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Flex Circuits and Photonics: A Pairing for the Future, and the Here and Now

Though their roots arguably go back more than a century, over the course of the last three-plus decades, flexible circuits have played an ever-expanding role in the world of electrical and electronic interconnections. The reasons are as many and as varied as the creative designs that circuit engineers continue to come up with. In recent years, flex circuit technology and manufacturing methods are beginning to see more light, in a manner of speaking, both for data transmission and power generation with developments and improvements in the world of photonics, where flexible circuit manufacturing methods and materials are being brought to bear on this important and wide ranging field of technology. Presently, copper circuitry is regularly assailed in the R&D press as something that will soon fall by the wayside in favor of carbon in the form of layers of graphene and compounds containing nanotubes; however, a more likely scenario for the foreseeable future and beyond is that copper will remain a cost and performance-effective solution for most electronic applications. This does not mean that the carbon materials will not have a role to play; they likely will. It is more just an assertion that one should not underestimate the staying power of incumbent technologies.  

Copper is readily available, easy to design with and manufacture, solderable, and has very good electrical and electronic properties that have actually improved as material suppliers have learned how to make lower profile finishes that adhere well to smooth surfaces. More fundamentally, copper can carry electrons, which remain the primary means for switching transistors, and it is still widely used in the manufacture of the circuits on ICs.

While copper is considered quite suitable for short-distance data transmission, high-speed data transmission at rates of up to 40 Gbps over a distance of 30 inches and through two connectors, no less, is possible; it was demonstrated at my former company, SilconPipe, nearly 10 years ago. Still, it is a fact that copper is no match for photo-optical solutions in a number of areas. Glass fiber can carry extreme amounts of data over long distances in both directions at once. However, it is also a fact that copper and electrons remain the most popular medium for transmitting data on and between adjacent circuit boards.

Off the board, microwave transmission seems to be taking over as the medium of choice for data transmission with all manner of wireless devices (which are truly the ultimate form of “flexible circuit,” as has been suggested in this column in the past; after all, there is nothing more flexible than air). That brings us back to photons, which are making continuous headway into the world of electronics. One thing that all of these basic data carriers (electrons, microwaves and photons) have in common is that flexible circuits are being increasingly looked to for help in managing their data transmission function.

Photons have some very attractive properties which are not lost on those tasked with keeping the massive amount of information whirling around the globe moving smoothly, and for the most part, error-free, between distant communicating devices. Those who have been in the industry more than 15 years will recall the huge surge of interest and investment in optoelectronics in the late 1990s. The interest was lead in large part by the belief that glass fiber was the only viable option for future high-speed interconnections involving large amounts of data.

Not surprisingly, the driving force for looking at optical solutions was telecommunications, which was being pressured to provide ever greater bandwidth to satisfy the exploding demands of internet service providers as they strained to handle the immense traffic that was being generated. In that regard, nothing much has changed; what has changed is that the once waning interest in photonics is coming back. Copper solutions managed to continue to carry the day in spite of the assertions that it was not capable, but there are limits, as mentioned.

While engineers are presently finding and will likely continue to find ways to increase the performance limits of copper, there are clearly important benefits to using photons over electrons. One of the key benefits briefly touched on earlier is low signal loss over long distances (which some suggest is greater than 1 meter). Laser generated photons can travel many miles through glass fibers before the signal is attenuated to the point where it must be boosted. A related benefit is that relatively little energy is required to transmit that information over the distance. Moreover, light can be multiplexed and simultaneously sent and received at high bit rates and at different wavelengths. Other attractive benefits of using photons include the fact that light is virtually immune to electromagnetic interference and provides excellent signal integrity over great distances.

The one major lingering concern is that, lacking a photonic transistor, electrons must be converted to photons and back to electrons for them to convey the messages to and from different IC chips. With that said, some recent announcements by big name companies indicate important progress is being made. Where do flexible circuits fit in to all of this? The answer is essentially the same as it is for electronics: as transmission cables or wave guide assemblies. One method of integrating optical transmission capability into flex is to route and bond optical fibers to a flexible base material and then encapsulate them to hold the optical channels in place. Another method has involved the use of formed channels into which is disposed an optical polymer. The latter method has the attraction that it can likely be better adapted to large scale production. In either case, the most important feature will be making the connection and assuring good alignment at both ends to assure optimum transmission.   As was alluded to in the opening paragraph, flexible circuit manufacturing methods are also being brought to bear on the use of photons to generate power. This is, of course, a reference to the used of the equipment developed for flex circuit  applied to the manufacture of solar cells. It has been estimated that the amount of solar energy striking Earth every year is equal to more than 274 gigawatt-year (GWyr) hours or 20,000 times the amount of energy used by the human race per year. Passive collection of just a small portion of that huge amount of lost energy seems like a nice objective to go after as an alternative to oil and coal, the burning of which, most now concede, is contributing to global warming. For this and financial reasons, venture capitalists have been funding the techniques that are being developed to capture solar energy and convert it into electricity, and web-based processing methods and equipment, originally designed to serve flexible circuits, are seeing expanded use and at some point, one can only hope, increased production.

Direct printing of photovoltaic cells on flexible base materials of foil and plastic has yielded positive results and despite the fact that there has been significant fallout among early contenders. Fortunately, there are a number of startups still in pursuit of flexible circuit-based solutions and they are still in the hunt to provide cost and performance levels required to make the switch. Since the first solar cells were produced a half-century ago, there has been a steady decline in costs and increase in performance as efficiency has climbed steadily; it is expected by some investigators that eventually, they could well reach cost parity with the most common used for the generation of electricity for the grid, which include oil, natural gas and coal. As evidence of solar technology’s gains, one need only look at homes in sunnier regions of the U.S. to see the increasing number of solar installations on homes. As prices drop, it will undoubtedly accelerate. To sum it up, one might say, “Flexible circuits…they’re not just for electrons anymore.” Joseph Fjelstad, founder and CEO of Verdant Electronics is an international authority and innovator in the field of electronic interconnection and packaging technologies with more than 200 U.S. patents issued or pending. He is the author of Flexible Circuit Technology 4th Edition and author, co-author or editor of several other books including Chip Scale Packaging for Modern Electronics. He has also authored numerous technical papers and articles. He frequently presents seminars on PCB, flex circuit and chip scale packaging technologies at industry conferences. You may contact him at 408-836-2856 or by e-mail.