High-Speed Back and Midplane Connectors Continue to Evolve
By Bob Hult, Bishop & Associates Inc.

Connectors that provide links between daughtercards mounted in a rack have come a long way over the past 20 years. Connector construction has evolved from simple one-piece edge connectors to two-piece open pin field configurations that provide higher pin counts and reliability.

The current crop of high-speed backplane connectors are highly sophisticated components consisting of molded contact wafers, some with integrated metallic shields to control crosstalk. These wafers offer tightly controlled impedance, as well as design and manufacturing flexibility.  













These advances have enabled connectors to leap from megabit to multi-gigabit performance as the industry transitioned from single-ended to low- voltage differential signaling. The industry has very quickly transitioned from 3.125 Gb/s to systems that operate at 10+ Gb/s.

The race among the leaders in the high-speed backplane connector segment appears to be highlighting several predominate themes.

1. Performance scalability to closely match requirements of current and future iterations within the same connector family. Within each connector family a series of good, better, and best variations are offered that provide somewhat higher/lower bandwidth, increased signal density, or lower cost. Backward compatibility allows upgrades without changing PCB artwork. In some cases, the right-angle daughtercard receptacle has been upgraded to allow a boost in performance of backplanes in existing systems. Connectors that utilize advanced PCB footprint routing offer another variation. Bandwidth bump continues as connectors originally designed for 5-6 Gb/s have morphed into 10-12+ Gb/s connectors.  

2. Modular connector assemblies that allow nearly custom configurations using a collection of standard components. Insert molded contact wafers are able to maintain tight control of dimensions, essential for signal integrity at higher speeds.  Pin counts can be adjusted by selecting the appropriate number of wafers. Wafers designed for high-speed, low-speed, and power can be easily combined to satisfy very specific applications. Design engineers win, as they are able to fine-tune their design to match their specific needs.  

3. Advanced backplane connector families often include a full range of packaging options, including mezzanine, midplane, orthogonal midplane, and card extender configurations. Designers can package the entire system with compatible products, simplifying the design, documentation, and manufacturing process.

4. Demonstrated performance headroom to provide confidence that connector technology will be able to keep pace with future anticipated bandwidth needs. Backplane connectors rated to 40 Gb/s are currently on the market, although broad market need for connectors operating at this speed may be three to five years into the future. We are currently in the process of transitioning from 5-6 Gb/s to 10 Gb/s systems. Achieving reliable 10 Gb/s channels is not a trivial accomplishment and many OEMs are still in the learning process. It appears that 25 Gb/s systems may be the next target, which will present additional challenges to every aspect of the system design.

5. Suppliers are utilizing advanced manufacturing techniques to achieve higher published bandwidths. In some cases, suppliers are utilizing multiple dielectric materials to fine tune resonance or specialized geometry to minimize skew. Several of the highest performing backplane connectors are integrating capacitors into the contact wafer for DC blocking, resulting in improved noise margin. This also frees up valuable space and simplifies routing on the PCB.

6. Mechanically and electrically identical second sources that assure supply at competitive prices. Although they may not be announced at the introduction of a new interface, advanced connectors are often the results of early collaboration between two suppliers. This is less true for some older connector families, but becomes critical when major OEMs select a connector for a device that is expected to evolve over a long product life.

7. Although not universal, connector families are often offered in both 100- and 85-ohm differential impedance. Most suppliers are seeing increased interest in 85-ohm connectors, which not only matches the impedance of emerging silicon chips, but also offers the potential of thinner PCBs and less complex trace routing. This adjustment requires a change in connector materials and/or geometry, which may be costly. Those product families that are not currently offered in 85-ohm versions will likely be made available in both impedances as customer demand builds. 100 ohm is expected to dominate the market well into the future. 

8. Interest in orthogonal midplane architecture is growing. Most backplane suppliers offer an orthogonal option, which can effectively shorten the length of signal traces between daughtercards, reducing distortion created by the backplane. Molex recently introduced an orthogonal direct connector where daughtercards mate orthogonally with each other, eliminating the midplane entirely. Without the midplane blocking cooling airflow, the thermal strategy of the system is simplified, but issues of mechanical alignment and power distribution introduce new challenges. 

9. Defining a backplane connector by a published Gb/s bandwidth is a common measure used in the industry today, but it is increasingly becoming irrelevant. As system speeds rise, the optimization of every aspect in the channel, including PCB materials, plated thru-hole design, and trace routing, becomes essential. Many “high-speed” connectors can perform at 10+ Gb/s, if the link is sufficiently short and proper design rules have been applied, but noise margin that works in one circuit may result in unacceptable noise and crosstalk in another. A more relevant measure would be to quantify performance of a defined backplane link that is one meter in length and passes through two backplane connectors. More useful measures of a successful channel include levels of insertion/return loss, crosstalk, and noise.

10.  Leading suppliers in the high-speed backplane connector market have created an extensive set of technical support tools to assist customer implementation of these advanced interfaces. Application parameters of each application are unique and can have significant influence on the ability of the channel to perform to specification. Connector suppliers offer advanced signal integrity support including design support, circuit simulation, evaluation boards, and troubleshooting analysis, which is critical to smaller OEMs that may not have this level of internal resource.

11. The connector industry has probably reached the practical limit of high-speed signal density of backplane connectors. Connector contact centerlines could be reduced, but limitations on routing density make this impractical unless surface mounting becomes more widely accepted. Making the compliant pin smaller could reduce noise attributed to the PTH, but concerns about mechanical durability as well as aspect ratio become issues.

Industry-leading manufacturers of high-speed backplane connectors are gearing up to address the emerging roadmap to higher speeds being driven by portable devices, including smartphones, tablets, and high definition video on demand. The bandwidth required to support these devices is pushing the speed of backplane connectors in routers and servers to higher levels. Industry standards organizations such as Ethernet, InfiniBand EDR, PCIe Gen3, and Fibre Channel are scrambling to create updated standards that have become the vehicle that delivers this bandwidth in a uniform and verifiable format. Faster is better across many categories that range from military to consumer products. Designers will find use for increased bandwidth, which has become the expectation in next-generation products. New connectors are in the pipeline that will continue to address higher speed and better noise margin.  

Longer-term, the bandwidth roadmap appears to make reliable 25 Gb/s channels that extend 27” to 39” long the next target. It is unclear if traditional backplanes will evolve to orthogonal midplane architecture or if discrete cables plugging into the back of a backplane can deliver a more cost-effective, high-speed interconnect solution. Another approach would be to deliver increased bandwidth using multiple lower speed channels via higher density backplane connectors. Another potential solution could be the use Pam 4 signaling to keep bandwidth limits within comfortable levels.

After 25 Gb/s channels become mainstream, the outlook for copper becomes fuzzier. Channels that operate at 40 Gb/s per differential pair may be technically feasible, but they may not be the most cost-effective choice. The prices of components associated with fiber optic alternatives, including fiber, transceivers, and connectors, continue to drop. At the same time, advanced signal conditioning devices required to extend the reach of high-speed copper links adds cost and complexity. The physics of electronic and optical circuits do not change, but the economics of both are in constant flux.  

The following chart outlines some of the leading high-speed backplane connector products currently on the market. Stay tuned, as this list is expected to grow over the next few years.

Bishop & Associates Inc. © 2012