ICT Northern Seminar, Darlington, UK, March 2, 2010

Citation

(2010), "ICT Northern Seminar, Darlington, UK, March 2, 2010", Circuit World, Vol. 36 No. 3. https://doi.org/10.1108/cw.2010.21736cac.003

ICT Northern Seminar, Darlington, UK, March 2, 2010

Article Type: Conferences and exhibitions From: Circuit World, Volume 36, Issue 3

Spring had not quite arrived, but the snow had melted away and the crocuses had begun to open in picturesque Middleton One Row. While the sun was setting over the Tees Valley, Institute of Circuit Technology Technical Director Bill Wilkie welcomed delegates from Scotland and the north of England to the Devonport Hotel for the ICT’s first Darlington Seminar of 2010, which included presentations from Lamar Group, Park Electrochemical, Electronics Yorkshire, and the IeMRC.

First to speak was Geoff Layhe from Lamar, with a very informative introduction to thermally conductive laminates – why they were required, and what were their properties and processing characteristics. He began by calculating the heating effect of a 1 A current on a 0.18 mm circuit trace in 18- μm copper. In the absence of a means of dissipating the heat, the temperature could rise locally by about 350°C. This was far above the decomposition temperatures of typical laminating resins. High-current devices, such as automotive electric power steering systems, inverters and DC power converters demanded efficient thermal management, as did high-power LED lighting.

A practical alternative to heavier copper, or external or internal heat sinks, was a family of materials known as insulated metal substrates (IMSs), typically composed of a copper foil bonded to an aluminium substrate with a thermally conductive dielectric. IMS materials were available with copper foil thicknesses of 35, 70, 105, 140, or 210 μm, dielectric thicknesses of 80-180 μm, and aluminium thicknesses of 1, 1.5, 2, or 3 mm. The dielectric was loaded with inorganic filler, which increased its thermal conductivity (and, as a secondary effect, its dielectric constant). Thermal conductivity was measured in Watts per metre-Kelvin (W/mK), and commercial IMS materials were available with values from 2 W/mK, suitable for low-power LED applications, up to 8 W/mK for high-power LED and power conversion. About 14 W/mK materials were in development. By comparison, FR4 laminate had a thermal conductivity of 0.4-0.6 W/mK, and aluminium 237 W/mK.

Circuitisation was generally by conventional single-sided print-and-etch technology, the aluminium being protected with a peelable film. Drilling of mounting and tooling holes could be carried out on standard drilling machinery, but with a lower chip-load and slower in-feed than would be used for FR4. Two-flute routers gave a considerably better finish than diamond-cut routers for profiling. Some aluminium grades were suitable for scoring.

For applications like power transistor modules, requiring significantly greater thermal conductivity than could be achieved with the most efficient IMS materials, the alternatives were direct bonded copper on either silicon nitride (up to 68 W/mK) or aluminium nitride (up to 180 W/mK).

George Wheadon from Park Electrochemical explored some of the challenges and practical realities of sequential lamination, with particular reference to a failure mechanism known as “eyebrow cracking”, seen in typical HDI applications, especially in conjunction with low-Dk materials and lead-free reflow profiles. The failure tended to occur as a de-lamination at the interface between the resin plug in a buried via and the adjacent pre-preg layer, and was a consequence of z-axis expansion mismatch. The effect was common if the vias had been plugged during standard vacuum lamination using conventional FR4 prepreg, and could be significantly reduced by using prepreg based on heavily filled phenolic-cured resin, specifically designed for low-thermal expansion. Alternatively, the vias could be pre-plugged in a separate operation, although this was a high-cost option, with significant capital investment required.

During the succession of re-lamination operations involved in building an HDI board, there was a risk of progressive embrittlement of the resin. Whether or not to fully cure the sub-laminates prior to re-lamination remained a dilemma. Wheadon demonstrated that by careful choice of appropriate materials and press cycles, HDI boards constructed by three-times sequential lamination would consistently withstand nine reflow cycles at 260°C followed by three additional cycles at 300°C with no evidence of eyebrow cracking. Properly controlled storage of prepreg was critically important, and an extended pre-vacuum stage in the re-lamination cycle gave reliability benefits.

Eric Hinsley introduced Electronics Yorkshire (EY), a not-for-profit organisation funded by Yorkshire Forward and the European Regional Development Fund, with directors drawn from the electronics industry. EY’s objectives were to provide dedicated training to companies and individuals in the electronics sector, to maintain a technology centre offering a broad resource of leading edge equipment for inspection, test and repair, and to operate a membership network for promoting collaboration and complimentary services between electronics businesses, to the benefit of individual companies and the UK electronics industry generally. EY was one of only three IPC accredited centres in the UK capable of training operators and trainers, with courses ranging from industry standard soldering to advanced surface mount technology with BGA placement. Environmental issues took a high priority and EY had recently run the Envirotronix programme to promote environmentally responsible manufacturing. EY was also committed to encouraging an interest in electronics amongst young people in order to enthuse future engineers at an early age, and had won awards for their Educational Outreach Programme.

Professor Martin Goosey, Industrial Director of IeMRC and newly elected Chairman of the ICT, gave the concluding presentation, entitled “Advanced PCB interconnection technology – materials challenges”. He summarised the many new requirements being demanded of PCBs, including smaller features and increased interconnection density, enhanced functionality, higher frequency operation, improved thermal performance, and environmental compliance, as well as cost, productivity, and yield improvement. Recent innovations in PCB technology included new and enhanced substrate materials, laser techniques for drilling and imaging, increasing microvia complexity, integration of passive and active components, optical functionality, and additive digital circuitisation techniques. These innovations were driving the development of new material technologies and processing techniques and presented many new challenges and opportunities for material suppliers. Although FR4 continued to be the industry’s mainstay base material, it was not suitable for some of the more advanced applications and there was a growing need for higher performance substrates. Goosey referred to the Prosurf Technology Road Map to illustrate projected future requirements. Considering flame retardancy in the context of environmental legislation, there remained some concerns about TBBPA as a constituent of FR4 resins, and halogen-free materials would inevitably be promoted in the future. It was ironic that, besides gratifying the environmental lobby, the removal of TBBPA could enhance the thermal stability of epoxide resins.

A project being supported by IeMRC had taken a radical new approach, and was evaluating glass as a substrate for high-density interconnect. CMZ glass had expansion characteristics similar to those of silicon and offered dimensional stability, electrical insulation, and high-temperature capability as well as the potential for optical interconnect. The project aimed to create multilayer substrates from thin glass sheets, using laser machining for microvias and tracks and electroless metallisation to provide conductivity. Examples were shown of functional circuits already produced to demonstrate the technology.

Bill Wilkie closed the formal proceedings with a vote of thanks to the presenters, and to Lamar Group for supporting the event. Informal technical discussions amongst delegates continued well into the evening.

Pete Starkey

I-Connect007