Smaller, better, cheaper, and faster are the buzzwords for high-speed backplane connectors in the future. Not to mention plastic versus air shielding.
By Bob Hult
High-speed backplane connectors continue to evolve as market demands fuel development of new interfaces. The transition from single-ended parallel architectures to serial differential signaling has dramatically altered the backplane connector landscape, enabling multi-gigabit transmission thought impossible just a few years ago. Recovery from the 2000 to 2003 market meltdown has generated new application requirements and stimulated a flurry of product extensions as well as new interface families. In some cases, connector suppliers are actively filling gaps in existing product lines, which allows users to more precisely match the needs of their applications to the performance of the chosen interface. In other cases, manufacturers are introducing new connectors that address emerging requirements.
Connector suppliers are addressing a series of trends that will shape backplane connector requirements for several years to come. Increasing bandwidth is the first. Just a few years ago, most systems were focused on achieving speeds of 3.125 Gbit/s (XAUI), but systems now being designed demand 5 to 6 Gbit/s, and must have headroom to allow performance to 10 to 12 Gbit/s. The ability to upgrade a system without changing to a new backplane connector system is very attractive. It is unclear where the next step may be, but advances in SERDES signal-conditioning features, along with new signaling protocols and PCB materials, seem to indicate that the use of copper interconnects may be extended to 20+ Gbit/s in the future.
The market’s demand for smaller, better, and cheaper end-user products impacts system design in several critical areas. First, smaller connectors put contacts on tighter centerlines that can increase crosstalk, degrading signal integrity at high frequencies. A smaller contact typically has a lower current rating, a real problem as system power demands continue to increase. Many components in densely packaged systems generate excessive heat that must be conveyed out of the box. All components, including connectors, must be designed to minimize any obstruction to cooling airflow. Examples include a backplane power module that permits airflow through the connector by FCI (Etters, PA), and the Aptera connector from Teradyne (Nashua, NH), which offers an exceptionally low profile to reduce air blockage.
 FIGURE 1. The Z-Pack HM-Zd high-density (HD) connector (below) and the GbX standard backplane connector (above) feature internal metallic shielding. |
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The debate between metallic and air shielding systems rages on. Traditional high-speed connectors have used grounded metallic shields between signal lines to establish impedance control as well as reduce crosstalk. The Tyco Z-PACK HM Zd, Teradyne GbX, and 3M MetPak HSHM connectors all use internal metallic shields (see Fig. 1). The FCI AirMax Virtual Shield connector has shaken up the market by eliminating the shield. Combining the use of air dielectric with staggered and edge-coupled contacts, AirMax VS has injected an alternative approach that reduces the weight and cost of this interface (see Fig. 2). Others contend that only a grounded metallic shield can provide adequate crosstalk management while adding mechanical robustness to the interface. Tyco is introducing the Z-PACK Max, a new high-speed backplane connector that uses plastic dielectric and no internal shields to reduce cost while maintaining a robust interface (see Fig. 3). Market experience, bandwidth demands, and cost pressures will eventually sort out these issues.
 FIGURE 2. The AirMax VS connector uses air dielectric, along with staggered and edge-coupled contacts, to eliminate metallic shielding. |
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 FIGURE 3. The high-speed Z-PACK MAX uses plastic dielectric and no internal shields. |
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Termination technology
The transition from connector to PCB footprint has been recognized as the greatest source of signal distortion attributed to backplane connectors. Most of the leading high-speed interfaces use press-fit compliant-pin termination technology today. The plated-through-hole footprint generates crosstalk, while unterminated stubs at the bottom of the hole create lump capacitance that further reduces signal fidelity. Surface-mounted connectors that incorporate pad and small diameter vias have been suggested as a possible solution to these problems.
The ERNI ERmet zero XT connector uses surface-mounted signal contacts and through-hole ground pins (see Fig. 4). FCI has announced a ball-grid-attached version of the AirMax VS connector, while Teradyne is launching their Ventura high-density connectors with surface-mount termination. Tyco Electronics recently announced the Z-Pack HM-Zd high-density (HD) extension of the Z-Pack HM-Zd family, which will be available in both compliant-pin and surface-mount versions. Other connector suppliers are expected to develop alternative attachment mechanisms such as compressive contacts for their high-speed interfaces, especially as bandwidth requirements exceed 12 Gbit/s.
