Plastic and Printed Electronics: Interconnects and Manufacturing Challenges

IeMRC Seminar, Loughborough, UK, 19th March 2012.

With research, development and commercialisation activity in plastic and printed electronics continuing to gather momentum, it was appropriate that the Innovative Electronics Manufacturing Research Centre (IeMRC), who support an extensive portfolio of projects in this technology area, should organise a dedicated seminar at their base in Loughborough University in the East Midlands of the UK.

IeMRC Research Co-ordinator Dr Darren Cadman welcomed delegates, gave a brief overview of the aims and objectives of IeMRC and introduced a comprehensive and well-balanced programme of presentations addressing the manufacturing challenges of printed and plastic electronics.

Keynote speaker for the morning session was Jerome Joimel from Plastic Logic Ltd, with a paper entitled "Developing a high volume manufacturing process using plastic electronics – challenges on the way from lab to fab"

Plastic Logic, originally a Cambridge University spin-off, had established a manufacturing facility in Dresden, Germany, in 2008 which had been in full production since 2010 and was presently producing hundreds of thousands of units per annum of plastic logic displays based on E-ink with an all-plastic backplane technology. Joimel related some of the challenges encountered in developing a volume-capable process and how they had been overcome, beginning with equipment differences, process specifications, materials characterisation and production control, then aspects of training and know-how for a new team whose background was mainly in silicon semiconductor manufacture. He discussed manufacturability from the point of view of achieving consistency and repeatability, with reference to material analysis and measurement techniques, quality, reliability and lifetime behaviour issues, and yield enhancement strategies.

As an example of a commercial product currently in stable volume production, Joimel described the characteristics of the Plastic Logic 100, a lightweight large-format electronic textbook designed for the education sector, with the significant benefit that the display was all-plastic and therefore shatterproof. Future application areas included interactive signage, phone displays, smart cards and sensors.

Professor Martin Taylor of Bangor University gave a fascinating presentation on "Printed electronics: Device production, characterisation & simulation", first describing the roll-to-roll vacuum deposition process developed at University of Oxford, which was capable of producing reliable working thin-film transistors by evaporation and in-situ e-beam polymerisation of diacrylate monomer on to a 75 micron polyester substrate followed by evaporation of a pentacene layer. Good results had also been observed with pentacene and dinaphthothieno thiophene thin-film transistors on polyethylene naphthalate substrates.

Current work at Bangor focused on characterisation and simulation of a range of thin-film transistors, capacitors and inverters of different geometries and aspect ratios, using UTMOST-4 modelling software. Although some of the semiconductor theory may have been beyond the comprehension of many in the audience, Professor Taylor explained clearly how key parameters could be extracted, and commented on the success of early exercises in circuit simulation.

Martin Wickham from the National Physical Laboratory gave the first public-domain report on the progress of a collaborative project named ReUSE: Reusable, Unzippable Sustainable Electronics, an innovative enabling technology for the fabrication of sustainable multi-layer electronics assemblies. He set the scene with some remarkable statistics concerning printed circuit waste in the UK. It had been estimated that about 85% of scrap PCB assemblies ended up in landfill, of which 70% was non-metallic content amounting to around 1 million tonnes annually, and that WEEE – waste from electrical and electronic equipment – was growing three times faster than other waste streams. Typically, only high pin-count socketed devices were physically removed from electronic assemblies, the rest of the assembly being shredded for precious metal reclamation and subsequent disposal to landfill.

The aim of the ReUSE project was to develop an interconnection technology based on special polymer layers and binders designed to allow straightforward, end-of-life disassembly with easy reuse and recycling. A series of technology demonstrators had been constructed and Wickham took as example an inverter assembly for an electroluminescent lamp. This consisted of a thin flexible circuit assembled with standard tin-finished components by normal SMT techniques using an isotropic conductive adhesive and bonded to a rigid base. The assembly had been extensively tested for reliability and proved to be fit-for-purpose. However, Wickham showed that at nominal end-of-life a simple immersion in hot water allowed the assembly to be "unzipped" – the components to removed without damage to leads or terminations and the flex circuit to be removed from the rigid base. He believed that recovery levels in typical assemblies would be improved to at least 90%, and that the ReUSE technology would lend itself readily to rigid, flexible and 3D structures.

