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Flexible Thinking: Flexible Structures for Data Transmission

Information in today's rapid-paced, data-hungry world is vital to the success or failure of social networks, armies, businesses and national entities. Actually, this basic fact of the importance of information flow has changed very little over the centuries. In earlier times, however, the flow of information was normally very slow, placed by the means used to transport it. Hand-written letters, for example, contained lots of detail but the systems used to transport them were slow. It could take from weeks to months or even years for information to arrive from distant places if the information traveled on foot, horse and boat.

While considered primitive by modern societies, African peoples mastered rapid communication over modest distance by the use of drums, which conveyed simple messages from one to many and the information traveled at the speed of sound. Similarly, the Native Americans also understood the importance of speed of information and sent messages at the speed of light, in the form of smoke signals.

These were simple useful methods, but they could be interrupted by daily events. Smoke signals are of little use at night, for example, and a high wind could obscure the sound of drums. Electrical signals, first through wires and then wirelessly, overcame the limitations imposed by nature and made transmission of information over distance in near real time an everyday "miracle." In fact, with the invention of the telegraph, messages could traverse continents or oceans in a virtual instant and with the introduction of such more than a century ago. While we are jaded by our modern telecommunications capabilities, one thing remains true: The sense of urgency surrounding information transmission has steadily risen over time.Today electronic information and the handling systems that manage and distribute it are critical elements of every society. These vital systems process, compile, store and transmit data around the globe. Because not all information handling needs and requirements are the same, system performance needs can vary from application to application. However, just as in earlier times, there remains a desire to make certain that the data is transmitted clearly, cleanly and securely. Today, a number of different interconnection technologies are capable of meeting these requirements including optical and various electrical cabling methods. As might be expected, advantages and disadvantages accompany each of these varied solutions. Optical methods, for example, is very attractive for long-haul data transmission and it is very secure and immune to virtually all of the concerns associated with electrical circuits such as noise, crosstalk and electromagnetic interference.

A recent announcement by Intel of advances in this area suggest that it could make its way down to the level of the chip within the next several years. On the other hand, since there is no optical transistor for use in these devices (nor yet one perceptible on the horizon), there is an inevitable need to convert an electron into a photon for transmission and then, at the far end, the need to convert the photon back into an electron. This is called the E-O-E conversion (short for electrical-optical-electrical). There is also a need to be scrupulously attentive to thermal management of the lasers used for data transmission as fluctuations in temperature will affect the wavelength of the light transmitted and result in the far end transceiver's inability to decipher the data sent. Such thermal control is manageable but it comes at a cost.

For these and other reasons, flexible circuit cables are becoming increasingly popular. It is clear that in systems where various components must be moved relative to one another, flexibility is vital. Flexible circuits are ideal for such interfaces. However there is a balancing act involved and there is more than one master to be served to successfully get an increasing number of both circuits and circuit lines into the system while creating them in a mechanically robust and electrically reliable manner that is highly manufacturable. One way to assure sufficient flexibility is to reduce thickness, but this also makes the circuits more vulnerable. The addition of a ground plane is a normal requirement for improving the electrical performance of the circuit as it serves as a both shield and reference for the increasingly common controlled impedance circuits required for system performance.Over the years, a variety of different ground plane solutions have been employed in flexible circuit design and manufacture including screen printed silver inks in sheet or mesh and copper either as a foil or as a photoimaged and etched hatched ground plane. In addition to the metal and metal matrix solutions used, various sprayed coatings of colloidal graphite and very thin sputtered coatings of copper or even gold have been used. However, flexible circuits remain vulnerable to breakage, especially in applications where dynamic bending of the circuit is required, and though hatching is suitable for shielding, its ability to provide controlled impedance has been questioned. Still, interest in the hatched ground plane has waxed and waned over time and it is presently on the rise again. The hatched structure breaks up the fields, making definitive impedance control more difficult and less predictable, though there was an attempt to do so in the 1980s.

Figure 1: Image filed with Pat. No. 7,377,034 depicts a new method of data signal management in a flex circuit.

There was also an offset ground and signal trace structure that was described as a sort of quasi edge-coupled variation to circumvent the flexibility and I-beam construction concerns. One clever solution recently described in US Pat. No. 7,377,034 is a method which could reduce the problem of data signal management in a flex circuit while maintaining reasonable flexibility. The fundamental idea is to run in the ground layer continuous ground wires that are contiguous with a hatched ground plane of the circuit. The ground wires are aligned with their associated signal wires providing improved signal integrity.

An example would be a flexible circuit having a number of differential pair traces and a mesh ground plane having ground wire traces with each signal wire pair aligned over the top of the signal traces. Figure 1 shows the patent's concept in graphical form. The concept, by appearances as understood, is a best-of-both-worlds type solution, though managing alignment is obviously very important and the I-beam concern is not abated. But if the circumstances are not extreme or are reasonably well managed, it may not be a problem.In summary, flexible circuit cables offer some significant advantages for facilitating the movement of data between elements of a system that must also be moved or flexed. Moreover, while amenable to standards, flexible circuits can be easily customized, opening the door to a range of potential solutions to problems that cannot otherwise be easily addressed or solved.

Verdant Electronics Founder and President Joe Fjelstad is a four-decade veteran of the electronics industry and an international authority and innovator in the field of electronic interconnection and packaging technologies with more than 250 U.S. and international patents issued or pending. He is also the author of "Flexible Circuit Technology" and author, co-author or editor of several other books and more than 300 technical papers, articles and columns. To contact Joe, click here.