ICT Evening Seminar - "Multilayer Laminating".
27th April 2005

The Institute of Circuit Technology held a well-attended evening symposium at the premises of Rohm and Haas in Coventry on 27th April 2005, on the theme of multilayer laminating. ICT Chairman Andrew Hall welcomed delegates and introduced presentations from Alan Hanson of MacDermid, Geoff Layhe of Lamar and Dave Wayness of Rohm and Haas.

Alan Hanson discussed the impact of oxide and oxide-alternative bonding processes on high-frequency conductor loss, explaining with the aid of some complex mathematical modelling the signal-carrying characteristics of printed-circuit-board-level interconnects in the context of ever-increasing transmission frequencies. Because of the tendency for the signal to be carried in the skin, rather than in the core, of the conductor at frequencies in the 20GHz range, experiments had been designed to study the effects of surface roughness produced by different multilayer bonding treatments on a centred-stripline construction. After extensive analysis it had been concluded that, although low-etch oxide-alternatives gave slightly lower loss factors, surface roughness effects were relatively small in comparison with the consequences of reductions in conductor cross-section resulting from re-work.

Geoff Layhe defined a series of essential conditions, all of which required to be properly understood, carefully selected and closely controlled in order to achieve successful multilayer bonding. Using plain language, he presented a clear and comprehensive cook-book which explained the attributes of different types of press platens, press pads, tooling plates, separator plates, release films, copper foils, core materials and pre-pregs. He discussed the individual and combined effects of variations and tolerances on the properties of the resultant laminate, with particular reference to factors influencing the rate and extent of resin flow and the influence of glass-cloth style on dimensional stability. The common-sense conclusions were to understand the choices available, to find out by experimentation what parameters gave the most acceptable results, to use them consistently, and not to alter them without a clear understanding of the potential outcome.

Dave Wayness referred to the increasing requirement for circuit boards in automotive applications to handle high currents, necessitating the use of very heavy copper, typically in the thickness range 200 – 400 microns. His presentation described techniques and materials which had been developed to overcome the problems associated with the encapsulation and planarisation of these extremely high-profile conductors. A UV-curable two-pack acrylic-based filler formulation was now available which had a long pot-life and could be applied either by open-stencil screen printing or by a horizontal coating machine with a high-volume, low-pressure fluid head. Process development had so far been concentrated on the planarisation of surface layers prior to solder mask coating, but there was scope for adapting the concept to the encapsulation of thick-copper inner layers in planar transformers and similar multilayer builds.

In his closing remarks, Andrew Hall acknowledged the kind hospitality of Rohm and Haas Electronic Materials in hosting the event.

Pete Starkey
ICT Council

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