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Estimated reading time: 9 minutes
The European Angle
By Pete Starkey
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An Inside Look: ICT's 39th Annual UK Symposium
The Institute of Circuit Technology (ICT) Annual Symposium has become a must-attend conference and networking event for the UK printed circuit industry. Technical Director Bill Wilkie chose a splendid venue for the ICT’s 39th Annual Symposium--the Heritage Motor Centre in Gaydon, Warwickshire, in the Midlands of England, home to the world’s largest collection of British motor cars from the classic, vintage, and veteran eras.
The UK PCB industry was remarkably well represented, with an impressive collection of fabricators and suppliers (including a few personalities from the industry’s classic, vintage, and veteran eras…) gathered together to absorb up-to-date knowledge from eminent presenters, to renew acquaintances, and to share information, gossip, and opinion with their peers.
ICT Chairman Professor Martin Goosey welcomed the gathering and remarked upon the continuing growth of ICT's membership, now well into the 300s, and the wide and increasing range of services to members: The annual symposium, evening seminars, foundation courses, a technical journal and website, as well as collaboration in research projects such as ASPIS, coordinated by ICT and now in its final stages.
Professor Goosey introduced a well-chosen programme of technical presentations, commencing with an up-to-the-minute review of PCB substrate materials for thermal management from Ventec Europe Technical Support Manager Ian Mayoh. He explained that, besides general trends to miniaturisation, higher density, and embedded technologies in multilayer circuits, a major driver is the continuing growth in high-power LED lighting applications. Insulated metal substrates (IMS) enable the reduction of system costs through the reduction in size or elimination of cooling fans and heat sinks. Compared with standard FR4 laminate, which have a thermal conductivity around 0.25 Watts per metre Kelvin (W/mK), IMS materials are currently available with thermal conductivities between 1 and 5 W/mK, and 7 W/mK materials are in development. Mayoh stressed that the importance of understanding the actual thermal impedance of a material, which is a function of both its thermal conductivity and thickness, is the meaningful practical consideration in determining the suitability of an IMS for a particular application, and a cost-effective solution can often be found without having to use the highest thermal conductivity (and most expensive) material. He discussed the relative attributes of glass-reinforced and non-reinforced materials in terms of thermal and mechanical performance, formability and cost/reliability trade-offs, and advised against specifying a material purely on data-sheet information without carrying out proper trials.
Next to speak was Stuart Hayton, sales and marketing director for Mutracx, with a thought-provoking discussion of the dilemma faced by companies choosing whether or not to pursue “sustaining” or “disruptive” technology routes. Hayton offered sobering examples of big names with successful histories who concentrated on putting too much emphasis on customers’ current needs and failed to adopt new technologies or business models to meet customers’ unstated or future needs--losing their leading position as a result. Quoting Clayton Christensen, Hayton explained that in their efforts to provide better products than competitors and earn higher prices and margins, suppliers often “overshot” their market and gave customers more than they needed or were ultimately willing to pay for. He illustrated this with his own mobile phone, which he said had many more functions than he ever wanted and some which he had not even known to exist. For established companies, it was easier to rationalise the decision not to invest aggressively in disruptive technologies because these tended to have a lower initial profit margin for the supplier than contemporary technology and were often conceived and commercialised in different market areas. Indeed, in most cases failure could be attributed to classical “good management.”
A specific case in point was Kodak’s failure to realise the opportunity presented by digital photography and to cling to film whilst their technology was effectively obsoleted by their digital competitors. And film had a particular relevance to PCB imaging--Hayton described how the technology had progressed from hand-laid artwork and process cameras, through vector and laser plotters to laser direct imaging (LDI). He does not consider LDI to be a truly disruptive technology, since it still involves the wasteful use of photoresist and its associated processing. And although LDI has been commercially available for over 15 years, it still only represents 5% of the world PCB imaging market, the remaining 95% relies on silver halide film. The real disruptive technology emerging is ink-jet primary imaging, which eliminates 11 of 15 process steps in the imaging of inner layers, and only places resist where it is actually needed. The technology itself is not new--it was developed to a high level in the graphic arts industry, but brings a new value proposition to the printed circuit industry and offers the capability for inner layers be ready for etching within five minutes of the image data being output from the CAM station.
The ASPIS project, mentioned by Professor Martin Goosey in his introduction, is in its final stages and Professor Karl Ryder described how his research group at University of Leicester had previously evaluated immersion gold processes based on ionic-liquid chemistry as a potential means of overcoming the hyper-corrosion effects observed when aqueous gold chemistries are used in the ENIG process. A series of immersion gold formulations were prepared using Ethaline 200, an ionic liquid composed of ethylene glycol and choline chloride in 2:1 molar ratio, and gold in the form of gold chloride, gold cyanide, and potassium cyanoaurate, and these were used to deposit gold on a standard electroless nickel surface. Quartz crystal microbalance techniques were used to study plating rates, scanning electron microscopy, and atomic force microscopy to study surface morphology, and wetting balance measurements to test solderability. The roughness of deposits from ionic liquids was less than the equivalent from aqueous solutions, and there was less evidence of corrosion of the electroless nickel. Soldering tests indicated that coatings from ionic liquids wetted faster and more reliably than those from aqueous processes. Spun out of the ASPIS work, a current project supported by Innovative electronics Manufacturing Research Centre (IeMRC) is exploring applications of ionic liquids as soldering fluxes, where their ability to readily dissolve metal oxides without the use of acids potentially offers significant advantages.