 FIGURE 4. The ERNI ERmetzeroXT features SMT termination that can accommodate 10 Gbit/s signals. |
Most card-cage systems today feature traditional backplane architecture with the backplane located at the rear of the card rack, and daughtercards plugging in from the front. An alternative is the orthogonal or cross-connect design that employs a midplane, with daughtercards plugging in from both sides. Since the front and back daughtercards are typically oriented 90° to each other, signals can be passed directly from one daughtercard to another without the losses associated with interconnecting traces on the midplane. High-speed signal integrity is degraded as the length of the signal path is increased, so keeping these interconnects as short as possible is a real advantage in some systems.
Properly designed, the midplane becomes a mechanical support structure for the connectors and the only copper traces on the midplane are for low-speed signals and power distribution. In some cases, the midplane can be eliminated entirely, and replaced by a metal frame that supports the orthogonal connectors. Orthogonal designs have been used in selected applications for years, but they can create real challenges in daughtercard access as well as thermal management. Recent increased interest in the performance advantages of midplane architecture has stimulated new connector development efforts at several of the leading suppliers, which will likely be announced in 2006. Other suppliers are closely monitoring this market potential for future development.
Mezzanine connectors are becoming more common as a way to provide a greater degree of user customization based on standardized building blocks. The PMC mezzanine card standard is now being supplemented by the new Advanced Mezzanine Card (AMC) specification to support new generations of equipment. Several suppliers, including Tyco and Teradyne, have expanded their backplane connector family by developing vertical receptacles that enable high-speed stacking applications. Other manufacturers such as FCI offer the MEG-Array and GIG-Array mezzanine connectors that also feature BGA attachment for high-speed performance. Samtec is another supplier who offers a wide range of mezzanine connectors capable of multi-gigabit speeds.
Connector durability has been an issue for years, as a single bent pin in a backplane can shut a system down and necessitate costly repairs. As signal densities continue to increase, the resulting smaller pins become more susceptible to damage. Connector suppliers have responded with more durable alloys, shaped features in the pin, and better pre-alignment hardware. The Tyco MultiGig RT connector family addresses this concern by replacing the pin and socket with a card-edge design. The new Aptera connector from Teradyne also uses a two-piece edge connector configuration to add robustness to the separable interface (see Fig. 5).
 FIGURE 5. The design of the new Aptera connector conserves board space while addressing durability of the separable interface. |
The applied cost per mated line, which includes the impact on the PCB design and layer count, remains a key determinate in the selection of any connector today. Prices in the high-speed backplane connector market continue to slide as users are squeezed between demands for greater performance and lower costs. Some users continue to design in standard 2-mm unshielded connectors that have become commodity products. Designers that require relatively few high-speed lines and do not have stringent density requirements often find these connectors a low-cost solution for at least the short term. Assigning more ground pins can push these interfaces to speeds well past what they were originally designed for.
Development of high-performance connector systems has become a costly and time- consuming process requiring state-of-the-art resources in design, materials, simulation, measurement, and fabrication. Characterization of multi-gigabit systems is evolving from simple time-domain reflectometry and single-line crosstalk measurements to the use of S parameters and analysis of eye diagrams. Large OEMs have become accustomed to demanding nearly custom interfaces that require the tooling of product extensions with a limited customer base. At this point, many of the leading connector players have chosen to participate either as an originator or second source, but the cost-to-reward ratio may cause some suppliers to rethink this strategy. As the price per mated line continues to drop along with available market share, this specialized market segment may be left to those suppliers who have committed the resources necessary to stay in the game.
Industry leaders will gain valuable expertise as they continue to address the issues of greater bandwidth, increased density, durability, surface-mount termination, and thermal management. These characteristics can be applied to other connector types, providing valuable competitive advantages across their entire range of products. As even commodity interfaces ramp up their speeds, the ability to develop new interfaces to support greater bandwidth may result in a two-tier connector industry consisting of those with and those without high-speed design capability and product offerings. This may ultimately accelerate supplier consolidation long anticipated within this splintered industry.
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BOB HULT is the director of product technology for Bishop & Associates. He can be reached at 1691 Vigilante Ave., Bailey, CO 80421-1053; Tel: (303) 816-5589; Email: r.hult@worldnet.att.net.