Conductive inks and adhesives are key materials in plastic and printed electronics technologies, and the majority rely on silver as the conducting element. Dr David Hutt from Loughborough University reviewed developments in the use of copper as a more abundant and lower-cost alternative to silver, and also explored the feasibility of combining the operations of forming conductors and assembling components. A fundamental difference in the nature of copper compared with silver was that its oxide was non-conductive, so the principal initial challenge was to produce an oxide-free copper powder and to preserve the surface against further oxidation. This had been achieved by treating de-oxidised copper powder with a process that coated each particle with a self-assembled organic monolayer, which enabled it to be stored for several weeks in a freezer. Conductive pastes had been formulated using 10 micron powder with one-part and two-part epoxy resins at 85.7% by weight metal loading. These had been stencil printed on glass and cured at 150ºC in an inert atmosphere, with conductivity comparable to typical air-cured silver-loaded resin. The copper-loaded material did not give good results if cured in air. Best results were obtained with the one-part epoxy formulation. Inert-atmosphere microwave curing was an alternative which gave good conductivity in a shorter cycle time. Conductor resistance had been observed to increase during reliability testing in 85ºC / 85%RH damp heat conditions, but conformal coating overcame this effect. Dr Hutt showed examples of functional circuits which had been successfully produced in a combined circuit formation and component interconnection operation.

The UK government recognised the significance of "plastic electronics" – a term encompassing all classes of printed and organic electronics – as a sector in which Britain had potential for leadership in research, development and manufacturing, creating opportunities for employment and economic growth, and had published a guide to promote the UK's capabilities, designed to be used by commercial and academic groups, within the UK and overseas, looking for development partners or suppliers. A new edition of the guide was shortly to be published by the Department of Business, Innovation and Skills: The BIS Guide to Capability: Plastic Electronics in the UK 20012. Chris Williams of Logystyx UK Ltd was engaged in the compilation of data for the guide, and explained that any organization in the UK involved in plastic electronics was entitled to a free entry, provided their details were supplied before the deadline of 31st March 2012.

Dr Steve Jones of Printed Electronics Ltd admitted that he was out of his comfort zone – Professor Martin Goosey had asked him to deliver the afternoon keynote from a "philosophical", rather than his usual technical, point of view. And he gave an inspirational presentation: smooth, professional and well-researched, on a theme of invention and innovation, as he described some of the obstacles to be overcome in taking printed interconnects from R&D to commercialisation. Defining "invention" as the creation of a novel idea, and "innovation" as the commercialisation of novel ideas, he explored the flow from discovery, through engineered product to societal benefit, remarking that since the 1970s and 1980s, not much had happened in the way of true innovation; the 21st century was characterised by incremental improvements. "Radical or transformational innovation produces substantial improvements, radically alters or even creates markets. Printed electronics sits here!"

Reviewing the decline of manufacturing in the UK, and the risk-averse cultural resistance to change of the few remaining "big companies", Dr Jones discussed the attitude of governments past and present, and how manufacturing had finally been recognised again as an fundamentally important area to be helped and encouraged, with the Technology Strategy Board and Research Councils supporting technology, sector and cluster development. Digitisation and modularity had made it possible to separate R&D and design from production. Few vertically-integrated companies remained and global technology brands had shown that manufacturing could be outsourced and off-shored without damaging their ability to innovate. "Who makes electronics?" he asked: "EMS and ODM companies. And how can printed electronics compete with them?" He believed that the UK was well placed from a technology standpoint. The TSB and academic institutions had been supportive and it was acknowledged that the UK and Germany were leaders in the field of printed electronics.

Whilst acknowledging the need to be heroic and passionate and to explore boundaries, Dr Jones hoped to avoid being remembered for having died heroically or being found lying face-down in the desert with an arrow in his back! Collaboration and co-operation were what was important for small companies and he referred to the business strategy of his own company, which was involved in many collaborative ventures and for which EMS and ODM offered opportunities to develop products in the short term. The potential for printed electronics was huge: not just to replace conventional electronics but to transform where electronics could go.

