EIPC-IONMET Workshop: Ionic Liquid Plating Technologies in Development. Silver and Gold Final Finishes for PCBs

Citation

Starkey, P. (2010), "EIPC-IONMET Workshop: Ionic Liquid Plating Technologies in Development. Silver and Gold Final Finishes for PCBs", Circuit World, Vol. 36 No. 2. https://doi.org/10.1108/cw.2010.21736bac.003

EIPC-IONMET Workshop: Ionic Liquid Plating Technologies in Development. Silver and Gold Final Finishes for PCBs

Article Type: Exhibitions and conferences From: Circuit World, Volume 36, Issue 2

University of Leicester, UK, 24 November 2009

Ionic liquid solvents offer an opportunity to overcome the limitations of aqueous chemistry in metal finishing processes and to avoid the use of toxic and corrosive materials, by a radically new technology with the potential to transform the scope and competitiveness of the metal finishing and printed circuit manufacturing industries.

IONMET is a funded project under the European Community’s FP6 programme, aimed at the development of new techniques of metal deposition and dissolution using ionic liquids, with the international collaboration of 34 SME and academic partners from 11 EU countries, to develop technology and oversee technology transfer of metal finishing processes based on a generic group of ionic liquid solvents.

Significant potential applications exist in printed circuit technology, particularly in protective and solderable finishes. Nano-structured immersion silver and immersion gold processes based on ionic liquid technology are at an advanced stage of development at the University of Leicester, and are being evaluated by PW Circuits, a locally based PCB fabricator.

With the co-operation of the European Institute of Printed Circuits, a technical workshop was conducted at the University of Leicester on 24 November 2009 to enable members of the PCB industry to understand the significance of the IONMET project, the characteristics and environmental benefits of ionic liquids, their applications in metal finishing and their scope as the basis of solderable finishes for printed circuits, to observe practical demonstrations at laboratory and pilot plant scale, and to discuss opportunities for future co-operation and evaluation.

Delegates from the UK, Ireland, Germany, The Netherlands, Switzerland, Czech Republic, Singapore, and the USA were welcomed by Professor Andy Abbott, Head of Chemistry at the University, whose team have been responsible for many innovations in ionic liquid technology.

Professor Abbott invited Michael Weinhold, Technical Director of EIPC, to make the opening presentation.

After summarising the contribution made by EIPC in representing and promoting the interests of the European PCB industry in the global electronics arena, and keeping the industry up-to-date with technology through its membership of the World Electronic Circuit Council, its links with trade associations in the USA, Japan, China and other Asian countries and its participation in standards-writing activities, Weinhold explained EIPC’s function in the IONMET project: the dissemination of information through the organisation of conferences and technology workshops in different languages. Of the 47 countries in Europe, 27 were under EU regulation, representing 492 million inhabitants with 22 official languages. Europe’s PCB industry represented 7.2 per cent of a global market of US$51Bn, North America 8.1 per cent, Asia 84 per cent.

Considering solderable finishes for PCBs, in terms of surface area OSP represented 51 per cent of world usage, followed by ENIG at 17 per cent, and HASL at 16 per cent. Immersion silver was 8 per cent and immersion tin 6 per cent. Square metre costs of solderable finishes ranged from US$0.45-0.75 for OSP up to US$2.10-7.50 for ENIG, excluding the cost of gold metal. Immersion silver cost US$2.10-2.85.

Putting a cash value on the total world cost of PCB solderable finishes, Weinhold calculated a figure of US$400 million without counting the cost of gold metal. When gold cost was included, this figure rose to US$1.1 billion. Europe’s 7.2 per cent share amounted to US$28.8 million excluding gold metal.

Having defined this global market perspective, Weinhold reviewed the risks, ecological, technical and financial, associated with established finishes, referring to environmental impact and energy consumption, and phenomena like “black pad” with ENIG, “creeping corrosion” with immersion silver and whisker formation using immersion tin. He believed that immersion silver had great potential to reduce PCB fabrication costs, provided that creep corrosion could be controlled. It had been demonstrated that immersion silver was not prone to electrochemical migration failure, and UL had reversed its earlier position and no longer required testing PCBs with silver surface finishes for electochemical migration. In Weinhold’s opinion, IONMET immersion silver could offer environmental as well as technical benefits, although its functionality under production conditions remained to be proven. He ended with the question: “Will the environmental and/or cost pressure be sufficiently high to force the PCB industry to use more cost-effective surfaces finishes?”

Professor Andy Abbott and Dr Karl Ryder gave a presentation entitled “Ionic Liquids and Deep Eutectic Solvents for Metal Finishing Processes” in which they explained the fundamental characteristics of ionic liquids, some examples of their chemistry and methods of preparation, and how they could replace acidic aqueous systems in metal finishing applications. In principle, these materials were non-molecular ionic solvents melting at less than 100°C, often at room temperature or below. They were electrically conductive, exhibited unusual solvent properties and had extremely low-vapour pressure, therefore they did not evaporate. Most liquids were thermally stable at temperatures greater than 200°C and were immiscible with many organic solvents. They were composed wholly of ions, with the general characteristic that at least one of the ions was large with a low degree of symmetry, factors which reduced the lattice energy of the crystalline form of the salt and hence reduced its melting point.

