Although a few industrial backplane architectures are already geared for rugged Mil/Aero applications, some design considerations are often required when designing rugged COTS products.
What is Rugged COTS?
Despite some controversy around the acronym, COTS stands for Commercial-Off-The-Shelf. COTS components (such as backplanes) can be and are being used increasingly in Mil/Aero applications. If a few changes are applied to the product in order to ruggedize it, we call the result “Rugged COTS”. Some of the standard architectures already contain ruggedization features, although they haven’t been written with Mil/Aero applications in mind. The majority of Mil/Aero applications have special design considerations due to environmental issues (dust, dirt, moisture, temperature, etc.), shock and vibration, quality processes, and more. We’ll take a closer look below at some of the design implications when COTS backplanes are being used for special applications.
Key Open Standard Architectures
To understand Rugged COTS backplanes, we first need to look at the various architectures being standardized. Eurocard backplanes are some of the most common in the Mil/Aero arena. Its beginnings go back to Motorola’s VERSAbus in 1979, based on the 68000 microprocessor. This bus formed the electrical basis for the VMEbus (Versa Module Eurocard) along with the preexisting Eurocard mechanics. Eurocard — originally specified by IEC60297-3 — is a term that collectively represents the format based on the DIN 41612 and IEC 603-2 connector standards, the IEEE 1101.x series of standards for PCB and subrack mechanicals, and finally the DIN 41494 and IEC 297-2 rack standards. All the mechanical hardware was readily available and proven to be robust even back in the early 80s. The VMEbus was born in 1981 as a result of the joint efforts of Motorola, Philips/Signetics and Mostek. Subsequently, VME was adopted as a standard by IEEE (IEEE 1014-1987), IEC and the VMEbus Manufacturers Group (now VITA).
In 1994, the VME64 bus was introduced, expanding VME from 32-bit to 64-bit, increasing bandwidth from 40 Mbytes/s to 80 Mbytes/s. VME64 extensions (VME64x) was introduced in 1997, featuring an enhanced DIN connector with 5 rows and expanded I/O capabilities as well as a P0 connector that offers a high density of pins (using the same 2mm HM connector as CompactPCI). Also, a 3.3V power plane, more 5V power, rear plug-in units, hot-swap capability, EMC front panels with injector/ejector handles, and geographical addressing were introduced, along with 160 Mbytes/s bandwidth.
VME and VME64x have for years been the staples of the Mil/Aero community for their ruggedness, reliability, well-understood technology, and ubiquitous nature. But, other form factors (Figure 1) have played a key role, including:
CompactPCI: This standard is based on the same rugged Eurocard form factor and the IEEE 1101.10/11 mechanicals, but employs 2mm HM (hard metric) connectors, with PCI being the core electrical portion. CPCI allows great flexibility with choices of 3U/6U heights, 32- bit/64-bit bus width, and 33 MHz/66 MHz speeds. PICMG 2.1 (hotswap), PICMG 2.5 (H110 telephony bus), PICMG 2.7 (dual system slot), PICMG 2.9 (system management bus), PICMG 2.16 (Ethernet) are other key benefits. CompactPCI is used in both terrestrial and airborne applications, with its high amount of IO pins, wealth of off-the-shelf products including embedded systems, and high reliability.
VXS: In this architecture a switch slot runs the fabric (Ethernet, Infiniband, Rapid IO, etc) across the P0 portion of an otherwise standard VME64x backplane. A high speed connector (characterized to over 6 Gbps) replaces the 2mm HM in the P0 section. Some VME/VME64x designs have upgraded to VXS. In theory, VXS brings slot-to-slot bandwidth rates to 3050 Mbytes/s.
VPX: A newer, flexible architecture using the same connector family (MultiGig RT-2) as VXS. VPX allows VMEbus implementation to be optional. Mesh topology is common with high bandwidth and more I/O possibilities. 3U or 6U heights are available. VPX has very rapidly gained acceptance in the Mil/Aero community. Many new designs are going to VPX. Slot-to-slot band width rates to theoretical 5000 Mbytes/s.
