Look at the person sitting next to you on your next business flight. While your travel kit may not necessarily include them, chances are his or her briefcase arsenal includes a PC or PDA and a cell phone not much bigger than a family-size pack of chewing gum. The growing market for such downsized and portable products, which leapfrog the capabilities of their predecessors of only a few months, is driving down the size of the connectors within--much like what is being seen in the companion-component technologies of relays and switches (DN 11/3/97). Material advances, as well as new fabrication techniques and design configurations, are driving the downsizing effort.

Currently, PCI-standard bus connection needs are being met with ingenious strategies for increasing processing density in products. Packaging-improvement devices include the Elco (Huntingdon, PA) Mezzanine Card Connector System that enables a host CPU board to be customized by mating PCI cards to it in parallel stacks 8 to 15 mm high. These board-to-board connections feature a rugged leaf-contact configuration for secure mezzanine-board mating, according to John Ashman, director of engineering. Other PCI adaptations include the company's Compact PCI connectors, which repackage up to five rows of 2-mm pitch interconnects, with shielding, for right-angle configurations.

As mobile communications and other devices, such as PCs and PDAs, pack more functionality into increasingly smaller packages, Ashman notes metal shielding for impedance matching may not be adequate in the future. The reason is the need for signal conditioning as clock speeds increase. "Standard pin-socket impedance is becoming a major portion of the impedance budget a designer has to contend with," he says, since impedance for other components is decreasing. Also, for many current connectors, impedance is not consistent from connector to connector while, in the future, pin-to-pin impedance will have to be consistent to allow signal tailoring for each line.

As clock-speed requirements over 300 MHz kick in for many connectors, impedance matching will encompass not merely line-to-line matching but signal tailoring. Thus each line will require an inductor, capacitor, or resistor. "And all of these will have to fit within the same envelope as the basic connector," adds Ashman. "You can't increase size in impedance-matched connectors since volume is critical." In some portable devices, the I/O connector is already the largest component after the display screen. "Until now, impedance matching has been a luxury only for the military and Cray because of cost," says Ashman. As a first step, a unique low-cost fabrication process will allow Elco to build such circuitry line-by-line for production multifunction connectors beginning in 1998.

Power management critical. As devices shrink, available power must be husbanded. Lifetimes can be lengthened by improving basic battery longevity, lowering power-interconnection resistance, and cutting on-board power consumption. Power-using parts, for say a PDA, include a microprocessor, hard drive, and display. As flash memories replace hard drives and display efficiencies rise, connectors may become a major drain above what is now roughly 10% of the power budget. Currently standard-contact connectors have a resistance around 3{OMEGA}, with higher-cost gold contacts at about 1{OMEGA}. In the future, Ashman notes common connector resistance will have to be in the m{OMEGA} range as well as rugged enough where many disconnects and reconnects occur, as in docking or recharging stations.

Materials and new designs are leading the downsizing charge. New flowable polymers with higher dielectric constants can be molded in thinner sections for tighter-pitched pins. Wall thicknesses approaching 0.005 inches are possible, down from three times that. And locating mating contacts on the thinner shear (cut) edges rather than the wider stamped side reduces the width of the pins, Ashman adds.

Size is not the only factor influencing connector design. As electronics increasingly replace mechanical systems in many applications, designers are finding they are not an exact one-for-one substitute. In appliances such as washing machines and dishwashers, mechanical systems are typically located within top fascias and doors, where the conditions include harsh heat, humidity, and shock. To survive in these environments, electronics, including connectors, must be more robust and adequately sealed.

AMP (Harrisburg, PA) has recently introduced splash-proof and immersible seals for its existing Mate-N-Lok connector line. The system uses a Teflon^TM seal template around the wires coming out the back end of each connector half, and an insert with triple ridges for the actual connector interface. With the Teflon parts, the sealing capabilities are not destroyed if the connector is undone. As an accessory that can be fit to an existing connector, the cost is only $0.15 per mated line, as opposed to the $0.50 per line for dedicated connectors.

A new adaptation of the Mate-N-Lok concept, the Mini-Universal 2, has a molded pin on each side of the connector block in which the wires are inserted. Each pin rides within in a corresponding slot on each side of its wire-locking cap, which forms the mating interface. Even before they are snapped together to form a ridged connector for mating, the cap and block cannot be removed from each other and lost. A unique injection-molding machine with movable dies on two sides, as well as the top and bottom, allows the production of the attached pin and slot pieces in a single mold.