Ionic liquids were not new. For example, ethyl ammonium nitrate, melting point 12°C, was first described in 1914. But, it had been during the last decade that research and development had accelerated to a point where a huge number of novel ionic liquids and associated applications had been developed. Eutectic-based ionic liquids were relatively low cost and easy to prepare, and had the advantages of low water sensitivity and high metal solubility. Typically, these were composed of organic cations with halide anions and various complexing agents to make anionic complexes. An example was a eutectic mixture of choline chloride, produced commercially in high volume as an animal-feed additive, and urea or ethylene glycol. Chemistry of this type formed the basis of the immersion silver process and the related immersion gold process, the metals being added as chloride salts.

Other metal finishing application examples were an electroplating process for trivalent chromium, based on a choline-chloride ionic liquid, a chloroaluminate process for the electrodeposition of aluminium, and an elecropolishing process for stainless steel and nickel alloys. Demonstration equipment was in operation in the University’s Scionics Laboratory, which delegates had the opportunity to observe and to discuss with members of the research team. Metal finishing aside, a project was ongoing to produce a rechargeable battery from a combination of a zinc cathode and an electronically conducting polymer using an ionic liquid electrolyte, with the potential benefits of low-toxicity, low-cost, light-weight and high-energy density.

Professor Abbott commented upon the recyclability of ionic liquid process solutions. Metals could be precipitated from spent or contaminated solutions simply by adding water. Then, taking advantage of the low-vapour pressure of the ionic liquid, it was a straightforward procedure to remove the water by distillation.

He stressed that the primary objective of the IONMET consortium was not the selling of ionic liquids but the delivery of drop-in technology, an example being the process in commercial operation at Anopol Ltd for electropolishing fixings for racing car engines. With five years experience of ionic liquid electropolishing, from Hull Cell to production scale, Anopol’s Daniel Wheeler gave his user-view, stating first the perceived disadvantages: higher raw material costs, more difficult optimisation of operating parameters and more technically demanding solution analysis and control, compared with conventional sulphuric acid/phosphoric acid chemistry. On the positive side, the ionic liquid process was more environmentally friendly, substantially more energy efficient and offered lower process temperatures and shorter process times. An additional benefit was that both 300- and 400-series stainless steels could be processed under the same conditions. And the recyclability of the solution was seen as a specific advantage.

Having visited the ionic liquid demonstrator facility in the University laboratories, delegates travelled to the South Wigston factory of PW Circuits, where MD Cecil O’Connor and Technical Manager Jason Griffin gave a working demonstration of the IONMET immersion silver pilot plant which they had recently commissioned, and a comprehensive account of the challenges they had overcome in bringing the process from laboratory to production scale. Work had already been processed for in-house assembly trials, with good results, and PW Circuits were about to commence customer trials. The pilot plant would be made available for the preparation of third-party evaluation and qualification samples, by arrangement with EIPC or the University. A small auxiliary immersion gold plating process line, which shared pre- and post-treatment tanks with the silver line, would shortly be brought into operation. Immersion gold samples had already been produced in PW’s laboratory for exploratory wire-bonding trials.

In order that solderability could be independently verified, PW Circuits had submitted representative PCB samples finished with IONMET immersion silver to the International Tin Research Institute for testing. Tom Perrett, representing ITRI’s Soldertec Global laboratory attended the workshop to explain the test procedures and deliver the results. Perrett described the function and operation of the MUST II solderability wetting balance and the significance of wetting time and wetting force values, with examples of good and bad wetting, delayed and impaired wetting. Under test conditions characteristic of a typical lead-free soldering process, the IONMET immersion silver samples had shown rapid and a consistent wetting force. A clear affinity had been observed between the solder and the PCB pads, and he pronounced the samples to be “perfectly solderable”.

Project Leader Dr Khalid Shukri of Genacys moderated the open discussion which concluded the workshop programme. All agreed that the workshop had met with expectations, provided awareness, information and explanation, and generated interest in applications of ionic-liquid-based finishes in the PCB industry. It was commented that the immersion silver process required further characterisation and documentation before it could be considered market-ready. Dr Ryder agreed: a programme of work was in hand to identify working limits and to address the process control issues. The PW Circuits pilot plant was already providing significant feedback, and there was in-depth knowledge elsewhere in the IONMET project team in the control of related processes. The suggestion was made that partnership with a chemical supply house might be the most effective route to achieving a commercial product. In this connection, and in response to enquiries from process supply companies represented at the workshop, Dr Shukri advised that the intellectual property in ionic liquid applications was owned by the joint venture company Scionix, who would be pleased to consider proposals for future co-operation and to explore opportunities for commercialisation of ionic liquid technology. The demonstrator facilities at the University were freely available for the evaluation of new applications.

In his closing remarks, Dr Shukri commented that although the FP6 IONMET project itself was drawing to a close, work would continue on green replacement technologies exploiting the unique properties of ionic liquids to achieve results which could not be realised by traditional methods.

Pete StarkeyEAB member, Circuit WorldNovember 2009