By Bill Keller
Interconnect system technology that can meet the requirements of next-generation signal-processing equipment has been developed.
As the world continues down the path of the Information Age, one constant necessary for advancement is the need to increase data transmission speed. Telecommunications, data storage and high-end computing systems are now operating at speeds of 1 Gbps and greater, which is an order of magnitude faster than was available just a few years ago.
Problems arise when information gets bottlenecked during board-to-board transmissions. The problems occur with connectors and, more specifically, interconnect systems. The small dimensions contribute to signal loss at the contacts and the increased lead density increases crosstalk. Information may be moving fast through today's networks but it may not move fast enough unless interconnect systems keep pace with the processing speeds of today's integrated circuits.
Industry Dilemmas
An interconnect system is analogous to the ramps of a large interstate highway system. Data can be carried along a seven-lane highway with minimal traffic, but it slows down dramatically when the information has to travel on and off the highway along a single lane of traffic. Current digital systems commonly operate at speeds faster than 1 Gbps, and analog systems at speeds up to 2.4 GHz. The speed and amount of data transmitted between boards is enormous. That is the situation the telecommunications, networking equipment, high-end computing and data storage marketplaces are facing.
The telecommunications and networking equipment industries provide the greatest impetus to design and develop advanced interconnect systems that can keep up. The burgeoning wireless industry needs products that can deliver great signal density and fast rise times. By 2005, interconnect systems will need to handle over 1,000 signal contacts per card — a significant increase over the 100 signal contacts per card typically available today.
Interconnect System Meets Demands
A two-year initiative resulted in an interconnect system that can meet the demanding requirements of next-generation signal-processing equipment. Computer-aided design/computer-aided manufacturing (CAD/CAM) was used to design the product from the initial concept to a working prototype. Robert Bradley, Lead Design Engineer, initiated a series of theoretical, computer-generated models to assure the design was feasible.
Design engineers went outside the division and enlisted expertise from the company's printed circuit board (PCB) division because of its expertise in manufacturing components with very small, critical features and tight tolerances. Critical components such as the eye-of-the-needle compliant sections, which have extremely tight tolerances, were prototyped and tested.
One requirement for the interconnect system was that it needs to operate in a variety of harsh environments for as long as 20 years. High-reliability telecommunications requirements are some of the most stringent in the industry. Tests the system has to pass include temperature cycling, mixed flowing gas, high humidity, and shock and vibration.
Interconnect Design
The primary benefits are realized by the X-shaped design of the signal contacts and grounding pins. One of the problems with increasing signal density is the increase in crosstalk. The X-shaped design is well-suited to a number of grounding schemes, which lowers the overall electrical noise created when very fast signals pass through the interconnect system. The ground contact in the center of the X reduces electrical noise in the four signal contacts surrounding it. In effect, the ground contact serves as a barrier, dampening the stray electric fields that contribute to crosstalk.
 Figure 1. The XCELL interconnect system's signal and power modules use guide modules for proper mating. |
The interconnect system has dedicated signal, power and guide modules (see Figure 1). Each guide module offers a polarization key and electrostatic discharge (ESD) clip. The guide pin aligns the connector bodies and signal contacts, which is especially critical because of the density of the pins and the reduced size of the contacts. The polarization key allows the designer to give each card a unique identity, and makes it almost impossible to plug a card into the wrong slot. The ESD clip provides a chassis ground throughout the system, eliminating the possibility of damage from an ESD during card insertion and extraction.
The backpanel signal module incorporates the X-shaped construction of four signal contacts surrounding a hollow-molded post that houses a ground contact in the center. It connects to a PCB using an industry-standard pin attachment.
To simplify manufacturing, the design of the receptacle connector, which receives a mating daughtercard, is based on a building block approach (see Figure 2). Signal contacts are stamped in lead frames, electroplated, then overmolded with a plastic wafer. Ground contacts are installed between two molded pieces to form a wafer assembly. Six wafer assemblies are installed into a plastic housing and a protective cap is installed to complete the mating signal module. Signal, power and guide modules are then installed on an aluminum stiffener.
 Figure 2. The receptacle connector houses six wafer assemblies, providing 144 signal contacts. |
The interconnect system has six levels of contact engagement: one level of contact in the guide/ESD module, one at the center ground contact, two at each power blade and two at each signal contact. At no time is there greater than a 1.1 mm contact separation between any of the leads with a minimum contact wipe of 2.0 mm. This design allows for hot-swapping of cards, eliminating the need to shut the system down for repairs or changeouts.
PCB Routability
An important benefit of the interconnect system is PCB routability. The PCB interconnect system can be terminated in as few as five layers in the PCB. The wiring plan uses a wide routing channel of 0.060" to accommodate four 0.005" traces and spaces between each contact cell. The "skin effect," which is the tendency for electrical signals to travel near the surface rather than through in the center of a conductor, is an issue in routing the copper traces on the board to the connectors. The design reduces the skin effect by permitting two traces to run to each contact rather than one.
Another benefit of the design is the reduction in crosstalk between signal contacts. When using a four-to-one ground-to-signal pattern with a rise time of 500 ps, engineers can expect a crosstalk value of 6.7 percent, increasing to 14.9 percent at 100 ps rise time. Using a pattern with three grounds per contact area, crosstalk can be reduced to 0.3 percent at 500 ps and 1.4 percent at 100 ps.
Conclusion
Considering the demand for DIN 41612 and CompactPCI, 2 mm, hard-metric connectors, this interconnect system represents a welcome addition to the marketplace. Thinking ahead, other possibilities include the use of the grounding pin for coaxial input/output, and, because the next level of data transfer is at the optical level, a fiber optic module that combines copper and fiber into one unit.
Interconnect systems are an integral piece of communications, data storage and computing networks. They must handle tremendous amounts of data at ever-increasing speeds. Components are expected to transmit information in a reliable and rapid manner for many years.
BILL KELLER is Director of Account Development, Electronic System Packaging Div., Litton Winchester/Retconn, 400 Park Rd., Watertown, CT 06795; (860) 945-5000; Fax: (860) 945-5191; Web site: www.litton-wed.com.
SPEC SHEET
End Applications:
Communications, data storage and computing networks
Related Products:
Board-to-board connectors, PCBs
Main Point:
Interconnect systems are an integral piece of communications, data storage and computing networks. They must handle tremendous amounts of data at ever-increasing speeds. Interconnect system technology has been developed that can meet the requirements of next-generation signal-processing equipment to transmit information in a reliable and rapid manner.
