ICT Evening Seminar - " Microwave Technology". (Full Review).
13th June 2002
Martin Goosey introduced to an attentive audience a well-balanced programme addressing the topic of microwave circuits from the perspectives of a specialist materials supplier, a niche-market PCB manufacturer dedicated to microwave work and a main-stream high-end PCB manufacturer.
Tom Brown of Holders Technology gave a comprehensive overview of material properties, which took away much of the mystique from the selection of materials for microstrip and stripline circuits. Design options were discussed for applications from the various requirements of telecom infrastructure, through radar, military and avionics, medical and optical, and space and satellite equipment. The cellular transceiver was chosen as a model on which to base comparisons.
The characteristics of a wide range of PTFE and non-PTFE materials were described in detail, with FR4 properties as a point of reference. Dielectric constant and loss tangent were the most obviously important parameters, followed by temperature coefficients of expansion and of dielectric constant and the effects of elevated temperatures on reliability. But manufacturability and the trade-off between cost and performance were equally significant in choosing the right material for a particular application.
Tom discussed materials for single and double sided designs, then considered multilayer constructions and the significance of the choice of bonding material, not only with regard to maintaining the desired electrical performance but also the processing considerations of laminating, drilling and through-plating operations. Of particular interest were mixed-dielectric builds which are increasingly used where digital and microwave functions are combined in the same board, for example where a PTFE antenna is integrated with an FR4 multilayer interconnect. The properties and relative merits of high and low temperature bonding materials, thermoset and thermoplastic, were explained with particular reference to sequential lamination.
Having reviewed heat transfer issues, and the thermal characteristics of filled and metal-backed materials, Tom concluded his presentation with a selection chart which tabulated the broad spectrum of available microwave material options and enabled material properties to be matched against functional requirements.
Gary Morgan of Labtech gave an insight to the practical realities of manufacturing circuits for diverse microwave applications, beginning with the statement that microwave circuits are not PCBs as such, but distinctive microwave components in their own right. In Labtech's market sector they represented very high added value and were made to particularly fine dimensional tolerances. Materials were susceptible to mechanical damage and demanded careful handling. Several of the operations involved in processing them were unfamiliar to the conventional PCB manufacturer and demanded critical process control.
He listed the many proprietary materials used at Labtech, each of which had specific attributes appropriate to individual designs. Apart from substrate characteristics, a significant consideration was copper cladding thickness, which determined the balance between power-carrying capacity and the control of etched line width.
Labtech had extensive experience of processing pre-bonded metal-back materials: copper, brass and aluminium in thicknesses up to 12mm, and the three-dimensional machining capability to enable waveguide features, or cavities for mounting components, to be cut directly into the metal. Special metal-finishing procedures had been developed to facilitate through-hole plating on these substrates, aluminium being the most difficult.
Gary described manufacturing process sequences for a series of specific examples, including PTFE multilayers, mixed-dielectric constructions with blind, buried and through vias, post-bonded to aluminium carriers, copper-cored multilayers and circuits with embedded components.
Further to Tom Brown's comments on choice of bonding film, Gary listed a series of applications for different thermoplastic and thermoset materials, and explained how silver-loaded epoxies could be used to achieve electrical and thermal conductivity.
Various metallic finishes were specified for microwave circuits, including immersion tin, brushed tin-lead, hard gold, pure gold, electroplated nickel, electroplated silver, electroless nickel and electroless palladium with immersion gold. The electroless finishes were preferred wherever possible, for reasons of thickness uniformity andconductor-edge protection.
Whereas Labtech's output included an enormous range of effectively hand-crafted specials, Mark Beesley described how Circatex had to consider different priorities in selecting materials to support increasing requirements in the telecom, automotive and consumer electronics sectorsfor high-frequency functionality.
In a presentation entitled "PTFE – what does it stand for from a volume manufacturer's point of view?" he looked at the challenges involved in deciding which materials satisfied electrical and thermal performance criteria without seriously compromising processability, affordability or availability. Moisture-absorption characteristics, with regard to conductive-anodic-filament growth, were of particular concern in automotive applications. Other issues were flexural strength, heat dissipation, z-axis expansion and glass transition temperature. High Tg did not necessarily infer good thermal stability. The low conductor peel strength of certain materials could have dramatic effects on yield, not only for the board fabricator but also for the assembler.
An important characteristic determining process yield of a material from Circatex's point of view was movement in the x-y plane, and its consequent effects on registration.
Their dimensional analysis technique had demonstrated that x-y distortion in high frequency materials was typically non-linear, and therefore extremely difficult to compensate in tooling.
Certain of the thermoset and blended materials could be satisfactorily conditioned for through-hole plating by conventional permanganate de-smear, but PTFE builds demanded non-standard, off-line treatment. Compromises in drilling and hole preparation were necessary for mixed-dielectric hybrid builds. Some materials had been bserved to be incompatible with finishing processes; leach-out of organics during plating could lead to nickel inhibition and affect solderability.
Mark commented that, ten years ago, PTFE laminates were the only available low-loss substrates suitable for frequencies of 1 GHz and beyond. In recent years the laminators had responded to market demand such that there was now a broad range of materials alternative to PTFE which could satisfy the demands of high-frequency applications whilst remaining processable with acceptable yield on conventional PCB equipment. Although their ultimate electrical properties might not equal those of PTFE, intelligent selection of the appropriate material could provide a satisfactory combination of performance, reliability and cost.
In answer to the question posed in the title of his paper, Mark concluded with the suggestion that, despite the unprintable opinions of past generations of process engineers, PTFE really stood for "Performance That
Fulfils Expectations..."
All present enjoyed an informative evening, and the generous hospitality of Shipley Europe in hosting the event is gratefully acknowledged.
Pete Starkey
ICT Council