INDUSTRY FOCUS MILITARY, AEROSPACE & DEFENCE


In military & aerospace applications, printed circuit boards (PCBs) need to be rugged enough to withstand the application demands, as


Mark Jeffrey, technical director at Recab UK, explains


ELECTRONIC COMPONENTS: MEETING THE CHALLENGES OF MILITARY EQUIPMENT


P


CBs consists of a flat sheet of insulating material and a layer of copper foil, laminated


to a substrate. The copper is divided into separate conducting lines – tracks, circuit traces or pads – through chemical etching, allowing connections to pass between the layers of copper between solid conductive areas. PCBs are used to mechanically support and electrically connect electronic components in all but the simplest of electronic products. A large number of embedded computers are


used in harsh environments, and here they can be subjected to extremes of temperature, shock and vibration. Examples are in extreme rail applications in systems like media converter boards, and in the military for video surveillance, ground to air communications, and temperature and pressure measurements, and in-air applications. When designing PCBs for extreme applications,


heat is usually the biggest consideration. So, the components need to be laid out in a way that those which generate heat are not near others which can be affected by it. Designers also need to consider that some


manufacturers will state that a product is rugged because it has a wide temperature range, which generally means that it can function properly over a temperature range of -40 to over 85˚C. This goes back to the early days of the embedded computer market when wide temperature versions, if available, were harder to buy and much more expensive. Today, most common components have wide


temperature rating. However, wide temperature operations are about more than just the components themselves. Variabilities in timing – in memory components, for example – or value (in capacitors) are still affected by temperature. As a result, sufficient margin must be built into the design to guarantee suitable performance. Given that the high temperature range presents


a challenge for today’s latest embedded processors, Diamond Systems, a supplier and


34 SEPTEMBER 2020 | DESIGN SOLUTIONS


partner of Recab UK, recommends a 15˚C operating margin. So, to achieve an 85˚C ambient operating temperature, a processor rated for 100˚C maximum is recommended. Heat dissipation is another critical element in


ruggedness, particularly as most PCBs are used in outdoor applications and subjected to extremes of temperature. Lower temperatures are typically less of an issue, due to the natural self-heating of electronics. Embedded processors can be more limited at high temperatures. Another requirement is that some power


signals to the board’s inputs and outputs (I/O) must be isolated. After all, extreme environments can present extreme voltages in some cases. Recab UK tries to ensure that all its customers’


analogue or digital I/O requirements are satisfied, within the desired voltage widths and ranges. This is where power filters designed for use with direct current to direct current (DC-DC) connections can be used according to the RIA12 standard for protecting electronic equipment from transients and surges in DC control systems.


MILITARY APPLICATIONS Diamond Systems’ own rugged boards include the Athena II which is used in a large volume military vehicle roof-mounted remote weapons station program. However, in order to meet the applications demands, modifications were made to the board both to ruggedise it and make it more reliable – with enhancements including conformal coating to minimise the effects of moisture and fungus, and an additional pin header was installed to improve the PCB’s rigidity. The company has delivered over 18,000 units of


this product over a 13-year period. As an example of good design practice, even though the processor and companion chip were rated for only zero to 70˚C, Diamond Systems achieved virtually 100% yield from burn-in testing – a full -40 to more than 85˚C burn-in for each board – and experienced zero returns for temperature


performance over the life of the program. Unsurprisingly, I/O requirements in military


applications are tougher than most other industries. While power suppliers are roughly the same compared with other extreme applications such as rail, more filtering is required on the I/O to avoid electromagnetic interference (EMI). Temperatures are also wider. In one case, a customer approached Recab UK


with a requirement for a motherboard with multiple serial I/O and Gigabit Ethernet channels. This was for a military application, specifically for capturing video streams and controlling devices through serial ports. The customer had several key requirements


including that the technology worked with a modern Intel Core i7 processor and various types of serial ports. They included four RS-232 serial ports, which are most popular for PCs but limited in distance and susceptible to noise; ten RS-422 serial ports, suitable for longer distances and noise resistance; and two RS-485 serial ports, that are similar to RS-422 but can have multiple commanding and multiple listening devices but is easier to programme. There was also a requirement for nine Gigabit Ethernet (GbE) ports, three mini PCI express slots for expansion, with a global positioning system (GPS) built-in. These requirements needed a significant


amount of I/O that no off-the-shelf motherboard would be equipped to provide. Recab UK turned to a COM Express solution, a highly integrated and compact PC that can be used in design applications like an integrated circuit component. A Kontron type 6 COM Express (COMe)


module with a VxWorks board support package (BSP) was chosen as the core module. A BSP is a layer of software containing hardware- specific drivers and other routines that allow a particular operating system, in this case the VxWorks real-time operating system to function in particular hardware environments.


/ DESIGNSOLUTIONS


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