MicroTCA: Less is Sometimes Better
By Bob Hult, Bishop & Associates Inc.

Relentless market pressure to offer greater functionality in smaller envelopes and at lower cost is the name of the game in today’s electronics industry. Designers in every market segment, from consumer to military, are looking for increased value in these tough economic times. Systems that conform to a packaging standard can minimize the need for scarce engineering resources, as well as shorten design cycle time. Engineers can take advantage of access to multiple competitive sources and be assured of plug compatibility. A widely adopted open standard can evolve to address new technology, providing a clear migration path to avoid obsolescence. A wide variety of special interest groups, industry consortiums, as well as formal standards writing organizations, have been actively responding to industry demands for open standards that define backplane architecture.

For many years, manufacturers of telecommunications equipment typically designed their products using proprietary architecture, assuring themselves of a captive market for hardware upgrades. The cost of this approach, as well as customer demand for greater flexibility, created a fertile environment for the introduction of telecom equipment packaging standards.

The PCI Industrial Computer Manufacturers Group (PICMG) is an industry consortium that has been particularly successful in introducing packaging standards focused on the telecom equipment market. PICMG standards are specifically focused on the unique requirements of the telecom central office, but it appears that systems that are designed around PICMG specifications are gaining traction in applications beyond their original scope. MicroTCA is the latest iteration in a series of specifications created by PICMG that now address applications that range from military to medical.

One of the key characteristics of a PICMG specification is the inclusion of defined interconnects. As system speeds approached the gigabit range, traditional open pin-field backplane connectors on 0.10” centerlines began to demonstrate increased crosstalk and skew. Design engineers had the option of assigning more pins to ground for improved isolation, but that solution seriously impacted signal pin density. The adoption of high-speed serial, low-voltage differential signaling made every element in the channel a factor in the total performance of the link. In order to develop a specification that offered plug compatibility and assured performance, PICMG defines a specific connector verified to meet the performance requirements of the specification.

Compact PCI was one of the first in the series of standards that addressed high-speed passive backplane architecture. Originally introduced in 1995, PICMG 2.0 Rev.3.0, ratified in 1999, defines a series of 2mm hard metric backplane connectors with specific pin counts and physical locations on 3U and 6U backplanes, midplanes, and daughtercards.

Designers were able to populate the board according to its function, while assuring plug compatibility in an expanding universe of board and system vendors. CompactPCI featured the ability to hot-swap daughtercards, a critical requirement of high-availability telephony systems.

A grounded shield was added to receptacles to improve the high-speed performance of the connector.

PICMG 2.11 addressed the need to efficiently bring increasing power to the system. This connector, subsequently tooled by a variety of suppliers, offers sequential mating, an essential attribute in telecom applications.

A defined 47-contact power connector allows a modular power supply to plug directly into the backplane.

The AdvancedTCA standard extended the influence of PICMG packaging specifications with its ratification in December of 2002. Advanced Telecom Computing Architecture (AdvancedTCA) switch architecture, as defined by the PICMG 3.0 Rev 1.0 specification, was designed specifically to support next-generation carrier class communications systems. The objective was to create a powerful, highly reliable and flexible system platform that would simplify and speed the development of next-generation products. This open standard defines a robust series of backplanes, daughtercards, as well as the cooling and power distribution systems.  Backplane interconnects are organized by zone and define a specific power connector, as well as a Zd-type interface for high-speed differential signaling.

AdvancedTCA chassis support multiple protocols at up to 40 Gb/s data rates.

The issue of power and thermal management resulted in the inclusion of a standard power connector that can deliver up to 300 watts per daughtercard.

A new AdvancedTCA 300 specification, designed around a 300mm shelf depth, defines a specific Multi-Beam XL connector from Tyco Electronics. The power entry module connector includes eight 55 amp power contacts and 24 signal contacts.

In an effort to provide a logical migration path and enhance the design flexibility of AdvancedTCA, PICMG introduced the Advanced Mezzanine Card (AMC.0) specification, which defines a field-replaceable mezzanine card offering multi-protocol interface options, with high-bandwidth in a hot-swappable package. The defined interface is a unique right angle “riser card”-type edge connector interface, in four different configurations.

AMC mezzanine cards are designed to piggyback on AdvancedTCA carrier daughtercards and can serve a variety of functions.

Front access mezzanine cards allow removal without shutting the entire system down, a critical requirement in high-availability telecom applications.

The AMC card edge interface was a significant departure from previous two-piece pin and socket connectors. Extensive circuit modeling, together with signal integrity analysis, enabled it to deliver up to 12.5 Gb/s data rates with this one-piece, cost-effective interface.

The resulting mezzanine card is a highly flexible vehicle for system expansion, I/O, and repair granularity. A broad range of AMC mezzanine cards by third-party suppliers support this expanding market.

Connector manufacturers have embraced the AMC concept by tooling a host of competitive receptacles that conform to the mechanical and electrical requirements of the specification.

The primary differences between these interfaces involve their method of attachment to the carrier board. Suppliers such as EPT, Harting, Molex, and Tyco Electronics utilize compliant-pin termination, while Yamaichi and Cinch use a unique solderless compressive contact.

