Long Live the Copper Connector
By Bob Hult, Bishop & Associates Inc.

The march to higher speeds and system packaging density continues to drive innovations in the connector world. Not long ago, engineers anticipated that copper connectors would be incapable of supporting multi-gigabit data transfer rates, forcing the transition to fiber optic alternatives. However, like many premature death predictions, copper outlived those estimates and beat the odds; now new copper backplane, mezzanine, and cable connectors offer capability to 10 gigabits and beyond.

To some degree, connector design has been refined to reduce distortion as the signal propagates through the interface. Isolation between differential pairs has been improved, reducing crosstalk, while skew has been essentially eliminated by careful pin assignment, or by the use of special plastic design and materials in the connector body.

The connector to printed circuit board transition had been recognized as a greater source of signal distortion than the connector itself. Stubs created by the barrel of the plated through-hole generate crosstalk and resonances that distort higher speed signals. Connector manufacturers responded by reducing the length and diameter of their compliant pins, while recommending the removal of the barrel below the point of contact to the signal layers in the board. New footprint designs further isolate signals, minimizing noise.

At the same time, advances in transmitter and receiver chips with integrated signal equalization features have extended the effective length of channels and greatly improved signal integrity. Connectors originally rated to three Gb/s are now being recommended in applications operating at six to 10 Gb/s

The industry had expected to adopt exotic PCB laminates to achieve reliable high-speed channels, but we’re finding that lower-cost standard or enhanced FR-4 PCB materials can often meet their system requirements.

Increasing demands for power have also influenced new interface development. Connectors are expected to be part of the thermal management strategy, and new lower-profile connectors offer reduced obstruction to cooling airflow. Most high-speed board-to-board connectors include integrated or compatible power modules.

Designing Now, for the Future
System designers are taking a long-term approach to new electronic product development. They are looking for connector families that satisfy immediate as well as next generation product requirements. Interconnect families that offer performance “headroom” provide a long-term migration path without making costly changes to the boards. The immediate need may be for a connector that supports 10 Gb/s data rates, but the next generation of equipment may require 16 or 20 Gb/s performance. Selecting a connector rated to 20 Gb/s provides plenty of design margin for the current system. Experience gained in the design process will speed the development of the next generation product, and use of the same connector minimizes changes to PCB design and established manufacturing processes.

Designers also want a complete interconnect system solution, rather than be forced to use interfaces from a variety of suppliers. The ability to offer backplane, midplane, orthogonal, and mezzanine connectors within the same family is a major factor in new system design. High-performance connectors today are typically released as a family of product, including low-speed, high-speed, and power options. In some cases, connector manufacturers have created good, better, and best versions within the same product line, giving engineers the ability to closely match system needs with the lowest-cost interface. At the same time, OEMs do not want to be captive to a single source for their selected interfaces. Over the past two years, leading connector manufactures have responded by cross-licensing their high-performance connector systems that are both mechanically and electrically identical. Designers can be assured of consistent supply as well as competitive prices.

High-speed connectors are also responding to technical changes emerging within the industry. Emerging standards, such as PCI Express gen 2 and the Intel Quick Path Interconnect, have adopted a system impedance of 85 ohms. This is a significant departure from the traditional 100-ohm differential impedance that has ruled since low voltage differential signaling became the transmission system of choice. Since every element within the channel must be tuned to the same impedance, connectors designed for 100 ohms are now in the process of being modified to offer both 100- and 85-ohm versions.

Recent announcements of new connectors illustrate the continuing evolution to higher performance connectors.

Amphenol TCS continues to expand their XCede backplane connector family with the addition of an 85-ohm version as well as compatible cable assemblies. Amphenol TCS currently offers a total of 10 backplane, five mezzanine, three co-planar, and one orthogonal connector family that support a variety of high-speed data rates and signal densities.

ERNI has upgraded their ERmet Zd connector with the introduction of the ERmet ZD PLUS version that boosts the published bandwidth to 20 Gb/s.

Additional suppliers are working on “plus” versions of their flagship high-speed interfaces.

FCI Electronics has been beefing up their high-speed interfaces with the introduction of the 85-ohm AirMax VS, and a coplanar option to their ZipLine connector system. A unique orthogonal configuration of ZipLine has also been developed.  

FCI is also expanding their SAS/SATA, and DensiShield cable assembly offerings.

Both Gore and Leoni have developed advanced high-speed cables using unique materials and construction that extend the limits of conventional cable.

Hirose USA announced the IT-3 mezzanine connector that utilizes an interposer assembly which provides a simple method of adjusting the stack height between the host and mezzanine boards. This connector is rated to 10 Gb/s, with the potential of moving to 20 Gb/s with a lower signal density footprint.

 

 

Molex is working on their ZXP interconnect technology that promises to minimize high-speed signal distortion created by internal grounding structures. Their Solder Charge technology improves the reliability of surface-mounted connectors. Molex recently announced the addition of an 85-ohm version of their popular IMPACT backplane connector family.

Samtec is actively expanding their extensive lines of board-to board and cable-to-board connectors with a focus on density and speed.

Tyco Electronics recently announced two new high-performance connector families that reflect industry demand for greater design headroom as well as advanced mezzanine connector performance.

The Strada Whisper backplane connector pushes the limits of copper with a series of improvements that allows the connector to be rated to 40 Gb/s.

The Strada Mesa mezzanine connector features 15 Gb/s performance and integrates low speed, high speed, and power contacts in a single housing.

Even standard commodity interfaces are being upgraded. The ubiquitous Universal Serial Bus 2.0 connector will soon be replaced by a new 3.0 iteration that will take the interface from 480 Mb/s to 4.8 Gb/s. Intel demonstrated a working prototype of USB 3.0 at the 2009 Consumer Electronics Show in January.

Demand for faster and denser connectors continues to drive the development of new copper interfaces that satisfy anticipated performance requirements. The result is delaying the day when fiber optic interfaces will be required.

Bishop & Associates comments:

• Manufacturers of high-speed connectors continue to find ways to expand the useful bandwidth of copper connectors. Fine-tuning the connector and the use of advanced signal conditioning technology have extended the life of existing connector families and stimulated next-generation interfaces.

• Many new high-performance connector families have been announced over the past two years in anticipation of new applications moving from design to large volume production.

• Designers prefer to stay with familiar copper interconnect technology and are developing channel design, simulation, and board layout expertise that continue to push the bandwidth to 10+ Gb/s.

• Design and simulation tools are evolving to support 10+ Gb/s channels.

• New backplane and mezzanine connectors that feature bandwidth headroom offer more design margin and a migration path to anticipated performance upgrades.

• Connectors announced or tooled offer performance from 10 to 40 Gb/s. Standards now in development define 40 to 100 Gb/s channels. Few engineers are willing to predict the ultimate performance limit of copper interconnects.

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.