A tour of VITA AFT cooling standards in 2023Story
August 03, 2023
For critical and intelligent deployed embedded modules, air-flow-through (AFT) cooling describes an approach that brings the coolant much closer to heat-generating electronics than standard conduction-cooled approaches. As a result, compared to conduction cooling, AFT increases the amount of power that can be cooled and decreases device temperatures.
With air-flow-through (AFT) cooling of deployed electronics, air is contained within the closed duct of the module’s heat exchanger, which is separated from the module’s electronics. The interface between the module and chassis requires an air seal to prevent air escape or contaminant ingress. AFT modules are typically thicker than similar conduction modules due to the AFT heat exchanger, which means that mezzanine cards need connectors to the host card with taller mated heights. This arrangement also provides superior cooling to the mezzanine card, because the primary side of the mezzanine is adjacent to the heat exchanger.
Figure 1 graphically shows the comparison between the two cooling approaches, with a clear reduction in thermal resistance provided via AFT.
[Figure 1 ǀ Air-flow-through (AFT) plug-in module and chassis (top, half module and partial chassis section shown), and conduction plug-in module in air-cooled chassis (bottom, half module and partial chassis section shown)]
In recent years, interest in AFT has gained renewed momentum in the VITA Standards Organization (VSO) and elsewhere. The following AFT standards and/or Working Groups are currently active in the VSO:
- VITA 48.5 Standard (Mechanical Standard for Electronic Plug-in units Using Air Flow Through Cooling) and VITA 48.5 Revision Working Group (Mechanical Standard for VPX REDI Air Flow Through Cooling, Multi-Pitch)
- VITA 48.8 Standard (Mechanical Standard for VPX REDI Air Flow Through Cooling, 1.0” to 1.5” Pitches)
- VITA 48.9 Working Group (Mechanical Standard for Electronic VPX Plug-in Modules Using Air Flow Through Cooling – Retractable Seals)
The VITA AFT standards provide excellent approaches for thermal management. In addition to the inherent advantages that AFT delivers, system designers also benefit from the VSO’s adherence to open standards, collaboration, rigor, and professionalism in developing the standards.
The following provides a concise introduction and overview of the VITA AFT standards:
VITA 48.5 was originally developed and standardized by Northrop Grumman, with the standard being released as ANSI-approved in 2010. It features a patented air seal that Northrop Grumman has used for decades. VITA 48.5 is now in the process of being revised and updated for wider applicability and better ease of use. For example, 3U OpenVPX modules will be included in the updated standard, whereas VITA 48.5 was previously limited to 6U modules. In addition, specific module pitches will be defined, more detailed guidance for heat exchanger design will be provided, the air seal IP licensing requirement is being reevaluated, and an air seal source of supply will also be included in the revised standard.
After VITA 48.5, VITA 48.8 was the next AFT standard to be developed in the VSO. Lockheed Martin developed the approach behind this standard, which has been ANSI-approved since 2017, with a revision in 2022. VITA 48.8 covers both 6U and 3U modules. It is unique among the various AFT standards in that it minimizes plug-in module weight by removing card edge retainers (aka wedgelocks) and replaces front-panel levers with jack screws. A recognized disadvantage of VITA 48.8 is that the air seal material and design are not clearly specified in the standard, so implementers are on their own to develop this critical piece of the AFT puzzle. On the other hand, for some system developers the freedom to define the air seal can be an advantage, since they can innovate and introduce superior air sealing for VITA 48.8 (although interoperability may ultimately suffer).
Adding to system designer options, a new AFT standard is now emerging. VITA 48.9, which features a new air seal, was developed by L3Harris. The VITA 48.9 Working Group in VSO is quite active, with 46 participants signed up, split more or less evenly between producers and users. VITA 48.9 will cover both 6U and 3U plug-in modules, and the new air seal is a two-piece design, one piece for the chassis side, and the other for the module side. There is also a push to standardize modules into pressure drop classes, and a lot of work is being done to bring the new air seal up to higher TRLs [Technology Readiness Levels].
Although there are common elements across each of these AFT standards, the truth is that they are not interoperable. Each approach has its pros and cons, and the companies behind the introduction of the respective standards obviously have an interest in using them for their own systems. Therefore, AFT implementers – in particular module developers – may need to be able to design, model, analyze, prototype, test, and build to each of the standards.
Outside of the VSO, Curtiss-Wright developed another approach for AFT cooling, called fluid-flow-through (FFT). A patented cooling method, like some of the other AFT standards, FFT also brings the coolant much closer to the heat-generating electronics, but with FFT the plug-in module is a common form factor conduction module. The coolant can be either air or liquid, with the heat exchanger being part of the chassis rather than located on the module itself. Like the other AFT cooling approaches, FFT has advantages and disadvantages. One of its key advantages is the ability to use a common conduction module form factor, and the associated interoperability and cost advantages that provides. A disadvantage of FFT is the increase in pitch and weight that it introduces, although it has been shown that a pitch of 1.1 inch to 1.3 inches can be achieved, and weight is actually less than some module-level designs for high power (e.g., copper frames).
One of the key benefits shared by all the AFT cooling approaches discussed here is the ability to innovate within the standard. This benefit combines some of the best aspects of MOSA [modular open systems approach], namely open standards interoperability and technological innovation. The VITA standards define and control the mechanical interfaces between the plug-in module and the chassis and other required components (e.g., levers, jack screws, etc.). Beyond those parameters, module developers have more or less free rein to design the heat exchanger within the required envelope, enabling cooling innovation to thrive.
Ideally, there would be a single AFT VITA standard that all module, chassis, and system developers would follow. However, the current reality is that three distinct AFT standards within VITA need to be considered – VITA 48.5 Revision, VITA 48.8, and VITA 48.9. Using a common circuit card assembly design for these standards would help minimize costs of AFT modules. Close collaboration with customer system developers is also highly recommended to ensure requirements are met.
An example of an AFT module is the VPX3-652 20-port Ethernet switch card, designed in alignment with VITA 48.8. This Layer-2 managed Ethernet switch provides as many as 20 ports of copper Ethernet connectivity. Models are available with 16 ports of tri-speed 1000Base-T backplane connections, or with 12 ports of 1000Base-T plus an additional 8 ports of 1000Base-X (SerDes) connections for mixed Base-T/Base-X environments. An additional 1000Base-T port is available on the front panel for air-cooled modules. All Base-T ports feature auto-negotiation and auto-MDI/MDIX to ensure trouble-free interfacing. Base-X (SerDes) ports are typically used for board-to-board interconnects within a system or can interface directly to optical transceivers for external fiber connectivity.
The VPX3-652 3U VPX Ethernet switch supports an extensive set of Layer 2 switching features. Its flexible management interfaces, including a powerful command line interface (CLI) for setup and configuration, as well as SNMP and Web management interfaces, accelerate development time to market and simplify maintenance support. In addition, special features like built-in test (BIT) aim the part squarely at those building tactical deployed networks.
Ivan Straznicky is technology director and a technical fellow at Curtiss-Wright, where his responsibilities include advanced thermal and packaging technologies. Ivan has three patents for cooling of rugged electronics and has written several technical papers and articles on the subject. He has a degree in mechanical engineering from McGill University in Montreal, Canada.