AdvancedTCA: Originated as a telecom-based architecture, it is used in some communications-based Mil/Aero applications. Thought of as non-rugged, the 8U × 280mm board format offers more real-estate and a powerful punch. Built-in system management and failover options are provided. The fabric can be run as mesh or dual star topology. Slot-to-slot bandwidth rates to theoretical 7500 Mbytes/s.
MicroTCA: Although Micro - TCA is non-rugged, a new “Rugged MicroTCA” specification is on the horizon. It uses AdvancedTCA’s small (75mm or 150mm high by approx 185mm deep) AdvancedMCs (Advanced Mezzanine Cards) to plug directly into the backplane via fast and reliable cardedge finger pads. Testing is being performed to modify the design for rugged apps to meet MIL shock/vibration levels by introducing a real cardedge mounting connector (to re place the current mating mechanism which is more prone to fretting/corrosion). Slot-to-slot bandwidth rates to theoretical 5000 Mbytes/s. Some built-in system management is optional.
The backplane architectures enumerated above (excluding at this point AdvancedTCA and MicroTCA) have a rugged Eurocard-based design, which can be enhanced for more severe environments.
Ruggedized Backplane Design Considerations
When designing for environments where dust, dirt, moisture, high humidity, extreme heat/cold and vibration are of concern, a plethora of techniques can be employed to ensure the backplane can withstand the operating and storage conditions. These include:
- Applying conformal coating to the finished assembly, before and/or after pressing connectors. This protects the backplane from severe environmental issues such as saltfog, dust, moisture, chemicals, temperature extremes, etc. Figure 2 shows conformal coating under blue light.
- Adding special stiffeners to rigid board and employing particular strain relief methods for the rigid-flex interfaces.
- Specifying a heavier copper plating for MIL applications in order to mitigate via barrel cracking due to prolonged mechanical stresses in the PCB (such as vibration, etc). The same applies if RoHS compliance is required, in order to prevent cracking due to thermal expansion in the laminate (which causes mechanical stress on the via barrels)
- Specifying increased pad-to-hole ratio in order to ensure the formation of proper solder joints for through-hole components; similarly, proper sizing of SMT component pads is crucial in obtaining robust solder fillets.
- When specifying the finish of the board it is important to keep in mind the reliability of the electrical connections of both the pressfit-type components and that of the solder-type components. Different chemistries between the finish of the board and the soldering material generally yield different reliability values for the solder joints thus formed.
- Assembly should be done per IPC6013 Class 3 to meet Mil/Aero requirements, while solderability testing can be performed per J-STD-003 class 3.
- Choosing components with wide operating and storage temperature ranges, as well as components which are not prone to failure due to repeated mechanical shocks and/or vibrations.
Once the assembly is complete, the backplane needs thorough quality and electrical testing. Particularly for high-speed designs, characterization of the backplane signals via signal integrity studies is more commonplace.
Airborne vs Terrestrial
Airborne applications tend to have more special requirements than those of earthbound applications. Random shock and vibration are key issues in both land-based systems and avionics. But, sinusoidal and high-frequency vibrations are a larger consideration for airborne designs. The continuous shaking pattern from a rocket, helicopter, or jet/plane can be very stressful for back plane components. The backplane enclosure is usually designed to meet MIL-STD-810-E for shock (includes levels to 35G) and random structural integrity vibration and MIL-STD-167 for vibration. MIL-STD-167 includes exploratory sweep (15s and each 1Hz interval between 4-50Hz), variable dwell (5min and each 1Hz interval between 4- 50Hz, and en durance dwell (2hr dwell at resonant frequency).
Unmanned vehicles, whether UAVs (Unmanned Aerial Vehicles) or UUVs (Unmanned Underwater Vehicles), commonly use backplane-based systems. They normally use a VME-based architecture with its real-time determinism and are expected to migrate more to 3U VPX because of its tremendous performance in a small size.
Looking to the Future
Backplanes will continue to need to be geared for rugged applications. Processes such as conformal coating, stiffeners, plating, and special assembly, are just a few of the key elements for Mil/Aero design. Expect to see VITAbased architectures like VME, VME64x, VXS, and VPX to continue to be favorites for rugged design. CompactPCI will also remain a popular choice for several applications and we may even see Rugged MicroTCA gain acceptance when it rolls out in the near future.