ANSI/VITA makes computer-on-modules mission-critical
Other aspects of rugged design can be addressed more easily: soldered components and suitable connectors withstand jolts and jars, and coating the PCB protects against dust, humidity and corrosive materials. Handling temperature, in turn, is a science, and is one of the biggest challenges for COM concepts. It is obvious that a standardized COM module with a well-designed cooling concept could take much heat out of the designer’s work, also by reducing design costs.
Most of the COM concepts available on the market are not standardized, lack rugged aspects or lag behind in interface technology. COM Express (PICMG COM.0) as the first PCI Express based computer-on-module standard specifies an operating temperature range of -40°C to +85°C but has its shortcomings in the details: shock and vibration are not fully taken account of, and cooling and EMC are still rather complicated issues.
A more recent variety of the computer-on-module concept known as ESMexpress builds upon the concept behind COM Express and takes its implementation to a higher level regarding rugged performance. Its specification is being prepared by a work committee including system vendors as well as avionics manufacturers and other rugged electronics experts to be standardized by VITA according to ANSI under the official designation ANSI/VITA 59, RSE Rugged System-On-Module Express.
ESMexpress sets out to make COMs a stable, cost-effective solution for a wider number of areas by addressing mechanical stability, rugged physical performance, and aggressive fanless cooling. To achieve this, the populated PCB with a size of 85 mm by 115 mm is mounted into an aluminum frame with a final size of 95 mm by 125 mm, which completely encloses the assembly.
This allows for conduction and convection cooling and enables power dissipation of up to 35 W. Instead of being limited to the -40°C to +85°C operating range, ESMexpress can offer performance up to -55°C through +125°C. (Figure 1 below)
The mechanical set-up of the COM module greatly contributes to the thermal performance of the overall design. The high pressure caused by the safe joints between the housing and PCB supports the thermal connection of the components.
The PCB edges transport heat to the frame and then to the housing cover. Also, the hottest components within the unit are coupled directly to the cover. The eight mounting screws for the carrier secure the entire physical assembly in place and ensure optimum heat transfer.
If a module needs additional cooling, the aluminum housing can be connected to an external heat transfer device (conduction) or combined with a heat sink for heat dissipation (convection). The closed metal case also ensures optimum EMC protection conforming to EN55022 for the electronic parts, hermetically sealing them on all six sides.To make it resist shock and vibration, the COM module has fully soldered connections and is rated to withstand 15 g / 11 ms for shock and 1 g / 10 to 150 Hz for vibration (sinusoidal). Eight screws and two robust 120-pin board-to-board connectors make the COM and its carrier a tight unit. The connectors are qualified to railway and MIL-compliant applications and specified for -55°C to +125°C. Supporting differential signals with up to 8 GHz, they provide ample connections for I/O intensive applications.
For particularly harsh operating environments, where dust or moisture is a potential concern, conformal coating is optional for added protection.
In addition to the 125 mm x 95 mm ESMexpress, an even smaller format using the same rugged-design concept is available in a 55 mm x 95 mm module size. This more compact COM format called ESMini is not specifically part of the VITA 59 standard but follows the same specifications for rugged performance and cooling as its bigger brother.
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