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Extreme boards!

“Extraordinary circumstances often bring along with them extraordinary strength,” wrote the political philosopher William Godwin.

The ethos extends to printed circuit boards (PCBs) for extreme applications whether that’s for military, rail or aerospace.

PCBs mechanically support and electrically connect electronic components in embedded computers that are now increasingly being used in harsh environments, where they can be subjected to extremes of temperature, shock, and vibration.

They are being deployed in extreme rail applications in systems like media converter boards, or devices that monitor vibrations and other data from train engines or the rails of the track, as well as in the military for video surveillance, ground to air communications, temperature and pressure measurements and in-air applications.

A basic PCB 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 which allows connections to pass between the layers of copper between solid conductive areas. It creates a more compact and reliable system compared with discrete wired enclosures, and also improves the overall reliability of the system.

Withstanding the heat

When it comes to PCB design heat is usually the biggest consideration especially when it comes to extreme applications and this entails laying out the components in such a way that heat generating components are not near other components, which may be affected by heat.

Another consideration is that some manufacturers will state that a product is rugged because of its “wide temperature”, which generally means that it can function properly over a temperature range of minus 40 to over 85C. 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 a wide temperature rating and the cost gap isn’t as big. But wide temperature operations are about more than just the components themselves. Variables in timing — in memory components, for example — or value (in capacitors) can still be affected by temperature.

Sufficient margin must be built into any design to guarantee suitable performance and that’s where designers will usually have to collaborate with an experienced PCB designer.

Given that the high temperature range presents a challenge for today’s latest embedded processors, one embedded design specialist - Diamond Systems - recommends a 15C operating margin. So, in order to achieve an 85C ambient operating temperature, a processor rated for 100C maximum will be recommended.

Any product development needs to be reinforced by extensive testing. Specifically, that requires the use of a temperature chamber that tests the PCBs, operating normally while specially designed software exercises the memory, at temperatures of between 40 and 100C. Testing can take place over periods of 24 to 48 hours to ensure there are no errors or out of the box failures.

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.

When designing a board it’s critical that a 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

When it comes to military applications and where there is a need for ruggedized boards certain modifications will have to be made to the board both to ruggedize it and make it more reliable. Enhancements can include conformal coating to minimise the effects of moisture and fungus, and additional pin headers can be installed to improve the PCB’s rigidity.

Diamond Systems’ Athena II board has shifted 18,000 units and, as an example of good design practice, even though the processor and companion chip were rated for only zero to 70C, the board actually achieved virtually 100 per cent yield from burn-in testing – a full minus 40 to more than 85C. As a result it 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 also need to be a lot wider.

In one example, 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.

As a consequence, 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 (RTOS), to function in particular hardware environments.

A carrier board was designed to meet the customer's requirements. Two quad-port Intel Ethernet controllers were integrated onto the carrier (in addition to the single GbE coming from the type 6 COMe module) to provide the nine Ethernet ports.

Two eight-port serial UART ICs were integrated along the appropriate level shifters to provide the serial I/O required. GPS was realised through a Trimble surface-mount GPS receiver and three miniPCIe slots (each with SIM slot) were also provided for expansion. Everything connected through standard IDC pin headers to the customer could make their own cables.

Going forward, the demand for capable PCBs is only going to increase driven, in no small way, by requirements surrounding the Internet of Things (IoT) and the need to mechanically support and electrically connect components.

Whatever the future holds, however, designing rugged PCBs will remain critical to ensure that extraordinary circumstances are equipped with electronic components that are up to the challenges they pose.

Author details: Mark Jeffrey is technical director at Recab UK

Author
Mark Jeffrey

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