The IEMRC 6th Annual Conference
Loughborough, UK, 21st September 2011
On a bright September morning, over 130 delegates from industry and academia gathered at Holywell Park Conference Centre, at the University of Loughborough in the East Midlands of England, for the 6th Annual Conference of the Innovative Electronics Manufacturing Research Centre, the UK’s internationally recognised provider of world-class electronics manufacturing research.
Academic Director Professor Paul Conway welcomed delegates, briefly reviewed the aims and objectives of the IEMRC, and was pleased to confirm that funding until 2015 had been secured. He introduced the keynote speaker Professor Jim Morris, who was directing research into electrically conductive adhesives, nanoelectronics and nanopackaging at Portland State University in Oregon USA.
Professor Morris gave a fascinating insight into some applications of nanotechnology in electronics packaging. “When is a metal not a metal?” he asked when describing the characteristics of metal nanoparticles. “You can’t talk about the conductivity of two atoms!” Provided they were larger than about 12 atoms in size, approximately 1 nanometer, metal particles did indeed exhibit metallic properties but because of their very high surface to volume ratio and high surface energy very small particles had a tendency to merge together by mechanisms such as Ostwald ripening, sintering or liquid-like coalescence, and these phenomena could cause problems in nanoparticle technology. However, melting-point depression effects could be potentially turned to advantage in lead-free soldering: there was a possibility of a 5% reduction in melting point of a solder paste if the particle size could be reduced to the 5 nanometre level, although making particles this small was a real challenge.
There was considerable scope for using printed silver nanoparticle paste as a means of electronic interconnection. In principle the particles were capable of being sintered at very low temperatures once the carrier material was removed and Professor Morris described a process based on jetting paste on to the substrate, drying, removing the carrier by immersion in methanol then sintering at 60ºC. There were also opportunities for filling PCB microvias by nanoparticle sintering.
Turning to the subject of carbon nanotubes in packaging applications, Professor Morris explained that single-wall nanotubes had properties far superior to those of multi-wall nanotubes, but could only be formed at temperatures around 900ºC – too high to enable them to be grown directly on silicon devices. Carbon nanotubes were capable of carrying high current densities and had the advantage over copper of no skin effect at high frequencies. Moreover they had zero, or even slightly negative, coefficient of thermal expansion. This offered potential advantages over copper in through-silicon-via applications. A further favorable property was excellent thermal conductivity, and aligned single-wall carbon nanotubes offered the possibility of a technology breakthrough in the heat-sinking of silicon devices.
However exciting might be the technological opportunities, Professor Morris advised caution in the manufacture of nano-materials, with regard to environmental and health and safety issues. For example, nano-silver had a very high level of toxicity in the aquatic food chain, and carbon nanotubes could present human a health hazard similar to that of asbestos. So products should be designed with safety in mind and researchers and manufacturers should endeavor to avoid litigation by ensuring that procedures were in place to protect the health of workers.
The second presentation came from Dr Hazel Assender of the University of Oxford, with a progress report on the IEMRC RoVaCBE Flagship project. The objective was to develop techniques for low cost, high-speed, roll-to-roll manufacture of organic field-effect transistors by adapting industrial vacuum evaporation processes already widely used in the packaging industry. Potential applications included flexible displays and tagging and tracking of consumer goods.
It had already been demonstrated that reliable working transistors could be produced, and novel organic semiconductor materials such as dinaphthothienothiophene (DNTT) were now being evaluated with promising results. Plasma curing of the insulator layer had proved to be more production-efficient than electron-beam curing and thermal annealing. Although organic semiconductors were reasonably stable in air, their shelf life could be greatly improved by encapsulation and work was in hand to determine whether established vacuum deposition methods for applying gas barriers to food-packaging film could be adapted for this purpose. Patterning processes for semiconductor and insulator layers were under development, and a fundamental aim of the project was to achieve web speeds of 50 metres per minute in an industrially realistic environment
Observations on Whisker Formation on Electrodeposited Metallic Coatings used in Electronics Manufacture was the title of the presentation by Dr Geoff Wilcox from Loughborough University who described some of the outcomes of the IEMRC-funded WHISKERMIT project, which sought to develop tin whisker mitigation strategies for high value electronics.
