The Evolution of Contact Design
By Bob Hult, Bishop & Associates Inc.

Electronic connectors are unique in the world of passive components—they must perform both electrical and mechanical functions. Unlike many other circuit elements that are buried deep within the box, such as capacitors and resistors, connectors undergo stress from thermal cycling and multiple mating cycles. The connector in a backplane assembly is often part of the mechanical support system for daughtercards. In external applications they must be able to resist nasty environmental conditions—in both the mated and unmated condition—and may be required to provide EMI protection while being ergonomically and esthetically pleasing. Designing an effective connector requires a delicate balance between potentially conflicting electrical and mechanical requirements.

The heart of any connector is the contact system it employs. The quality of the connector is measured by its ability to create and maintain a low resistance electrical connection. At a very basic level, the job of the contact system is to establish a metal-to-metal contact between two conducting surfaces. What may appear to be two smooth surfaces are actually very lumpy contours with peaks and valleys on the plated surface. Ideally, connector contacts should be creating broad areas of intimate contact, but in the real world, only tiny points of contact are actually established. Current flowing across the interface is restricted to these relatively small areas, resulting in greater resistance and the potential for localized heating of these spots. The objective of the contact engineer is to create as many of these points of contact as possible, while addressing mechanical concerns. Increasing the normal force or perpendicular pressure between the contacts can flatten these contact points. This results in a greater number of contacts and broader contact areas, but insertion and withdrawal forces may become excessive. Increasing this pressure can also result in greater penetration of the plating and exposure of the underplating, greatly reducing the mating cycle-life of the interface.

Connectors have utilized a series of basic contact designs that have proven to be both reliable and manufacturable. Contact styles—tab and receptacle, pin and socket, and beam-on-blade—have been utilized in connectors for many years. Within each type, there are variations on the basic design. Pin and socket connectors can include traditional screw-machined cylindrical pins mating with hollow spring-loaded sockets, and stamped-and-formed box receptacles that mate with square solid or formed pins. Beam-on-blade contacts can include a tuning fork design, bifurcated bellows, or cantilevered ski-tip type contacts, all mating with a flat surface.

The termination of the contact to either a wire or PCB has also slowly evolved. Discrete contacts were originally soldered to wire, but eventually, improvements in quality, productivity, and repeatability drove the transition to crimped contacts. Connectors mounted on a PCB have slowly evolved from wave-soldered to compliant-pin termination.  Advances in system packaging density, demand for greater power consumption, and high-speed performance are beginning to stimulate the development of enhanced or new contact forms; these new contacts go beyond the capabilities of standard designs to satisfy these emerging demands.

Backplane connectors optimized for multi-gigabit transmission have evolved from 0.1” and 2mm centerline open-pin field connectors to a tightly controlled impedance differential pair configuration, often with internal shielding structures to improve isolation. More recently, several connector suppliers, including Tyco Electronics and FCI Electronics, have introduced shieldless high-speed connectors that use the dielectric properties of air or plastic to reduce weight, complexity, and cost of these high-performance interfaces.

Many of these connectors utilize a conventional beam-on-blade contact interface, but Amphenol TCS recently introduced the Ventura™ connector, which features a unique matrix of plastic posts with contact beams located on both sides.

Unlike most new backplane connectors, which are designed for differential signaling, the Ventura is focused on single-ended signaling applications in the high-end server market. The contact configuration allows packaging density of up to 178 signal contacts per inch.

The Ventura connector is also one of the first large backplane connector systems to utilize a paste-on-pad surface mount attachment to the PCB.

To a large degree, the venerable edge connector contact has been replaced by two-piece contact systems, mainly due to concerns about reliability. The quality of an edge connector interface is dependent on the quality of the mating PCB pad, including plating, location accuracy, and board chamfer, none of which are within the control of the connector supplier. Two-piece connectors solve this problem. Edge connectors were being relegated to low-tech/low-cost applications but we may be seeing a reversal of this trend.

The Advanced Mezzanine Card and Micro TCA backplane specifications from PICMG define a high-performance edge connector.

Although appearing to be conventional edge connectors, they are the result of extensive high-speed simulation and analysis, and feature a bandwidth as high as 12.5Gb/s.

Continued concerns about resistance to the one-piece edge connector will likely result in the development of a right-angle male daughtercard header, but the lower cost of the one-piece design will keep it as a viable option in many applications.