Back to the technical agenda, it was the turn of Dr Paul Reip of Intrinsiq Materials Ltd to give an update on developments in nanoscale metal inks for printed electronics. Intrinsiq Materials had spun out of Qinetiq, the British global defence technology company, formerly the UK government Defence Evaluation and Research Agency and its particular expertise was the ability to manufacture nanoparticles, with over 20 years of experience in nano-copper: understanding the product and being able to produce it repeatably. David Hutt had earlier discussed applications of copper as an alternative to silver in the context of micron-particle formulations for stencil-printing; Dr Reip looked at a parallel scenario using nano-copper as a silver alternative in ink-jet formulations. Again the primary obstacle to be overcome was the tendency of copper to oxidise, greatly accentuated at nano-particle sizes. Intrinsiq had developed a proprietary organic coating, typically 1 nanometre thick on 35 nanometre particles, which inhibited oxidation. Jettable ink formulations had been validated on both XAAR and Dimatix print-heads and successfully printed on multiple substrates. Photonic curing techniques had been employed, using pulsed UV or laser, to cure the ink in milliseconds with no substrate damage, which offered opportunities for high-speed roll-to-roll processing. In response to market demand, screen printable versions had been formulated which had bulk resistivity values approaching that of pure copper, at between one third and one fifth the price of equivalent silver-based inks. Besides nano-copper inks, Intrinsiq had developed jettable nickel and silicon products, with applications in displays, smart media and photovoltaics.

David Watson from Heriot-Watt University described a process for laser direct writing of metals on plastic substrates, with particular reference to nano-silver on PMDA-ODA polyimide film, which could achieve line widths of 10 microns and less. The silver-on-polyimide process involved six steps, the first of which was hydrolysis of the polyimide surface with potassium hydroxide, followed by immerion in silver nitrate solution to replace potassium ions with silver ions. In the next stage, a solution of methoxy polyethylene glycol was sprayed on the surface and dried. This functioned as a kind of photosensitiser by acing as an electron donor to reduce silver ions to nano-silver particles when subsequently imaged by selective exposure with a UV laser. After exposure, unexposed silver ions were removed and the surface was re-imidised by immersion in dilute sulphuric acid and the work was annealed to coalesce the silver particles into a form suitable to act as a seed layer for subsequent deposition of silver from a proprietary electroless plating bath. Current investigations were directed at the characterisation of of a range of poprietary polyimide films, and the optimisation of the laser exposure and annealing conditions. A potential "green" alternative to methoxy polyethylene glycol as a photo-reducing agent was natural chlorophyll, extracted from spinach and sensitive to blue visible light rather than UV, resulting in less substrate degradation.

The concluding paper came from Dr Steve Wakeham of Plasma Quest Ltd., and was entitled: "HiTUS – an enabling technology for the plastic electronics industry." HiTUS was a technique for sputtering thin films, which differed from traditional ion-beam-source and magnetron deposition systems by generating a remote plasma in a side arm adjacent to the deposition chamber. Ions generated from this plasma system had low energy and required the application of a bias on the target, enabling a high degree of control on the deposition variables. Key advantages of the system were that it operated at low temperature and was therefore compatible with plastics, and that it offered high deposition rates with controllable stress, increased adhesion and increased densification. It was capable of depositing low-stress, fully-densified ultra-thin coatings on to temperature-sensitive plastics. Some examples were ultra-thin gold on PET, PEN and BOPP films, dielectric mirrors on PEN, multilayer colour filters on PEN consisting of 19 to 25 alternate layers of silica and hafnia, and the fabrication of photoluminescent and electroluminescent devices and plastic-compatible thin-film transistors.

In his closing remarks, IeMRC Industrial Director Professor Martin Goosey commented that although plastic and printed electronics were unlikely to replace silicon, there were many applications where they potentially offered innovative performance together with low-cost manufacturability.

IeMRC have established a reputation for organising and managing technical conferences and seminars of the highest calibre, and the team are to be commended for yet another outstanding event.

Pete Starkey

Technical Editor

I-Connect007

March 2012