Achieving conduction heat transfer for commercial AdvancedMCs in a rugged MicroTCA System
In order to be leveraged in rugged systems, standard commercial AdvancedMC (AMC) and MicroTCA Carrier Hub (MCH) boards in MicroTCA systems must be repackaged for conduction cooling. The steps, requirements, and limitations for ruggedization can be most readily achieved by emphasizing the clamshell design of the boards, interfaces to the conduction chassis, and simplified design of the I/O and power system.
MicroTCA is a very robust and highly redundant system developed initially for the telecommunications industry. Since the MicroTCA spec was released in 2006, there have been many different applications of these systems. However, while sophisticated features of this specification such as redundant switching hubs, redundant power, and advanced system management allow reliable use in telecommunications applications, they can overburden small rugged solutions. To meet the requirements of rugged applications, system designers can repackage standard commercial AMC and MCH boards for conduction cooling in a rugged MicroTCA system.
MicroTCA systems and commercial applications: A history
Initially, base station configurations were widely developed and deployed from this specification, but since then, lower cost and smaller configurations for industrial-grade and military-grade solutions have also emerged. This trend has been facilitated by the additional dot specification work provided by PICMG committee members in the form of working groups. One such group developed the MicroTCA.3 specification, which defined a conduction-cooled module based on the AMC specification for higher shock and vibration requirements, as well as harsh environmental conditions.
A key challenge to expanding the reach of these systems was in demonstrating the ability of the backplane connector system to meet, over its full life, extended environmental requirements. Rigorous independent qualification testing of the connector system, based on ANSI/VITA 47 and MIL-STD-810 methods for shock levels of up to 40 g and random vibration levels of 12 g, was successfully conducted and documented in Test Report #209623D (www.picmg.org/pdf/TEST_REPORT_REV_1.pdf); this paved the way for solutions providers to develop rugged systems using MicroTCA.
Typical rugged MicroTCA applications
One common application for rugged MicroTCA is in deploying a communications system onto a mobile platform. This application utilizes the already-developed solutions MicroTCA provides in the telecommunications space, which then has to be ruggedized to meet the harsh environmental conditions of a mobile, military environment. This open system platform provides an ideal solution for the next-generation, network-centric requirements within the military and delivers robust system management for Built-In-Test (BIT).
The small size of AMC boards also makes the MicroTCA specification a good fit for military applications. AMC is 75 mm x 180 mm in size, helping to meet the Size, Weight, and Power (SWaP) challenges of next-generation military systems. Putting these modules in a clamshell for conduction cooling also delivers an advantage for the military in that it provides Electrostatic Discharge (ESD) protection to the board. This protection allows for on-system field maintenance by the crew for a plug-and-play experience without the risk of damaging the boards, and meets Two-Level Maintenance (TLM) requirements, which allow the modules to be replaced in the field, instead of having to return the entire system to a field depot for repair.
Shortcomings of commercial AMC and MCH boards
While these new rugged applications have expanded the reach of the specification, some shortcomings and limitations of standard commercial AMC and MCH boards must be addressed before conduction cooling is applied to them for use in rugged MicroTCA systems. Features such as redundant switching hubs, redundant power, and advanced system management lend to system accuracy and dependability in telecommunications applications. However, many of these features are not necessary for compact, rugged solutions and may, in fact, overburden a small, specialized system.
The solution is found in simplifying the system and lowering the number of required modules. For example, a simplified power management scheme can be used to maintain centralized power management features via the MCH or control board while allowing the chassis to utilize an off-the-shelf power supply instead of a commercial-rated power module board. This approach helps to fast track a solution, because system designers can pre-certify a power supply that meets the environmental and power requirements for an application and then bundle this with a manufacturer’s simplified power management architecture. This solution eliminates extra cost and new design risk to the program. Additionally, multiple power output modules are not required, as is the case with other system platforms, since +12 V is the primary power output voltage for MicroTCA.
