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SPECIAL REPORT problems regarding poor adhesion of PTF silver conductors. Thermoplastic Gothenburg and Copenhagen, 23 and 24 October 1990 polymer silver has good flexibility. Multilayer PTF is feasible, but only with two layers, and a pinhole-free The text which follows reports on a co-operative project supported by dielectric of at least 30 microns is essential. One does not usually solder the Nordic Fund for Technology and Industrial Development. to such circuits an d crimping demands careful attention to cleanliness and freedom from dielectric. This one-day seminar, repeated in Sweden and Denmark, had its origins The paper also included reference to PTF resistors on flexible substrates, in a most interesting co-operative project which was supported in the four but a bending radius of less than 25 mm must be avoided. Ingenious designs Nordic countries by a total of eleven companies and research organisations. of a touch panel potentiometer (with high sensitivity to 'finger-rolling') A series of six projects, each relating to a particular aspect of Polymer as well as light-emitting facilities (LED or EL) show that membrane switch Thick Film Technology, was agreed by the participants, and the work technology has by no means exhausted its potential for development. programme was carried out mainly in national research institutes but with Martin Udden of IVF, Sweden, discussed the capabilities for PTF in cross a considerable input from industrial collaborators. over and EMI shielding requirements. He referred to the reduction of silver The programme involved a total expenditure of over £1 million and the migration between parallel PTF silver tracks covered by a polymer dielectric costs were shared between the four Scandinavian governments and the when each track is printed on a wider PTF carbon track. Also, in the case industrial supporters. At the conclusion of the project work, which lasted of a PTF silver cross-over on a polymer dielectric, silver migration is reduced for about two years, a series of reports was issued and the seminars were if a layer of PTF carbon is printed on the dielectric under the silver bridging planned to review progress across the whole range of PTF technology and conductor. In discussion it was generally agreed that the former effect related its applications. In addition, a book* has been produced, also on a to electric field gradients ending entirely at carbon 'electrodes' and in the co-operative basis, providing explicit design and lay-out guidelines together second case the carbon layer likewise protected the PT F silver from any with details of a number of different industrial applications where PTF direct involvement with field intensities. The speaker also referred to technology has proved itself to be viable. evidence that the properties of PTF silver conductors could be improved The chairman of the seminar in Sweden was Martin Udden of IVF (the if UV curing was used. host Institute for the meeting) and in Denmark Henrik Hvims of Dr Leif Halbo of EB Technology AS, Norway, spoke on the subject of ElektronikCentralen, where the second seminar was held. Otherwise the design guidelines, but also referred to examples where his company (a PCB same group of seven contributors spoke on each of the two occasions, which manufacturer without experience of hybrid circuits) had experienced also included a short visit to selected facilities in the laboratories of the significant cost reduction by changing, in part, to PTF technology. host organisation. Dr Halbo pointed out that, if a conventional PCB has a plated-through- In Sweden the seminar began with an introduction and welcome by Carl hole with an annular ring of copper on what will be the PTF side of the Gunner Bergendahl of IVF, who had spent some time in Japan examining board, it is essential to extend the copper from the ring to some other region attitudes to and applications of PTF . He summarised the main driving force away from the hole where a PTF silver track can be reliably terminated of electronic design as amounting to Thinner, Smaller, Shorter and onto the copper. It is bad design to have this junction at or around the Lighter—in all of which objectives PTF could play and was playing a part. hole. This and other examples of good design practice relating to the In emphasising that, far from developments being technology-inspired, they geometry of PTF conductor, resistor and dielectric patterns in relation to were invariably market-led, he summarised his overall impression that in copper foil were presented, and these are all described in the book which Japan they (a) combine technologies, (b) refine them, and (c) then sell them! was referred to above and which was edited by this speaker. He also His message could with advantage be continuously echoed throughout the emphasised the attraction of the printed silver-through-hole technique and whole of Europe. resultant cost savings. The latter result in part from the possibility of The first main contributor was Søren Nørlyng of Brüel & Kjaer, changing