Potential applications include high-performance backplanes and sockets, and high-speed transmission line interconnections.
By JOSEPH FJELSTAD & GARY YASUMURA
Electrical interconnection technology is increasingly being recognized as the gatekeeper of the performance of electrical/electronic systems. In the broad view, electrical interconnection technology includes all of the various structures and technologies that are employed to reliably usher an electronic signal from one component or circuit element to another in an electronic system.
Thus, the family of electronics interconnection technologies includes various technologies: plated through holes, blind and buried plated vias, solder balls, wire-wrapped metal pins, metal springs and beams, micro and macroscopic wire welds, carbon nanotubes, isotropic and anisotropic conductive adhesives, and-curiously enough, under some circumstances-even non-conductive adhesives and pastes.
Perhaps the most recognizable of electronic interconnection technologies is the ubiquitous electronic connector, which makes all-important electromechanical connections between circuit board assemblies.
For many years, these vital pieces of electromechanical hardware wizardry have been reliably interconnected to PCB plated through holes, most often either by means of solder or by press-fitting compliant connector pins into the holes of precisely fabricated PCBs. With the advent of surface mount technology (SMT), it quickly became obvious to some connector suppliers that connectors could be designed to take advantage of the new assembly technology, and surface-mounted connectors came upon the scene. Then hybrids of these two assembly technologies followed in their wake.
 FIGURE 1. Traditional interconnections have involved bending beam-type contacts. Both pin-in-hole (left) and surface-mount configurations have been employed. |
These solutions have served long and well, but as the industry moves to ever-higher processor speeds and data rates, the venerable plated through-hole and the annular pads associated with them have a propensity to create capacitance and disrupt signal integrity. It has, in fact, been calculated that adding a 150-mil-long via stub to a signal trace will result in an additional 65 dB of further losses at 9 GHz, compared to a backplane trace with no vias (“Abracadabra: Making system interconnect disappear with FPGAs,” Electronic Design News, September 14, 2006).
Thus, electrical stub removal or avoidance is clearly quite important.
Traditional approaches
In recognition of and response to the problem, astute backplane circuit manufacturing engineers, armed with this better understanding of the fundamental issues, have developed techniques to help ameliorate both the capacitance problem and the electronic stub, which is a common source of signal reflections. One of the more common methods used for high-speed backplanes is to back-drill the holes; however, while today’s manufacturing tools are quite good, a nagging concern is that it is a secondary operation, prone to error.
Often, common electromechanical interconnection devices (i.e., connectors) incorporate resilient or spring structures that, even though separable, serve to maintain contact force at the point of connection between electrical components. The spring or resilient conductive material and the contact designs made with them may be compared and contrasted for their relative ability to produce the desired deflection for a given applied force suited to the application’s needs.
In general, spring conductor structures are ideally designed to:
- Establish and maintain sufficient mechanical contact force for the intended application;
- Require the smallest amount of deflection to attain this contact force;
- Have sufficient resiliency as to create little or no permanent deformation;
- Employ the smallest volume possible.
But in addressing each of these attributes, spring structures are often quite complicated and difficult to manufacture, particularly when they are very small. The complexity of resilient interconnection structures typically increases when electrical components are disposed at angles to each other, as this design typically necessitates curved or irregularly shaped interconnection structures. In addition, the bends and twists of a connector’s contacting elements can, like plating vias, also degrade signal integrity. And perhaps just as important in today’s ultra-competitive environment, such requirements can increase the connector cost.
In a basic setup in which an edge card connector is mounted on a motherboard, the connector is designed to accept a vertically oriented plug-in card that bends the spring conductors to produce the desired contact force to hold the card in place and establish electrical continuity. Conductors typically are cantilever beams bent near the ends to offer a curved contact surface to facilitate insertion of the daughter card. The fixed ends are attached to the horizontally oriented substrate.
The contact forces at the free ends of the cantilever beams are sufficient to bend the beams and make contact. Unfortunately, cantilever beams do not store energy uniformly throughout their length. Instead, the greatest stress of a beam (or stored energy per unit volume) is at the fixed end of the beam, and stress is lowest at the free end where the electrical contacts are located.
