The Ripple Effect: Mezzanine Connector Options
Proliferate to Meet Increased Bandwidths
By Grace Showers, David Sideck, and Stephen Smith, FCI

Advancements in mezzanine connector design are being driven by increasing demands for more communications bandwidth for data center networking. Many of today’s rack-mount server, storage, and switching equipment that use a high-speed backplane or midplane also have a need for high-speed connections to a mezzanine card attached to one or more of the blades or line cards in the system. These mezzanine cards are commonly used to enable optional features, squeeze additional functionality or capability into an individual card slot, or provide a direct connection between boards in adjacent slots. An example application is shown in Figure 1. As for backplane connectors, system designers likewise demand excellent signal integrity performance when high-speed signals are routed through a mezzanine connector in a link.

Figure 1: Computer blade with an optional mezzanine card that can be used
to provide additional high-speed fabric connections (e.g., FibreChannel, Infiniband,
or Ethernet) to a midplane in a blade server chassis. Image courtesy of FCI

System designers are also faced with size constraints posed by rack sizes that aren’t getting any bigger, connector crosstalk concerns at tighter contact pitches, and power and cooling concerns resulting from a greater number of channels. Many equipment designers now consider increasing the channel speed from 10 Gb/s to 25 Gb/s per lane to be a more viable path to increased bandwidth density, rather than simply continuing to scale the number of 10 Gb/s lanes in backplane connectors and I/O ports. To that end, various industry organizations are now discussing the development of specifications to enable higher-speed signaling.

One such organization is the Optical Internetworking Forum (OIF), which aims to foster the development and deployment of interoperable products and services for data switching and routing using optical networking technologies. Recognizing the need for the entire component-level infrastructure to be upgraded to support higher system capacity, the OIF initiated the Common Electrical I/O 25 Gb/s (CEI-25) and 28 Gb/s (CEI-28) projects. These ongoing projects will define electrical specifications for up to 28 Gb/s signaling for short-reach chip-to-chip or chip-to-module applications, and 25 Gb/s signaling for long-reach backplane applications. These signaling rates will allow the development of narrower 4x 25 Gb/s interfaces in place of 10 x 10 Gb/s interfaces for 100 Gb/s. Such interfaces will enable smaller package sizes, lower pin count components, connectors and optical modules, and lower power dissipation.

Other industry organizations at the forefront of advancing higher-speed interconnect technologies commonly used in e data center are the Infiniband Trade Association, IEEE 802.3 Working Group for Ethernet, and INCITS T11 Technical Committee for Fibre Channel. Mezzanine connectors currently being developed in anticipation of 25-28 Gb/s requirements are utilizing both proven technologies as well as innovative breakthroughs that offer increased electrical performance and design flexibility.

Connector Design Features to Extend Electrical Performance
Many currently available connector products utilize a series of ground-signal-signal patterns for differential pair pin assignments within a connector column. If an additional ground contact does not follow the last signal pair at the end of a column, this approach can affect the electrical performance of these signal pairs. Some high-performance connectors now under development add an extra ground pin to the column to provide a ground-signal-signal-ground pattern for every differential pair, and assure consistent performance characteristics for both the outer and inner pairs. It is also beneficial to stagger the position of the differential pairs in adjacent columns to help minimize differential crosstalk.

There are also inherent electrical performance advantages that result from the use of a ball grid array (BGA) connector attachment instead of a press-fit connector termination. With BGA attachment, a via can typically have a smaller diameter than a press-fit via. This is important for high-density, array-style connectors where larger-diameter vias can make the connector footprint low in impedance, which can cause reflection issues in the link—a typical 100Ω differential channel, for example. The greater the distance between conductors, the less impedance drop is seen through the footprint. A smaller-diameter via effectively increases the spacing between conductors and raises the impedance to better match a 100Ω channel. An impedance mismatch can be more problematic in shorter channels where the channel loss is not sufficient to dampen the resultant reflections, making it a significant concern for mezzanine connections.

BGA termination also enables easy and reliable attachment of array-style connectors using conventional reflow soldering processes. FCI has produced and shipped BGA mezzanine connector solutions for over 13 years and has demonstrated solder joint reliability of greater than 22 years of life when tested in accordance with IPC-9701, formerly IPC-SM-785. The use of a BGA connector termination can also offer advantages in trace routing. With the TwinMezz®  connector (Figure 2) from FCI, for example, a designer can route all circuit traces on three board signal layers for even the densest connector configurations, which have six differential pairs in a column. To facilitate automated parts handling and placement, one should specify connectors with pre-installed vacuum pickup caps that are packaged in JEDEC trays.

Figure 2: View showing the ball grid array on the underside of a TwinMezz® board-stacking connector
and the open-pin-field design at the mating interface. Image courtesy of FCI

Signal Integrity Performance in 28 Gb/s Channels
Some available mezzanine connectors use shield-less technology to deliver low insertion loss and crosstalk. One such example is the TwinMezz® mezzanine connector from FCI that targets 25G+ Gb/s applications comparable to an OIF CEI-28G-SR short-reach channel. To assess electrical performance, channels simulated here consist of 300mm total trace length with 150mm of trace on each side of a mated connector set positioned in the center of the link. The traces have a width of 5 mils, and reside on a board substrate constructed of Nelco® 4000-13 material. Two channels are considered here: one with a connector stack height of 12mm, and the other with a connector stack height of 38mm. The specific connector configurations considered have six differential pairs per column and a column pitch of 1.3mm.

