Designing PCBs for extreme applications

Author : Mark Jeffrey | Technical Director | Recab UK

01 July 2021

Recab_Drone systems 1
Recab_Drone systems 1

“Extraordinary circumstances often bring along with them extraordinary strength,” wrote the political philosopher, William Godwin. This ethos also extends to printed circuit boards (PCBs) for extreme applications – be they military, rail or aerospace.

The full version of this cover story article was originally featured in the July 2021 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.

And as embedded computing systems become increasingly critical in such demanding applications, supporting everything from military video surveillance to real-time communication of air or land vehicle positioning data, many manufacturers are quick to claim that their boards and components are rugged. As demand for these systems grows, Mark Jeffrey, Technical Director at Recab UK, one of the UK’s leading suppliers of specialist PCBs for extreme applications, explains what ruggedised boards are really made of…

PCBs are used to mechanically support and electrically connect electronic components in all but all the simplest of electronic products. Even more impressive are the large number of embedded computers that are used in harsh environments, where they can be subjected to extremes of temperature, shock and vibration.

This can include for 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. Also, uses for military, like video surveillance, ground-to-air communications and 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 — called tracks, circuit traces or pads — through chemical etching. This 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.

Recab_Land defence
Recab_Land defence

Withstanding the heat

Heat is usually the biggest consideration when designing PCBs for extreme applications. In terms of designing the board, 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 it is “wide temperature”. It generally means that it can function properly over a temperature range of -40 to over 85 degrees Celsius. 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 and the cost gap isn’t as large as it used to be. But 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.

Sufficient margin must be built into the design to guarantee suitable performance – which is where it’s necessary to collaborate with an experienced designer of PCBs.

One such specialist is Diamond Systems, a supplier and partner of Recab UK. Given that the high temperature range presents  a challenge for today’s latest embedded processors, Diamond Systems recommends a 15 degrees Celsius operating margin. So, to achieve an 85 degrees Celsius ambient operating temperature, a processor rated for 100 degrees Celsius maximum is recommended...

Recab_Drone systems 2
Recab_Drone systems 2

Better isolation

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...

Military applications

Diamond Systems’ own rugged boards include the Athena II, as used in a large volume military vehicle roof-mounted remote weapons station programme. To satisfy these requests, certain modifications were made to the board both to ruggedise it and make it more reliable.

Enhancements included conformal coating, to minimise the effects of moisture and fungus, and an additional pin header was installed to improve the PCB’s rigidity...

Recab_Diamond Systems Aries PC104+ SBC
Recab_Diamond Systems Aries PC104+ SBC

The future

Going forward, Recab UK predicts an upswing in demand for ruggedised PCBs driven by requirements surrounding the Internet of Things (IoT) and the need to mechanically support and electrically connect components.

This is expected in military applications, but less-so in aerospace. Britain’s exit from the European Union may also help in driving more inward investment towards UK companies...

Read the full article in EPDT's July 2021 digital issue...

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