When repackaging a commercial AMC board for conduction cooling, certain steps and requirements must be followed, including modifying the clamshell design for boards and interfaces to the conduction chassis. To develop a clamshell, designers must secure and determine the following:
- A mechanical skyline drawing or 3D model of the board
- Any mechanical constraints within the board assembly that must be accounted for, such as mezzanine boards or other unique components
- All “hot” components that will be coupled to the clamshell
- Dimensional and tolerance information of the components to be coupled to the clamshell
A mechanical skyline drawing or 3D model is pivotal in the initial stages of the repackaging process. The model allows design engineers to easily identify the constraints and components that affect the success or failure of properly interfacing the AMC and clamshell.
Once a precise and accurate model is obtained, designers should determine any potential mechanical constraints or unique components of the board. A major design challenge is determining how to deal with any mezzanine boards used on the AMC. Securing these smaller boards and removing heat from underneath these boards may require additional mechanics and detailed assembly sequences. An ideal AMC would contain one or two large hot components that are already using heat sinks. For example, with a processor AMC the heat sink mounting footprint can be used to securely install the clamshell. In many cases, that heat sink spans the full surface area of the AMC already, and that model outline can be used to design the skyline of the clamshell.
At this stage, all hot components on the board that will be connected to the clamshell must be located. Designers should procure a board without the heat sink installed on the hot components, as removing a heat sink could potentially damage the component. The heat sinks are replaced with the clamshell, which is then responsible for removing the heat generated from the electronic components to the cold wall of the enclosure (Figure 1). The clamshell design integrates with the existing front panel from the commercial AMC, simplifying the assembly and maintaining support of any I/O connectors coming out of the front panel. The EMI gasket needs to be removed as it can cause interference in the adjacent module and is not required since EMI is contained by the clamshell and additional EMI containment is usually provided at the chassis level.
The clamshell is typically constructed with machined aluminum, and a thermally conductive gap material is used between the hot components of the board and the inside surface of the aluminum upper clamshell to account for component height tolerances identified in the initial design stage. There is also a thermal interface material that is used between the upper and lower halves of the clamshell where they are screwed together. This material accommodates the tolerance in the printed circuit board thickness, and also transfers heat to both halves of the clamshell.
Outside the AMC
Along with the clamshell, the interface to the cold wall must be addressed when redesigning commercial AMC boards for rugged MicroTCA applications. An uneven surface can account for higher thermal resistance and result in increased temperature of the AMC components. To achieve the flatness required for a proper mating surface of the clamshell edge to the cold wall, high-precision machining is critical. The Wedge LOK installed on the clamshell must be torqued to the proper value to ensure optimized clamping force of the clamshell to the enclosure’s cold wall (Figure 2). In most cases, the plating for these surfaces is hard anodized black, but other plating options are also available.
One of the primary requirements for a functioning system is the capacity for heat transferred to the cold wall to be removed from the system. This can be performed via natural convection, but approximately 75 W is the most heat that can be removed from a shoebox-sized enclosure. Since this system could consist of a processor card of approximately 30 W and a few peripheral boards at about 15 W each, convection alone isn’t always adequate. For higher power systems that don’t achieve sufficient cooling through convection, forced air cooling over the fins of the enclosure is often necessary.
Additional considerations include achieving ratings necessary to ensure a degree of protection against environmental concerns. The I/O connectors on the enclosure and type of gasketing material used for the removable panels must be rated for the environmental condition. Ingress Protection (IP) for sand, dust, water, and even full immersion can be achieved by using the right components and proper design for the enclosure. Service depth of the patch cables from the AMC boards to the removable panels must also be considered for proper assembly and maintenance of the system, while also accounting for the overall size and weight of the chassis. Alternatively, an I/O board can provide a better interface with cabling and simplify the assembly and services of the modules.
Multiplying MicroTCA markets
Today, the MicroTCA specification has expanded across many market segments, and systems engineers are finding creative ways to balance price and performance with this powerful architecture. Working groups within PICMG and other industry bodies have outlined the necessary steps to re-package the architecture for extreme environmental conditions, and work is being conducted today via the MTCA.4 specification, committed to defining the mechanics and interconnects for rear transition modules. This will ensure that MicroTCA can be used in most market segments, enticing more companies to invest in designing and building new AMC modules and system solutions. The ecosystem will continue to grow as companies find opportunities to use this increasingly versatile architecture.