The performance of each channel is compared to the four requirements as presently described in the current CEI-28G-SR Specification proposal. Figure 3 shows the Insertion Loss (IL), Insertion Loss Deviation (ILD), Return Loss (RL), and Integrated Crosstalk Noise (ICN) results for both channels. Note that the IL, RL, and ICN are consistent with the proposed requirements, which is a testimony to the excellent impedance control within the channel and the very low crosstalk for both stack heights. Although the ILD for both channels falls outside of the limits at the lowest end of the frequency scale, at and around DC, this is a result of the comparison of the ILD to a linear curve fit, and is not indicative of failure at high data rates. Furthermore, when the connector is completely removed, leaving only 300mm of trace, the channel still does not fall between the ILD limits at the lowest end of the frequency scale. A possible future means to address this issue might be the adoption of a different curve-fitting algorithm that allows the ILD to remain within limits, even at the lowest frequencies.

Figure 3: Insertion Loss, Insertion Loss Deviation, Return Loss, and Integrated Crosstalk Noise
for TwinMezz®  board-stacking connectors. Results for 12mm and 38mm stack heights are shown.
Image courtesy of FCI

Increased Signal Density
In addition to demanding improved high-speed performance systems to support future generations of industry-standard interconnect technologies, such as Ethernet, Infiniband, Fibre Channel, SAS, and PCI Express, equipment designers also want more compact mezzanine connector designs to conserve valuable board space and minimize obstructions to airflow. Mezzanine connectors can combine high-density designs with high-speed performance to offer increased signal throughput. There are signal modules configured with six differential pairs per column on a 1.3mm column pitch that provide the maximum signal density available in the industry today, delivering 25 high-speed differential signal pairs in a square centimeter, or 161 signal pairs in a square inch of board area. As shown in Figure 3, some combinations of density and stack height exhibit resonance-free crosstalk performance up to frequencies supporting bit rates of 28 Gb/s. Additionally, the use of an air-dielectric in some mezzanine connector designs provides the opportunity to further improve signal integrity performance at a given stack height by increasing the spacing of signal wafers to a column pitch of 1.8mm.

Importance of Design Flexibility
Mezzanine connector designs also need to anticipate potential needs for a wide range of board stack heights and circuit counts. Component height, clearance, and airflow requirements are important factors that a system designer must consider during the connector selection process, and their demands will impact the final decision. Figure 4 shows a few possibilities.

Figure 4: Connector stack height examples ranging between 12mm and 38mm.
Image courtesy of FCI

Some mezzanine connector systems can provide exceptional flexibility with options for molded-in or metal guides, integrated guides, and the capability to mix signal and power wafers in a single connector. Those with versatile, open pin-field designs (Figure 2) offer additional flexibility by allowing for mixed differential, single-ended, or power pin assignments within a single connector. Some designs also allow a system designer to choose between 1.3mm and 1.8mm column pitch, and among different signal counts to optimize signal density and electrical performance in a specific application. A flexible design also allows for future development of a slim-line version supporting two differential signal pairs per column. With proper orientation of the connector with respect to the direction of airflow, the reduced connector cross-section from the thinner form factor can be used to minimize obstructions to improve airflow and cooling efficiency. These features make mezzanine connector systems robust, reliable, and configurable enough for an incredible variety of mezzanine board-stacking applications.

To address applications that direct more power to a mezzanine card, some systems are capable of integrating power contact wafers and high-speed signal contact wafers within a single connector assembly. The optional power wafer contains two power contacts, each with 10 receptacle beams assuring multiple independent points of contact at the interface. The power wafer is rated to carry 19 amps, or 9.5 amps per contact. Designers benefit from the flexibility these wafers give them to customize mezzanine solutions to their specific needs by mixing signal and power wafers in one connector.

The industry trends driving increased I/O port and backplane connector bandwidth density are also impacting mezzanine connector requirements. Today, mezzanine connector systems are being asked to provide superior electrical performance at higher data rates, with higher signal density, and with the lowest insertion force compared to previous connector generations. Some innovative designs offer industry-leading signal integrity performance, making them capable of meeting the design requirements mezzanine applications demand at channel speeds in excess of 25 Gb/s. As industry standards continue to evolve, mezzanine connectors will remain an excellent choice for designers looking for flexibility, reliability, and performance in high-speed applications.

Nelco® is a trademark of Park Electrochemical Corp.

Grace Showers is the global product marketing manager for FCI. She can be reached at grace.showers@fci.com. David Sideck is FCI’s global marketing manager, and can be reached at david.sideck@fci.com; and Stephen Smith is a staff signal integrity engineer at FCI. His email is stephen.smith@fci.com. Visit FCI online.