ICT 45th Annual Symposium Review

Estimated reading time: 9 minutes

Anderson explained some of the science of wearable and stretchable performance, which required a balance between comfort and function, with reference to graphs of mechanical stress and electrical resistance versus strain. He also described a stretchable bi-layer thermoplastic polyurethane film composed of high-recovery and melt-adhesive layers supplied on a temperature-stable carrier. It was also designed so that a stretchable ink could be printed on the high-recovery surface and the melt-adhesive layer used to hot-press bond it to the fabric. A silver-loaded screen-printing ink had been developed with the metal in the form of fine flakes suspended in a thermoplastic elastomer resin. During the drying step, the solvent was removed, and the conductive particles packed together forming electrical pathways. He showed the results of 100% stretch testing and 80-cycle 10% strain testing—both of which maintained low electrical resistance—and functional wash testing based on 50 cycles of the ISO 6330 colour fastness test with minimal increase in resistance.

A typical volume production sequence for the incorporation of conductors into apparel was to screen-print circuits onto the polyurethane substrate, singulate the patterns by die-cutting or laser-cutting, hot-press-bond the circuits onto fabric, add any functional components, and then assemble the garments. Applications included sportswear for monitoring respiration and heart rate, heated industrial winter wear, and various medical monitoring functions and automotive heaters and sensors.

Dodging the showers, delegates made their way down the hill to the authentic reconstruction of a 1930s High Street and queued up to enjoy a traditional lunch at Hobbs and Sons Fish and Chip Shop. Some paused to admire the Sunbeams at Hartill’s motorcycle shop and had to hurry back uphill to return to the conference room and the 21^st century for the afternoon session.

"High performance automotive and power electronics begin with innovative materials," was the opening line of an enlightening discussion of insulated metal substrates and thermal interface materials from Robert Art, global account manager IMS/TIM for Ventec International Group. He listed the basic components of a typical IMS material—a metal base layer, a dielectric layer, and a circuit layer. Then, he looked more deeply into the realities of their properties and how they needed to be considered from a different perspective than those normally associated with FR-4 laminates when specifying materials for particular applications.

Beginning with glass transition temperature (Tg), for the gradual and reversible transition in amorphous materials from a hard and relatively brittle state into a rubbery state with increasing temperature, the popular perception was that this should be very high. This was true for achieving mechanical stability in regular FR-4 multilayers, but in IMS materials, a lower Tg was preferable. This enables the dielectric to act as a compliant interlayer between components with expansion coefficient typically 10 ppm/°C and the aluminium baseplate with expansion coefficient typically 23 ppm/°C, hence reducing any tendency to cracking of solder joints under thermal cycling.

Clearly, the primary reason for using IMS substrates was to conduct heat away—“LEDs get destroyed not because of current but because of heat”—but there was still some confusion in terminology (e.g., thermal resistance, thermal impedance, thermal conductivity, etc.) and different test methods could give widely different results. It was important that the customer carried out their own tests before specifying a particular material rather than relying on nominal data-sheet values.

Voltage-withstand testing depended on the application and was linked to the operating voltage of the system. UL required the test voltage to be twice the operating voltage plus 1,000, and users typically asked for higher values than that. Art emphasised that each test stressed the material, and the circuit should still work after testing rather than be tested to destruction.

He went on to discuss Ventec’s range of IMS materials, and the technology roadmap showing the typical applications for which they had been developed. In applications where solder-joint cracking was a critical reliability issue, the newer materials had dielectrics designed for elastic deformation and low-expansion aluminium alloys to minimise mechanical stressing.

Art examined a series of alternative design concepts for thermal management with IMS substrates and special surface treatments and demonstrated applications and benefits of electrically conductive and insulating thermal interface materials. He emphasised the importance of choosing the right material from a cost-performance standpoint but added a note of caution in advising OEM designers to avoid nominating too many different materials and causing inventory and stock-control problems for the PCB fabricator.

Taking the guesswork out of copper distribution in pattern plating operations was the subject of the final presentation by Robrecht Belis, manager of the Surface Finishing Business Unit at Elsyca in Belgium.

Achieving relatively uniform deposit thicknesses across complicated panel designs was a task that had historically relied heavily on the combined experience of pre-production and process engineers and had inevitably involved a lot of trial-and-error and compromise. Belis described graphical simulation software that could be straightforwardly configured to a specific plating geometry and electrolyte, panel size and pattern, and process parameters based on a virtual mockup of the real-life plating tank and flight bar. Alternative panel configurations and the effects of shields and robbers could be quickly evaluated. The software enabled a substantial improvement in plating uniformity and provided detailed information on copper thickness distribution and potential quality issues.

ICT deputy chairman Steve Payne, managing director of Cirflex Technology Ltd, brought the symposium to a close, thanking speakers for sharing their knowledge and the quality of their presentations, delegates for their attention and support, and particularly Bill Wilkie for organising an exceptionally interesting and relevant programme.

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