The new Aptera™ connector from Amphenol TCS is another departure from the traditional pin-and-socket contact design. In order to achieve the objectives of high-speed, packaging density and low-profile, a two-piece edge contact design was chosen. This modular connector system was specifically designed to offer a low profile, which allows a minimum slot pitch between daughtercards of 10mm, but also minimizes resistance to the cooling system airflow.

The daughtercard header features a straddle mounted surface soldered attachment system.

The vertical backplane receptacle utilizes compliant pin termination.

The L Series version of this connector is rated to 6Gb/s, with a signal density of up to 46 signals per inch.

Advanced high-speed edge connectors are now appearing in the personal computer market in the form of the PCI Express (PCIe) interface.






Bandwidth scalability is achieved by utilizing four standard connector sizes. Based on a 2.5Gb/s data transfer rate per lane, designers have the option of providing a mix of sizes based on anticipated system requirements. Contacts are located on 1mm centerlines and are available in 36, 64, 98, and 164 position configurations. Both vertical and right-angle entry versions have been tooled.

The transition to PCIe technology has begun, as new PC motherboards are being equipped with a combination of both legacy PCI and the new PCIe edge connectors. Graphics-intensive applications will particularly benefit from the enhanced capabilities of PCIe enabled systems.

Additional examples of high-performance connectors using an edge interface include HSSDC2, SFP, and XFP connectors.

Power connectors have traditionally been (physically) large interfaces with limited options for termination, pin count, and design flexibility. Existing power connectors tend to be proprietary designs, with relatively high cost per line. The introduction of the Server System Infrastructure (SSI) specification has stimulated the introduction of new modular power connectors that feature high performance ratings in a compact package.

The MulitBeam XL™ connector from Tyco Electronics, as well as similar designs from other suppliers, are examples of this new direction in power connector design. This connector features the ability to hot-mate, and includes options for signal contacts.
 

The power contact design is a folded blade that creates large contact areas to assure a low-resistant joint. This contact can be either solder or compliant pin terminated to a PCB.
 

RADSOK® technology from Amphenol updates traditional pin and socket contacts by adding an internal metallic structure. This generates multiple points of contact between the surface of the mating pin and the receptacle.

This results in the ability to increase the current rating of a standard contact by 50 percent, while maintaining low mating forces.

As electronic system speeds continue to increase, every minor profile variation in the circuit path can have a negative effect. The high-speed connector industry is currently focused on minimizing the lump capacitance created by the “stub” in plated through-holes terminated with compliant pin connectors.

 

The connector launch has been recognized as a greater source of signal degradation than the connector itself.  Alternative connection schemes, including surface mount and compressive connections, are being investigated. The current solution is to counterbore the lower portion of the PTH.

The separable contact itself is the next logical area to explore new designs that minimize impedance variations and stubs. Silicon Pipe, based in San Jose, California, has been investigating new contact designs that may improve high-speed connector performance. Traditional contacts often include features that create stubs and reflections.
 

Curved conductive elements create low-profile, redundant compressive connections with no stubs, while insuring adequate contact surface wipe to expose clean conductive surfaces.

Using a torsionally induced contact force beam, direct compression contact can be established between perpendicular surfaces.

Work is currently underway on new contact designs utilizing advanced alloys combining greater spring characteristics, as well as bulk conductivity.

Foxconn is developing a new orthogonal midplane connector concept that establishes direct board-to-board connections eliminating the midplane entirely.

Advanced particle interconnect systems can offer extremely dense connections without the need for wipe, opening the potential for unique new separable configurations.

Plated plastic contacts may offer particular advantages in high-speed circuits, where signal propagation occurs almost entirely on the surface of the conductor.

Enhanced or entirely new contact structures will likely move into mainstream production applications as evolving performance requirements push existing contact designs to their limits.


Bishop & Associates Comments:

• New connector and contact designs now in development within connector suppliers, as well as independent IP sources, are responding to user demands for higher performance interfaces.

• In some cases, existing contacts can be enhanced to improve performance.

• Edge card connectors are beginning to appear in several new high-performance applications, likely the result of pressure to reduce system component costs.

• The geometry associated with traditional contact designs may become a limiting factor as demand for greater bandwidth and system density increases.

• Innovative new contact designs, including surface mount, compression, plated plastic, and particle interconnects, may find greater acceptance over the next few years.

• Both connector manufacturers and designers must develop new test methods capable of verifying long-term reliability of non-traditional contact and attachment mechanisms, comparable to existing connectors.

Robert Hult
Director of Product Technology, Bishop & Associates, Inc.
Robert Hult has been in the connector industry for over 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, Chandler, Arizona, U.S.A. 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.