Not only tin – zinc and cadmium were also capable of forming whisker-like growths and Dr Wilcox quoted examples of catastrophic failure of electronic equipment caused by whiskers from mechanical and architectural components. The first documented evidence of failure attributed to whiskers was in the radios of World-War II Liberator aircraft after cadmium whiskers had formed on capacitors.
Whereas the tendency of electroplated tin coatings to produce tin whiskers had been significantly reduced by the alloying of tin with in excess of 3% lead, legislation banning the use of lead in electronics has removed this safeguard. Consequently, the threat posed by tin whiskers has risen to worrying levels, particularly where bright tin finishes were used. Shorting between conductors was not the only failure mode; whiskers could act as antennae and cause signal integrity problems in high-frequency devices.
The two main strategies in the WHISKERMIT programme were to determine how the chemical components and operating parameters of the tin-plating process could be modified to reduce the compressive stresses known to initiate whisker growth, and to formulate whisker-mitigating nano-structured polymeric conformal coatings. It was considered that the co-deposition of nano-particulates could reduce internal stresses in the plating, and it had been found that alkyd-based coatings were most effective at containing whiskers under accelerated testing conditions.
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IEMRC Research Coordinator Dr Darren Cadman moderated the second conference session and introduced Professor Marc Desmulliez of Heriot-Watt University who described new additive technologies for the patterning of fine metal tracks on flexible substrates, and discussed processes being investigated at Heriot-Watt for surface modification of Kapton polyimide film and reduction of embedded metal to form a seed layer for subsequent electroless deposition.
The first process sequence he described involved hydrolysis of the polyimide surface with potassium hydroxide followed by exchange of potassium ions for silver ions by immersion in silver nitrate solution. The next stage was to spray-coat with an 
alcohol solution of methoxy polyethylene glycol, which acted as an electron donor to reduce surface silver ions to silver nanoparticles upon subsequent laser exposure. Conventional photomasks or digital maskless imaging techniques could be used, with either helium-cadmium or diode lasers. After exposure, immersion in dilute sulphuric acid removed unexposed silver ions and re-imidised the Kapton surface. An annealing step was necessary to coalesce the silver particles into a condition suitable to seed the electroless deposition of silver conductors from a proprietary solution.
The alternative process involved a chemical, rather than a photochemical, reduction stage, a proprietary photoresist being applied in place of methoxy polyethylene glycol and imaged by conventional exposure and development, then using dimethylamine borane as a chemical reducer for the silver exposed in the surface. The resolution of this method was limited to 50-micron lines and spaces, whereas the laser method could achieve 15 microns.
Courageously facing an audience of solemn techies and academics, fashion designer Odette Valentine from Brunel University captured and held their attention as she introduced the concept of “soft-wear” and talked about style and fashion trends from a user-centred perspective. Her research objectives were to elucidate the nature of the new relationships required between the designer and the consumer which would catalyse the development of meaningful wearable technology, and to design and produce novel modular garment platforms demonstrating various levels of integration of electronic textile components. Wearable electronics could offer mental stimulation, measure body performance or provide safety and protection.
“Fashion is the science of appearances and it inspires one to seem rather than to be” - Design and aesthetics could overcome any failure in function, and it was not enough to invent cutting-edge technology; it needed to have a “wow” factor, like the iPhone for example.
Initial findings had been that designing for context was as important as designing for function. The was an ongoing role for the enthusiast in the development of wearable tech, and the framework had underlined the need to understand acceptance factors for the user’s context as well for the product.
In reality, however, despite early hype about the potential of wearable technologies, the integration of components into the kind of substrates found in current styles of clothing had lagged the development of the underlying technologies, so the practical benefits of wearable electronics had as yet been slow to emerge. A specific functional application was described by Professor Yiannis Vardaxoglou of Loughborough University in his presentation on wearable flexible antennas.