from a drilled FR-4 board to a punched FR-2 board as well as Denmark, who spoke on 'PTF—The Technology and its Possibilities, from the elimination of the costly electroless through-hole plating process. Illustrated with Applications from Brüel & Kjaer'. This company, with its Raimo Rikola of VTT, Finland, described the use of PTF on low volume but high technology products, must regard PTF as a thermoplastic substrates and concluded that optimum results came from complementary technology fitting in between PCB assemblies and hybrid the use of polyetherimide (e.g., Ultem 2312 with 30% glass fibre) or circuits. In one application they have used a PCB with an added hybrid polyethersulphone (e.g., Victrex 3601). circuit on ceramic, an active device as a COB on the PCB and then a number Then followed a paper on connection techniques by Claus Würtz Nielsen of PTF pull-up resistors directly printed onto the PCB . This process begins of Ferroperm, Denmark, formerly of AS Elbau. He dealt in detail with wih the acquisition of an appropriate PCB with etched copper conductors, silver-through-hole technology and stressed the importance of ink rheology holes etc. from a normal supplier, but the PTF resistors and associated with different requirements for print-through material and conductor track conductors are fabricated in-house. B & K began work on PTF in 1977 inks. Vacuum, as distinct from pressure, application has proved satisfactory and were particularly attracted by the facility of producing potentiometers for disc-collapse on both sides and both reliable connection and consistent on ceramic which were then integrated with the rest of the circuit. They conductivity are readily achieved. Boards of 1·6 mm were used with holes prefer to use UV cured inks partly to avoid atmospheric contamination! of 0 · 6 and 0· 4 mm and two sizes of annular copper. Tests including shock, Not surprisingly, this company makes use of PTF for a variety of vibration and damp heat on 110,000 silver-through-holes gave failures of developments based on membrane switch technology. These include Touch only 109 ppm and these were confined to the smaller copper ring on only Screen Overlays with two resistor overlays facing each other separated by two boards. a matrix of insulating dots where pressure at any point produces two voltage An alternative silver-through-hole process, only suitable for high volume signals which define the co-ordinates involved. Bought-in sheets of polyester, production, involves a set of pins dipped into the ink and then inserted sputter-coated with indium/tin oxide, are used in preference to the more simultaneously and rotated slowly in all the holes. expensive gold coatings. The writer was invited to present an overall view of the growth, current Other variants include electroluminescent coatings to provide back status and future potential of PTF technology and was privileged to join lighting for displays in automobiles and aircraft. Multilayer PCBs are used in at the conclusion of this joint project. Not only does the Nordic project but if, for example, the design requires four layers of a conventional deserve to succeed on the basis of the dedication of those involved but also multilayer PCB, it has been found that savings of over 50% in cost can in recognition of the foresight of the companies and national authorities be achieved with the use of a double-sided PCB and additional connections which have supported the considerable effort devoted to the projects. Other produced by PT F printing and curing. Some of the saving results from companies and indeed other countries throughout Europe will ignore the the replacement of 33 chip resistors by a single print of PTF to produce clear message from this project at their peril. Readers of this review may the wide-tolerance pull-up resistors required on the circuit. wish to read again the simple but stark comments from Carl Gunner The next paper from Kjell Ramberg of ISO AS , Norway, provided further Bergendahl reported earlier. examples of the effective use of PTF technology, this time by a smaller The writer returned to the UK greatly impressed by the work undertaken, company with a background in membrane switch manufacture. The paper extremely concerned at the continuing indifference of many managers and therefore concentrated on the use of PTF on flexible substrates. technologists to the value of PTF technology and amazed at a group of Polycarbonate foil is most frequently used, but cracking can occur after technologists from Scandinavia, each of whom presented clear papers in 500K operations and, while polyester provides a longer life, there can be English in a relaxed and fluent manner which surpassed the quality of presentation frequently achieved by UK and American technologists speaking in their native tongues. * This book, emitted 'Polymer Thick Film Technology: Design Guidelines and Applications', is reviewed on p. 63 of this issue of Hybrid Circuits. P . L. Kirby
Microelectronics International – Emerald Publishing
Published: Jan 1, 1991
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