High-Speed Backplane Design:
Higher Speed, More Density, Power Delivery
By Terry Jones and David Sideck, FCI

Much recent publicity has highlighted activities in industry organizations, such as the Optical Internetworking Forum (OIF) and the IEEE 802.3 Working Group for Ethernet standards, working toward the development of specifications to address future needs to scale high speed differential signaling from 10 Gb/s per lane to 25 Gb/s per lane (technically 25.8 Gb/s after allowing for overhead from 64/66 bit encoding) to enable a narrower 4 x 25 Gb/s link for 100 Gb/s over a backplane. While the next milestones on the roadmaps for other common data center interconnect technologies may be less demanding of the connectors, board materials, and other components in the channel, some legacy backplane connector systems may be challenged when data rates move beyond 10 Gb/s or 12.5 Gb/s. Some examples are 16G Fibre Channel and Infiniband FDR (Fourteen Data Rate) interconnects, where next-generation signaling falls between 14 and 15 Gb/s, and Serial-Attached SCSI (SAS), where 12 Gb/s is planned as the next step. While backplane connector technologies under development will demonstrate to the system designers the capability to develop and transmit data at very high speeds (25 Gb/s and beyond), the reality is that there will be interim steps that will be taken on the path to 25 Gb/s and beyond.

Evolutionary Path
Although equipment designers have the freedom to choose from among all available high-speed backplane connector technologies for entirely new platform designs, in other instances it may be preferable, advantageous, or perhaps even necessary to employ backwards mating-compatible interfaces to current backplane connector designs. The mating-compatible interfaces and capability to preserve critical pin assignments can provide opportunities for cost savings as new or upgraded equipment is deployed. For example, a backplane or chassis can be designed to allow the installation and continued use of legacy daughtercards, line cards, or blades that are already in the field, as well as new or future higher-speed module cards.

At the point when an equipment design requires both mating compatibility and electrical performance that extends beyond the limits of a legacy connector system, connector suppliers are challenged to implement connector design improvements to enhance the high-speed electrical performance of an existing connector system to cater to those needs. Just such a challenge from some customers led to the recent development of the AirMax VSe™ connectors by FCI (Figure 1). The new connector utilizes the same revolutionary AirMax VS® “shieldless” technology, which allows for flexible pin assignments and no common grounds within the connector, and provides the enhanced SI capabilities required for the new and future 12.5 Gb/s to 20 Gb/s chipsets. Right-angle receptacles and right-angle headers will support backplane, midplane, coplanar, or orthogonal midplane applications, and will provide users of the proven AirMax VS® connector system a migration path beyond 15 Gb/s per differential pair.

When introduced in 2003, AirMax VS connectors broke the dependence on metal shields and allowed the connector industry to accomplish superior high-speed electrical performance to 12.5 Gb/s, while providing design flexibility to system designers. That backplane connector technology advancement made it possible for individual contacts in a connector module to be allocated to differential signal pairs, single-ended signals, or low-level power as dictated by the system need. In addition, column spacing could be easily increased to enable more signal traces to be routed on a board layer, trading some signal density for reduced layer count and lower board cost for those applications that do not demand maximum signal density. At that time, operating data rates of 2.5 Gb/s or 3.125 Gb/s were not uncommon in production systems, and connectors designed to support signal speeds up to 12.5 Gb/s offered ample headroom to enable succeeding generations of equipment.

AirMax VSe connectors preserve the flexibility of the open-pin-field design and combine FCI technologies for a shield-less design with no metallic plates, closely edge-coupled differential pairs with innovative design improvements, and minimal changes to connector footprints that yield low loss and crosstalk. The measured data in Figure 2 was taken with a mated pair of connectors assembled onto small Nelco4000-13SI component test boards with 3.2-inch traces on either side. Individual crosstalk contributors to each pair from all adjacent pairs were measured, and power-summed crosstalk was calculated for each pair. The crosstalk values in this figure represent the connector’s maximum crosstalk (the worst-pair result) at each frequency.

