Bletchley Park, in Buckinghamshire UK, is the former stately home taken over by the British Government shortly before World War Two to be the secret base of the Government Code and Cypher School, which broke the reputedly unbreakable Enigma code and enabled British Intelligence to make sense of enemy communications: a historic and distinguished venue for the 35^th Annual Symposium of the Institute of Circuit Technology.

Opening the Symposium, themed “Printed Electronics”, ICT Technical Director Bill Wilkie welcomed faces old and new and reflected upon the continuing success of the Institute, and its steadily growing membership.

Keynote speaker Dr Steve Jones of Printed Electronics Ltd captured the attention and imagination of the audience with an enlightening overview of inkjet printed circuits and electronics, candidly subtitled “Trials, Tribulations and Some Success”. In his introduction, describing electronics in a context of interconnecting components to form a functional device, he observed that people tended to talk not about the physics of how and why electronics worked, but about applications and functionality. His personal view was summed up “If I can’t measure something, I know nothing about it – all I have is an opinion”, and his objectives in Printed Electronics Ltd were to further understand the fundamentals of inks, the critical details of the generation of ink drops and their 1-millimetre journey between print-head and substrate, and their subsequent interactions with the substrate and with each other. He emphasised that whereas inkjet techniques have become firmly established in graphic arts, the electron was not as tolerant of image defects as the human eye, and a far higher order of precision was required in electronics applications. He discussed the relative merits of binary versus greyscale printing, with slow-motion video to illustrate drop behaviour and substrate interaction. On the subject of electro-active inks, he had studied an enormous number of proprietary silver nanoparticle formulations, some of which had given promising results when inkjetted, some not, and parameters such as curing profiles had been shown to have significant effects on functionality. Many applications had been explored, some typical, some unconventional – the programmable dinner plate, is it dishwasher-proof? Fitness for purpose, durability and reliability were key factors, rather than conformance with established standards. Potential scope for novel applications was limitless, and there was a need for those with a background in conventional interconnection techniques to adopt a different mind-set when considering printed electronics: “We have functionality, Jim – but not as we know it!&rdquo

Dr Neil Kirby recounted his many years of personal experiences in thick film technology with a review of the origins and historical development of printed electronics, from ceramic thick film applications in aerospace in the 1970s to the new printed electronics of the 21^st century. A significant breakthrough had come with the development of low-temperature curing silver inks which could be used on polyester substrates, an early commercial example being the Sinclair ZX home computer in the early 1980s, featuring a membrane keyboard made using printed polymer thick film techniques. By the late 1990s, screen-printed interconnections, resistors and contacts could be found in mobile phones, electro-luminescent displays, remote controls for TV and video, calculators and disposable medical electronics such as electrocardiograph contact patches. Then came the sudden increase in interest in RFID in 1999, generated by proposals to replace barcodes for airport baggage tagging. Although RFID antennae could be mass-produced very cheaply by printing techniques, the cost of IC chips made the “one-penny tag” an unattainable goal. Printing of integrated circuits was a current mission for the printed electronics industry – not necessarily mission-impossible, but certainly mission-not-accomplished-yet. Dr Kirby discussed the evolution of materials, organic and inorganic, substrates and environmental issues, and surveyed current printed electronics applications in displays, lighting, smart packaging, sensors, batteries and photovoltaics, with many detailed examples. The future printed electronics industry had been estimated to be worth an annual £200 billion by 2020, most of which would be in new rather than replacement products, with materials development being the key issue. “Can do, can’t do” questions were largely a matter of personal attitudes – whether people would be disablers rather than keep an open mind and be objective about the opportunities presented by emerging technologies.

Dave Wayness of Dow Electronic Materials focused on inkjet from a material supplier’s perspective with a presentation entitled The Use of Inkjet Printing Technology for Fabricating Electronic Circuits, sub-titled The Promise and The Practical. The promise included elimination of photomasks, faster job turns, off-contact imaging for delicate substrates, image compensation and registration, higher yields and reduction in materials wastage. When it came to practical reality, inkjet fabrication of electronic devices was still very early in its maturing process and, echoing the comments of Steve Jones, although 300 dpi resolution was satisfactory for graphic arts it was far from adequate for electronics, which would require resolution of 750 to 5000 dpi for features to be continuous, uniform and functional. He discussed in detail the ways in which high-resolution images could be generated by multiple-pass and interlacing, taking as example a 75 micron line and space conductor geometry at 1250 dpi. Using an array of 15 heads, a 24” x 18” panel could be printed in 24 seconds. Dow had designed a hybrid UV phase-change etch-resist ink, for use with heated print-heads – liquid at jetting temperature but solid at room temperature so that flow-out was minimal once it hit the substrate surface – which gave excellent image definition. There had been significant increase of interest in solder mask imaging, particularly on designs with solder-mask-defined pads where image placement accuracy was critical, and Dow’s method of photo-tool elimination by inkjetting a UV-opaque negative image on to standard liquid photoimageable solder mask obviated all of the concerns regarding product qualification which had hindered the adoption of direct inkjet solder mask formulations.