In collaboration with industrial partners and specialists in textile fabrication at Nottingham Trent University, the project was exploring the most effective ways of manufacturing a fabric antenna and its associated electronics, and working to overcome the challenges of integrating the antenna into the fabrics of functional clothing and other products. Electromagnetic design rules had been revised to enable experimental antennae to be adapted to suit the needs of potential end-users, and progress was being made in the identification of suitable fabrics and yarns and in adapting textile manufacturing techniques such as embroidery to integrate the antenna into the host fabric. Challenges to be tackled included the minimisation of body interference, ensuring consistency of performance in harsh environments, and scalability to cost-effective mass manufacture.
After a very productive lunchtime networking opportunity, IEMRC Industrial Director Professor Martin Goosey introduced the third conference session. His first presenter was Dr Robert Blue from Strathclyde University, on the subject of polymer-based miniature sensors for explosives. A common feature of many explosives was the –NO2 nitro group and the objective of the project was to fabricate and experimentally evaluate micro sensors based on monomers engineered have a high affinity for volatile nitro-compounds and capable of being deposited electrochemically as thin polymer films to sub-micron dimensions.
Prototype capacitance sensors formed on interdigitated gold electrodes had exhibited a large, reversible response to nitro-bearing compounds as well as a low cross-sensitivity to other volatile organic chemicals commonly found in the atmosphere. Intended areas of application included transport hubs, sports arenas and shopping malls. Future work would be directed at the production of conjugated microporous polymers whose increased surface area would enable higher sensitivity and faster response.
“Power Electronics is a £70 billion direct global market, growing at a rate of 11% per annum, and the UK plays a significant global role both in design and manufacture”. Professor Bill Drury of Emerson-Control Techniques and the University of Bristol began his presentation entitled Power Electronics in the Low Carbon UK Economy.
Generally inconspicuous, power electronics was a critical enabling technology determining the performance of much larger systems, and was an essential component of renewable energy sources and the efficient use of electrical energy. Taking industrial drives as an example, Professor Drury explained that 60% of all electrical energy was used in industrial electric motors. Power electronic control could reduce energy consumption typically by 30-40%, and could be applied effectively in about 50% of applications, leading to an overall reduction in total energy consumption of 9%. Power electronics was a significant factor in transport: automotive, rail, marine and aerospace, and all sectors continued to grow strongly. For instance, global power consumption in automobiles was growing at 5000 GigaWatts annually, and rail transport in China by 22 GigaWatts.
But Professor Drury was concerned about the future of the UK power electronics industry. The market was worth £5Bn annually to the UK’s GDP, and more than 95% was exported, but the biggest ongoing challenge was an increasing skills shortage, as witnessed by a decline in the numbers of students in electrical and electronic engineering – 41% fewer students accepting places to study EEE in 2010 than in 2002, and 33% of engineering graduates taking non-engineering careers
The final presentation of the conference came from Professor Andrew Holmes of Imperial College London, who reported the progress of an IEMRC project investigating the feasibility of introducing a thermosonic bonding step into flip chip assembly using anisotropic conductive adhesives. The use of anisotropic conductive adhesives has previously been limited to a relatively narrow range of applications, because the interconnections relied on purely mechanical contacts, which might suffer from high joint resistances and reliability failures.
The project sought to replace mechanical contacts by metal-to-metal thermosonic bonds in order to reduce contact resistance and improve reliability. A dedicated thermosonic-adhesive flip-chip bonder had been developed incorporating an ultrasonic bonding head that could be rapidly temperature cycled by infra-red laser heating and compressed-air cooling. Assembly trials with dummy flip chips on glass and flex substrates with copper-nickel-gold bumps had demonstrated the thermosonic-adhesive bonder to be a highly versatile bond tool. Joint resistance measurements had been used to monitor performance and some issues with coplanarity and alignment had been resolved.
Before bringing the conference to a close and thanking all who had presented and attended, Professor Martin Goosey gave a brief update on the current status of IEMRC: two large multi-institute Flagship projects were in progress, a first round of standard projects had been awarded and started, and a second round was in review and due to be prioritised during October 2011. There would be a third call for proposals in 2012.
Another outstanding event! The 6th Annual conference of the IEMRC: content of the highest quality, eminent speakers, a large and attentive audience and a first-rate networking opportunity. A great credit to the team at Loughborough who organised it so seamlessly.
Pete Starkey
I-Connect007
September 2011