Gender bender. Likewise, Phoenix Contact (Harrisburg, PA) shows design legerdemain in its Inverted Mini-Combicon reduced-pin-spacing terminal-block connectors. Previously available for larger-spacing, higher-power control-panel applications, these PCB connectors can be gender-switched via adapters. Such interchangeability allows numerous board-to-board and board/wire mating arrangements and geometries, as well as wire-to-wire mating. The gender of the original cable or board connection need not be changed.

Finally, what connector developments might users of these devices be looking for? One perspective is offered by Robert Boyes, marketing manager for Poly-Flex Circuits (Cranston, RI). "For flexible-circuit manufacturers, directly interconnecting multiple boards is a driver for the coming years," he says. The reduction in thickness and increase in complexity of such circuits will dictate connectors that are "flex friendly"--that is, without any need for bulky and expensive additional packaging to adapt the "flex" to the rigid board or other electronic hardware. "Future applications will require very thin yet incredibly reliable connectors," Boyes concludes.

MEMS the word

Connector manufacturers continue pushing pin counts up and pin pitch down while needing to accommodate ever-increasing data rates. After the turn of the century, new technologies should be available to improve pin number and density, and clock rates, by several orders of magnitude, according to Barry Cammarata, director and senior technical advisor in the AMP Corporate Technology Office (Harrisburg, PA).

"Within five years, IC sockets will have I/O counts between 2,000 and 3,000," he notes. "With this large number of pins, the force requirements for insertion and extraction will be too great, requiring a switch to more complex and costly zero-force designs." And clock rates look to at least triple from current numbers into the GHz range--dictating accounting for signal characteristics, including cross talk, switching noise, and series and parallel termination--adding complexity to connector design.

One technology set that addresses these critical interconnection issues is MicroElectroMechanical Systems (MEMS) based on integrated circuit fabrication techniques. "While integrated circuits exploit the electrical properties of silicon, MEMS devices take advantage of both the mechanical and the electrical properties," notes Cammarata, "and include microminiature motors, pumps, switches, actuators, sensors, and mirrors--all of which can be integrated with CMOS electronic circuitry on the same chip."

MEMS micromachining methods include:

Bulk micromachining that removes silicon wafer material by etching.

Currently, to reduce connector-pin pitch, several companies are developing "Z-axis" contact devices having Z-axis motion, but no X- or Y-axis contact wipe. These are "contacts with bumps, dots, or butt contacts," and have pin pitch as low as 0.1 mm, says Cammarata. "MEMS microconnectors can potentially reduce pin pitch by a factor between 100 and 1,000 from that of Z-axis devices, while maintaining good mechanical and electrical characteristics. Typical MEMS devices exhibit 8 microns of topography which will define the small contact surfaces needed for microinterconnection."

MEMS microconnectors can work over a wide range of voltages, from below 1V expected in many future interconnections to hundreds of volts needed for some mechanical actuators. And their on-chip electronics can handle many of the demanding requirements of very-high-speed signals.

The next generation. But Cammarata says even before MEMS come of age, a successor is rapidly moving over the technology horizon. This new carbon--not silicon--based technology could reduce the size of electronic and mechanical components to molecular scale. It hinges on the physics and chemistry of fullerenes--a form of carbon in a spherical lattice structure of 60 atoms in hexagonal faces. The structure is called buckminsterfullerene (or "buckyballs") for its resemblance to the geodesic domes created by Buckminster Fuller. "The mechanical properties of fullerene devices suggest that they can match many of the mechanical functions of MEMS, but on a thousand-fold smaller scale," Cammarata adds.

While pure C₆₀ is an insulator, once buckyballs are doped with alkalimetals they conduct, and the packing structure becomes a 3-D organic conductor. "This suggests the possibility of forming monolithic, nanoelectronic devices or systems that have distinct regions of insulating, semiconducting, conducting, or even superconducting properties, in addition to photoconduction, luminescence, or magneto-resistance characteristics," says Cammarata.

As for interconnection within and between such systems, he notes fullerenes can include structures with increasing numbers of carbon atoms. "As more atoms are included, the sphere elongates and becomes a capped tube, with open-ended versions also possible. Called "buckytubes" or "nanotubes," they are stronger than steel and there is speculation that they eventually will be produced with any desired length, kilometers and beyond! Cammarata concludes that "nanotubes with electrical conductivity 10 to 100 times that of copper at room temperature are a possibility. Such nanotubes would be the ideal, ultimate small wire for interconnection in nanoelectronic systems."