Another IeMRC-funded project is concerned with the functionalisation of copper nanoparticles for applications in electronics manufacturing. Dr. John Graves described work being carried out at the universities of Coventry and Loughborough together with industrial partners with the objective of substituting commercially-available nanocopper powder for tin-palladium catalysts in electroless plating processes. It is necessary to coat the nanoparticles to improve dispersion stability, inhibit oxidation, and to promote adhesion to surfaces. The chemistry of self-assembled monolayers (SAM), of which there is in-depth experience at Loughborough, is being evaluated as a means of achieving an appropriate functional coating, and remarkably good initial results have been observed in the catalysation of glass and polymer surfaces. A particular challenge is the successful dispersion and de-agglomeration of the SAM-functionalised nano-powder, and Dr. Graves demonstrated how Coventry University’s expertise in sonochemistry helped overcome this problem using high-power ultrasonics.
From ultrasonics to megasonics, a related technique mainly used in the silicon industry, which operated in a higher frequency range (typically 0.8 to 2 MHz compared to 20 to 200 kHz for ultrasonics) and gave less-aggressive cavitation effects. Dennis Price from Merlin Circuit Technology reported his work with Heriot-Watt and Greenwich universities on the ASPECT project, studying the effects of megasonic agitation in the fabrication of high-aspect-ratio blind microvias. Their objective is to reduce the number of sequential bonding operations required to build complex, multilayer microvia PCBs by carrying out a single bonding operation followed by controlled-depth mechanical drilling to various levels to achieve interconnection to several layers in a single plating operation, rather than through a series of drill, plate, and build-up stages. The metallisation and electroplating of high-aspect-ratio blind holes presents particular challenges and conventional agitation cannot guarantee consistent results. Ingenious methods have been developed for modelling, observing, and measuring solution flow in and around via holes, and megasonic agitation has been demonstrated to facilitate the break-up and extraction of entrapped air bubbles. On a micro scale, acoustic streaming can give an enhancement effect in through vias although its influence is small compared with diffusion. Improved performance has been achieved due to the high level of bulk concentration next to the mouth of the via, supported by acoustic streaming along the board. Price concluded that, within microvias, megasonic agitation gives improved ion transport, mainly due to the presence of resonant bubbles rather than by acoustic streaming.
The focus moved from high-frequency sound waves to design considerations for high-frequency signal integrity in the presentation by Martyn Gaudion, CEO of Polar Instruments, who began by saying “Don’t always trust the data sheet” when considering impedance and loss. Regardless of whether impedance was modelled or measured, a multiplicity of factors should be taken into account when frequencies higher than 100MHz are involved, and results are likely to be inaccurate unless specialised 2D field-solver programmes are used for the calculations. A range of loss mechanisms comes into play as data-rates move into the multi-GHz range. Skin depth becomes a serious consideration and the surface roughness of copper conductors, particularly that resulting from bonding treatments, becomes a crucial factor. Solder mask can have a dramatic effect on impedance, and this can be further influenced by moisture absorption. It is preferred to route critical traces on inner, rather than outer layers. Even then, the natural dielectric inhomogeneity of woven-glass reinforced laminates is a limitation, although this could be mitigated to an extent by non-orthogonal routing. In general, losses can be reduced by the use of short traces, as wide as possible, and the smoothest copper consistent with adequate adhesion to resin. Gaudion discussed how to minimise crosstalk and explained mode conversion and field distribution in differential pairs. Finally, he demonstrated how to properly document modelled results into meaningful stack-up specifications and technical reports.
Back to practical aspects of PCB manufacture, as Chris Serre, managing director of Union Tool, described current technology and latest developments in drilling and routing tools, and discussed the technical and functional details of tool geometry, materials, and coatings. Another advocate of being careful of what to believe in data sheet statements, he listed the attributes and benefits of single-flute drills, which are capable of better registration accuracy, higher capacity for swarf removal, and longer life than their conventional two-flute counterparts and certain “parallel-flute” offerings. Furthermore, single-flute drill bits can be re-pointed on existing machinery. Proprietary lubricant coatings further assist in swarf evacuation, and Serre illustrated their effect using high-speed video. Taking the example of a 0.1 mm single-flute drill, an uncoated tool is capable of 6,000 hits and one re-point, giving an effective total of 12,000 hits, whereas the coated equivalent can be used for 16,000 hits before re-point and re-pointed twice, giving an effective total of 48,000 hits. Flute design is not the only physical consideration: The drill shank is a critical component in its own right. Traditionally, tools were made from solid tungsten carbide. New composite tools are becoming popular, with the shank made from stainless steel. This offers not only a significant material cost benefit over tungsten carbide, but it is better at absorbing spindle vibrations and run-out. Diamond-based wear-resistant coatings on routing tools give significant improvements in life and resistance to breakage, and with the increasing requirement to machine aluminium-backed IMS substrates, lubricant coatings substantially reduce the tendency to clog tools with aluminium.
When the symposium drew to a close, and Bill Wilkie had thanked presenters for their contributions, delegates for their attention, and Ventec Europe for their generous sponsorship, there seemed a certain reluctance for attendees to rush home. Many took time to browse the spectacular selection of museum exhibits on their way out of the conference area. A very satisfying and informative day all around.
Author’s Note: Pete Starkey is grateful to Alun Morgan for kindly allowing the use of his photographs.
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