Cost is another important consideration when system designers decide what evolutionary path to follow to enable their systems to meet increasingly stringent electrical performance requirements. By their nature, many high-speed research and development programs focus on the technical aspects of change. With the help of new board materials and/or specialized prototype components, models and simulations can be run to help the designer choose a future path and technologies for future systems. However, the commensurate higher costs of higher-performance board materials, connectors, and other components may not prove to be economically feasible when measured against cost targets that are dictated by the need to offer an acceptable and competitive market price for a commercialized system. The manufacturing reality is that the costs for the higher speed technologies must continue the downward trend for “dollars per gigabit” transmitted, and so the cost of the new technologies must also be taken into consideration. Enhanced high-speed connector solutions can enable designers to upgrade and extend the life of current product platforms while enabling some re-use of lower-cost PCB materials and technologies to help satisfy this requirement.

Orthogonal Midplane Technology Emerges to Simplify “Backplane” Connections
With the emergence and increasing adoption of orthogonal midplane system architecture, more communications systems designers are going one step further by adopting this packaging scheme to accomplish direct, efficient connections between multiple line cards and a common switch or communications card. With this architecture, vertical daughtercards on one side of a midplane have a direct connection to horizontal add-in cards on the opposite side of the midplane. The vertical headers are designed to be installed back-to-back and oriented at 90 degrees to each other. The headers’ signal pins share the same vias in the midplane, providing a direct, high-speed connection while eliminating the need for connecting traces on the midplane. Reducing the number of midplane signal traces also allows designers to reduce the complexity of the midplane design. The reliance on direct high-speed connections between add-in cards may also permit less costly and less exotic board materials to be used for the midplane. Some orthogonal midplane interconnects can support differential signaling at up to 20 Gb/s.

Currently, the architecture is most often found in modular data center switches due to the front and rear panel accessibility of those systems, as well as the ability to drive high speeds and improved signal integrity as a result of simplified backplanes. Orthogonal midplane technology also lends itself to high-density systems that can be difficult to implement in front of a backplane, due to space or size constraints imposed by standard 19-inch rack dimensions. Placing the switch blades behind the midplane can free space for additional port blades in front of the midplane.

With the ZipLine™ connector system, for example, FCI leveraged the proven shield-less technology pioneered with its AirMax VS^® backplane connectors to significantly increase density for high-speed orthogonal midplane applications. With six differential pairs per wafer on 1.8mm column spacing, the connector system provides 84.6 differential pairs per inch and up to 72 orthogonal crossover pairs, as shown in Figure 3.

Delivering More Power to Blades
With more multi-core processors and more memory on computer blades, and growing port density on switch port blades, additional power will need to be delivered to these add-in cards in a chassis. This need can be addressed either by integrating higher-power contacts in signal connector modules or by installing separate high-power connector modules on the card edge. Some high-speed connector designs provide the capability to include optional power wafers in a signal module to provide both signal and power contacts in a single connector. One such design (Figure 4) features a special six-contact power wafer, rated at six amps per contact, with the combined capacity to deliver up to 36 amps when a single power wafer is included in a six-pair (six differential pairs per column) connector module. Higher power delivery requirements can be addressed by adding more power wafers, but in doing so the current-carrying capacity needs to be de-rated accordingly. Separate modules (Figure 5) with high-capacity power contacts rated at 20 amps, 40 amps, or 83 amps per contact, and offering options for two contact lengths, address applications requiring higher linear power density or sequential mating of power and ground contacts.

Communications and networking equipment designers will continue to demand faster data links, so that even the latest backplane connector technology advances will continue to evolve, driven by the demand for ever-increasing bandwidth and density. Rack-mount servers, blade servers, external storage systems, and supercomputers will also be affected as lane speeds for PCI Express, Ethernet, Fibre Channel, SAS, and Infiniband continue to climb higher. Leading suppliers of high-speed backplane connector systems are continuing research and developments efforts that will yield both new products and product extensions offering enhanced electrical performance.

Higher speed, more density, power delivery. Adequate coverage of those topics demands more detailed discussion than we have time or space to offer in this article. For a more in-depth look at the mechanical and electrical solutions available from FCI please visit the FCI high-speed microsite, where you can find our latest white papers and find more technical information on high-speed backplane connector solutions.

For more information on FCI’s AirMax connector system, contact Dave Sideck at david.sideck@fci.com, or Terry Jones at terry.jones@fci.com.