Professor David Harrison’s Cleaner Electronics Research Group had been working for many years at Brunel University on methods for producing circuit interconnects and passive components on various flexible substrates using offset lithography, a faster and higher resolution technique than screen printing. More recently, research had been directed at methods for producing voltaic cells by similar techniques, so that printed electronics devices could be powered by batteries produced in-situ. Leclanché cell chemistry, using zinc and manganese oxide with an ammonium chloride electrolyte, had been chosen as the basis of a feasibility study, and inks with properties suitable for offset litho printing had been developed to prove the concept. Changing from a zinc-particle to a zinc-flake system had improved performance, as had the use of a carbon intermediate layer and silver current-collectors. Batteries had been produced with capacities of 10 milliamp-hours, capable of delivering a peak current greater than 150 milliamps, although shelf-life remained a major limitation, 10 days being typical, and this was being addressed in ongoing work. Professor Harrison demonstrated as a working example a musical greetings card where both the interconnect and the battery had been produced by offset litho printing.

Frank Eirmbter of SunTronic Electronic Materials presented a broad survey of inks and chemicals for printed electronics. He explained the principles of various printing techniques – letterpress, gravure, flexo, pad, screen, offset and inkjet, and how the different processes required inks with different characteristics. The only printing process traditionally associated with printed circuit manufacture was flat-bed screen printing, although rotary screen, rotogravure and flexography were now being applied to the production of hybrid devices and RFID tags. Offset lithography and inkjet printing were emerging as techniques for printed electronics. SunTronic were able to offer inks for all of these processes, and he listed many applications including antennae, membrane switches, capacitors, shielding , sensors, photovoltaics, RFID, displays, automotive, telecom, medical and diagnostic. Taking a membrane switch as an example, he described the manufacturing process and the specific products: graphics inks, silver conductive inks and flexible UV dielectric inks, used in fabricating the intermediate components. A second example he demonstrated was the construction of an RFID “smart label”. RFID antennae could be printed directly onto cardboard boxes using a flexo-printed water-based silver ink. Other applications illustrated included flexible displays, printed resistors, transducers and sensors, and development would continue on functional inks for a whole range of new applications.

The Lunaris system was a drop-in replacement for traditional photolithographic and laser-direct-imaging processes, taking the work direct from base laminate to etch-resist image in a single step. Océ’s extensive knowledge of drop flight and drop flow characteristics had indicated that a hot melt ink was most suitable for this application, and they had engineered a specific formulation compatible with their own design of heated print-head. The Lunaris machine incorporated 60 print heads, each of 260 nozzles capable of firing at 10 – 20 MHz. Although Océ’s heads were claimed to be the most reliable in the world, statistically a failure rate of 1 in 1 billion droplets was expected, due to air entrapment in a print channel. A unique feature of Lunaris was the ability to predict such failure, and in operation only 1 in 3 print heads was in use at any instant, the system switching to an alternative row of heads every 20 seconds, with an additional row of heads always available as a standby, so that the one-in-a-billion failure was eliminated and a perfect image was guaranteed. The machine had a production capacity of 60 double-sided panels per hour, and was attracting keen interest in the market.

After Professor Martin Goosey, ICT Vice-Chairman, had wrapped up the proceedings and thanked the presenters, delegates were invited to take a tour of the Bletchley Park site, in the company of knowledgeable guides who explained its significance in the history of military intelligence. A highlight was the opportunity to see Colossus, the world’s first semi-programmable electronic computer, meticulously rebuilt and fully operational, occupying a fair-sized room and with the cheerful glow of over 2500 thermionic valves. How things have changed…

Pete Starkey
ICT Council
June 2007