But if they were designed to store energy more uniformly throughout the conductors’ volume, the conductors could be smaller-a never-ending objective in electronics manufacture. A smaller size is also a goal of high-frequency interconnect designers who strive to increase the bandwidth of the interconnect device by shrinking physical interruptions and their related electrical constructs.
Torsion-based contacts
In response to the challenge, a new type of electrical contact force delivery, based on the principle of torsion, is now under development.
Torsion is a fundamental mechanical principle predicated on the twisting of a material as opposed to bending, which is common practice for nearly all standard connector contacts. Surprisingly, torsion has not seen much use in high-frequency applications even though it offers significant benefits.
 FIGURE 3. In an edge card connection, stress is largely concentrated on the base of the beam. |
In a torsion bar-type contact, the wire has a bent tip section at both ends. At one end, the tip section is bent so that it bends away from the axis of the torsion section to form a moment arm. At the other end, the bend is configured so that when both ends make contact to their respective contacts, they cause the length of the conductor to twist, creating a rotational force (torsion) within the conductor.
The advantage of torsion is that a torsion bar conductor can be made volumetrically smaller than the cantilever round wire beam of the same diameter, yet have the same electrical contact force when electrically mated. Another advantage is that torsion bar connector elements tend not to deform substantially during electrical mating. Thus, when a torsion bar conductor is an element of a transmission line, the transmission line’s characteristic impedance is also substantially unchanged. Moreover, the torsion bar conductor/contacts can be fabricated from drawn wire, the diameter of which can be held to precise tolerances so that the characteristic or differential impedance and the contact force remain within a small value range. The combined benefit is a structure that should improve high-frequency signal integrity.
From a manufacturing perspective, torsion bar conductor/contacts are comparatively simple to fabricate; a torsion bar conductor can be shaped easily in a four-slide bending tool, progressive die, or other manufacturing method, and placed on a holding reel for later assembly into connector housings. Torsion bar conductors are also much less sensitive to manufacturing-process variation associated with beam contacts formed by etching of metal sheet.
 FIGURE 5. Torsion contact and connector technology is amenable to the creation of arrays of coaxial connectors, illustrated here. |
For high-performance applications, the torsion connector is amenable to a virtually unlimited number of interconnections that are substantially coaxial. For example, a connector body may be molded or otherwise formed from an integral material or alternatively assembled from mating components. In a monolithic construction, the channel could simply be a through-hole in a material that serves as the connector body and through which the wires pass. The body on the connector could be made of a solid metal coated with insulation or of an insulating material that is coated with metal. In the former case, the wires could be inserted without an insulating cover (ground pins excepted), and in the latter an insulating layer would be required (ground pins excepted here as well). The choice of insulation and its mechanical and electrical properties would be matched to the performance requirements of the design.
 FIGURE 6. Sockets and mezzanine connectors can also incorporate torsion contacts, and in doing so will enjoy significant cancellation of self and mutual inductance. |
The concept of torsion contacts also extends to sockets and mezzanine connectors. Figure 6 is an example of one such construction. While the electrical path seems overly long, it is, in fact, relatively short. Moreover, because of the reversing of the path, much self- and mutual inductance is canceled out.
Stub-free mating
The novel torsion connector design offers significant advantages over more traditional approaches. This new design allows for mating in a virtually stub-free manner at both ends of the connector. Moreover, when round wire is used as the connecting element, it is possible to make a coaxial connection.
Prototype connectors are being prepared for testing at the time this article was being written; the results of those tests will be available once the tests are complete.
The advantage of torsion is that a torsion bar conductor can be made volumetrically smaller than the cantilever round wire beam of the same diameter, yet have the same electrical contact force when electrically mated.
For high-performance applications, the torsion connector is amenable to a virtually unlimited number of interconnections that are substantially coaxial.
JOSEPH FJELSTAD is founder and CEO of SiliconPipe (www.siliconpipe.com). GARY YASUMURA heads SiliconPipe’s research and engineering initiatives.
