi-gasket in order to prevent both interference from external devices and the emission of interfering signals. Within the system itself, interference between the various boards can be prevented by mechanically segregating the signal area and the power area as far as is possible.
Shock and vibration
Packaging systems for use in aviation must on principle be so designed that they can withstand both types of vibration - shock and vibration. Each of these has a differing impact on the components - up to 100g in MIL or naval systems. As a consequence card and connector contacts can be subjected to a high level of stress and contact surfaces can be damaged. To counteract this, the type of construction, the choice of material and components as well as the application and the installation mode of the system are extremely important criteria in the design of the packaging. Vibration, on the other hand, is defined as continuous oscillation with a variable force along one or more axes - as it occurs in helicopters. As a result, the components
or materials used can begin to resonate, components can break or be damaged mechanically and screws can loosen with time and become undone. Precautionary measures are needed to
protect the system from the consequences of shock and vibration. That is why, in the construction of its ATR Chassis, POLYRACK has interconnected the individual parts using dip soldering. This guarantees stability, avoids screw connections and at the same time enhances the heat transfer characteristics. A further option for damping shock and vibration is the use of passive isolators. These consist of a spring which absorbs the shock or vibration and a damping element to dissipate the energy.
Standards compliant The rugged MIL ½ Short ATR Chassis from POLYRACK fulfils all requirements with regard to robustness, thermal management, weight and EMC/ESD protection. In the development and manufacturing of the mechanics, the engineers at POLYRACK deployed new materials e.g. aluminium 606 - T651 and other certified materials. These enable lightweight construction, optimal heat dissipation, shock and vibration protection and the ability to function at high altitudes. The standard model is available in 13 sizes and versions that comply with all relevant norms and standards. Further customisation can be performed on the basis of these certified rugged platforms not only in terms of dimensions but also in terms of function, configuration and performance.
www.polyrack.com www.cieonline.co.uk
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