At this time it appears that the B+ version is the most popular configuration, with the majority of suppliers tooling this part.

HARTING also offers an AMC plug assembly that eliminates the need to add gold-plated fingers to the AMC mezzanine card. These plug assemblies create a rugged two-piece connector system designed to increase reliability.

Soldered onto the mating edge of the AMC module, the plug connector can reduce insertion forces, guarantee 200 mating cycles, and enables the use of thicker PCBs for extra routing layers.

The introduction of AMC mezzanine modules will likely boost adoption of ATCA architecture in high-end telecom central office infrastructure, and has raised interest in select military imaging applications, but emerging mid- to low-range applications simply cannot afford the cost and relatively large physical size of ATCA architecture.

Market demand led PICMG to create the MicroTCA specification, where the AdvancedTCA carrier daughtercard is eliminated, and AMC format cards plug directly into a size- and cost-reduced backplane.

MicroTCA is particularly attractive, as it takes advantage of the expanding range of off-the-shelf AMC modules and a reduced 4-U high by 300mm deep shelf size, and offers a nice balance between performance and system cost. Several different daughtercard form factors, as well as shelf, cube, pico, and back-to-back configurations, provide exceptional scalability to address a wide range of applications at reduced cost.

Equipment, including outside plant equipment such as wireless basestations, Wi-Fi/WiMax radios, optical networks, and media servers, are ideal applications for MicroTCA architecture. Additional applications in medical imaging and industrial control have been identified.

Systems that demand the high-reliability inherent in Advanced TCA, but are cost-sensitive, space-constrained, or do not need the full set of ATCA features, are finding MicroTCA to be a viable alternative.

The backplane signal interface has morphed into a 170-position vertical version of the AMC edge connector and uses the same serial fabric protocol.

Connectors are surface mounted or compliant pin attached to the backplane.

 A variety of manufacturers, including FCI, Molex, Harting, Tyco Electronics, and Yamaichi, have tooled this edge connector. The backplane launch has been identified as a critical aspect of the design, and as a result, many of these suppliers have taken different design approaches to achieve high-speed performance. Depending on the manufacturer, MicroTCA connectors are attached to the backplane via compliant pin, through-hole solder, compression, or surface mount, each using non-compatible footprint designs.

A companion two-piece power connector consisting of 24 individual 15-amp power contacts and 72 high-density signal contacts, is also included the MicroTCA specification. Connectors are typically attached to the backplane via compliant-pin technology. Power contacts are hot-pluggable to control arcing.

The timing for introduction of new PICMG specifications has often been less than ideal. Advanced TCA entered the market near the bottom of the 2000 electronics meltdown, when telecom was particularly hard hit. Manufacturers were understandably reluctant to adopt new system architecture, as their primary objective was to minimize risk in a down market. A slower than anticipated adoption of ATCA was the result. Industry observers are wondering if MicroTCA may also suffer a protracted adoption curve, given the global recession.

The influence of the weak economy may be softened by new applications that are appearing for MicroTCA outside of its initial telecom central office environment. The military, in particular, is attracted to the inherent redundancy of MicroTCA. Its reliability, feature set, and form factor are exactly what are needed in many mission-critical applications. Cost pressures have piqued interest in using commercial-off-the-shelf (COTS) technologies. Recognizing the military operating environment is considerably different than a central office, PICMG has begun to develop a ruggedized MicroTCA specification to withstand much wider temperature extremes, shock, and vibration. Additional features, including passive and active cooling schemes, ESD/EMP protection, and field repairability, are being considered. A ruggedized MicroTCA platform that can perform in military applications could open new commercial applications in energy exploration, transportation, and aviation. Each of these expanding market opportunities could speed the implementation of MicroTCA architecture across multiple market segments.

 Bishop & Associates’ Comments:

• MicroTCA offers a unique combination of features, including reliability, performance, and reduced size and cost, making it ideal for a wide range of emerging applications.

•  PICMG specifications have traditionally addressed telecom-related applications, but are now being considered in many unrelated markets that can utilize the reduced design cost and time-to-market features of these open specifications.

• Relatively long life cycles of central office equipment insure extended market potential for components defined by open standards designed to support this industry.

• Suppliers who manufacture defined PICMG connectors have proliferated and assure users of multiple sources at market prices.

• Sales of Advanced mezzanine cards are expected to nearly double over the next three years. AMC boards to date are primarily utilized as mezzanine cards attached to Advanced TCA carrier boards, but the bulk of the business is expected to shift to MicroTCA blades by 2011-2012.

• It is difficult to determine how the current global recession will impact sales of AMC and MicroTCA components, but new applications in non-telecom markets may accelerate demand as additional industries take advantage of these evolving specifications.

Robert Hult
Director of Product Technology, Bishop & Associates, Inc.
Robert Hult has been in the connector industry for more than 36 years. Hult began his career as a sales engineer for Amphenol. He joined AMP in 1972 and served in several management positions through 1996. In 1997, Hult joined Foxconn as group marketing manager for Intel in Chandler, Arizona, USA. Prior to joining Bishop & Associates, Hult was the regional application engineering manager for Tyco Electronics.

Hult graduated in 1968 from Bradley University with a Bachelor of Science degree in electronics technology and a minor in business.