Accessing enclosure expertise

Since its release in July 2006, the MicroTCA standard has been welcomed by users in the telecoms industry as a compact and cost effective alternative to AdvancedTCA technology for lower end systems. MicroTCA offers a number of benefits that make it an attractive solution and, right from the start, it aroused significant interest not only in the field of telecoms, but also other sectors. The fact that MicroTCA uses the same Advanced Mezzanine Card (AMC) modules as AdvancedTCA encouraged the rapid adoption of the new standard, since there were already many AMC modules on the market. In the AdvancedTCA standard, the AMC modules were defined for use as mezzanine boards, housed within special AMC carriers; MicroTCA, however, took these mezzanine boards and defined them as function modules that are inserted directly into the system. As well as reducing cost and complexity, this approach also meant that MicroTCA could offer greater flexibility and a smaller form factor. AMC modules themselves do not adhere to the traditional 3U and 6U Euroboard format, being somewhat smaller – with heights of about 75 and 150mm respectively for single and double modules. The continuing miniaturisation of components meant complete single board computers could be realised on these small boards and MicroTCA therefore made it possible to build small systems with high processing power. The potential benefits of MicroTCA technology immediately caught the attention of sectors such as industrial automation, test and measurement, railways and transportation, and defence. However, the prevailing environmental conditions in these markets differ significantly from telecoms, often placing more stringent requirements on shock and vibration resistance. Rugged MicroTCA In response to widespread interest in the original MicroTCA.0 specification, PICMG (the organisation responsible for developing AdvancedTCA, MicroTCA and various bus standards) started working on three secondary specifications. The first of these – MTCA.1 (Air Cooled Rugged MicroTCA), aimed at air cooled industrial applications – was released in March 2009. The PICMG working group is currently occupied with the specification for MTCA.3 (Hardened Conduction Cooled MicroTCA), which is for conduction-cooled military applications. Work on drafting MTCA.2 (Hardened Air Cooled MicroTCA) for air cooled military applications will start as soon as MTCA.3 is complete. The enclosures, racks and systems defined in the original MicroTCA.0 specification were required to pass shock and vibration tests at performance level DL1 to IEC 61587-1. This means acceleration rates of 7g for shock impact and an acceleration amplitude of 0.5g in the case of vibration loads. Performance level DL1 is designed for general industrial applications (for use in power stations, for example, or in applications where impact phenomena with low energy content are to be expected). The MTCA.1 specification then defined the requirements to be met by a system that is exposed to increased loads with regard to temperature, shock, vibration and ambient conditions. The specification was particularly aimed at systems used outdoors, in production locations, in railway and shipping equipment, in aviation and, to a lesser extent, in defence applications. The applicable IEC 61587-1 performance level DL3 defines peak acceleration rates of 25g for shock testing and 3g for vibration testing, four and six times higher respectively than is the case with MicroTCA.0. The stringent mechanical requirements of MicroTCA.1 could only be fulfilled by using an additional screw interlock to bolt the AMC modules on to the board cage. In practice, this meant the AMC module faceplates had to be extended at the top and bottom to create flanges for fastening on to the MicroTCA system. A particular challenge lay in the requirement for the interlock to be made without any force being applied to the connector on the MicroTCA backplane. When turned to tighten, a 'normal' screw would push the pcb on the AMC module toward the backplane, exerting force on the connector. To address this, Schroff has developed a working and tested AMC module fastener (figure 1) that satisfies the requirements of the MicroTCA.1 specification (interlock without exerting pressure on the connector). An expansion bolt with an internal cone fits into a bushing, which has been welded to the front panel. The screw is shaped to match the expansion bolt's cone. When the screw is tightened by turning, the bolt expands, forming a positive-fit connection with the bushing fastened to the front panel. As such, no force is exerted in the direction of the backplane connector. This patent pending solution is an integral component of the MicroTCA.1 specification. Conduction Cooled MicroTCA For extreme environments, a fully sealed and hardened enclosure is required and this needs a completely different packaging solution. The MicroTCA.3 specification is currently being developed to meet this requirement. In developing the specification – which will provide the basis for defence applications – the goal of the MicroTCA.3 committee is to avoid modifying the AMC module itself or changing the MicroTCA backplane connector. Although the MTCA.3 specification has yet to be finalised, it seems certain that AMC modules will be fastened to the system using Wedge-Lok technology. First, the AMC module is encapsulated into an aluminum clamshell. A skyline profile of the board is created and machined into the aluminum part, which makes contact with each hot component – similar to the way a heat sink would be designed and mounted. A thermally conductive paste is applied as a gap filler between the aluminum part and the board component. The standard AMC front panel does not need to be removed and is used within the assembly. The clamshell is then mounted with two Wedge-Loks. These Wedge-Loks provide the clamping force to the cold wall rails inside the enclosure, thereby giving the necessary resistance to the highest shock and vibration conditions, as well as conducting the thermal energy from the AMC to the exterior cooling fins of the enclosure. As PICMG continues to develop the MicroTCA specifications for use in demanding environments, this versatile technology will be adopted in an increasingly diverse range of applications. Author profiles: Christian Ganninger is MicroTCA, backplanes and psu product manager for Schroff GmbH, Keith Reynolds is technical/marketing manager for Schroff UK