Institute of Circuit Technology Harrogate Christmas Seminar 2018

Estimated reading time: 10 minutes

In “Everything’s Getting Hotter! Are High-temperature Electronics Pushing the Boundaries for Solder?” Martin Wickham, senior research scientist at the National Physical Laboratory, discussed the limitations of conventional solders. He also suggested alternative sintered polymeric interconnection materials and described the development of a high-temperature protective coating to enable organic PCBs to operate at higher temperatures. Wickham explained that RoHS2 could create an automatic expiration of exemptions for lead-based high-temperature solders if they were not renewed by requests from industry, and the choice of lead-free alternatives was limited to alloys like tin-antimony and gold-tin for operation above 200°C.

He described an alternative technology developed by the partners in the Elcosint project based on a low-temperature-sintering nano-silver formulation, which would allow a standard SMT production sequence to be maintained but enable operating temperatures of over 250°C. Thermal ageing testing had demonstrated Elcosint to have significantly better reliability than high-melting-point solder at a sustained 250°C, although some of the failures on the soldered samples could be attributed to the high reflow temperature causing degradation of the materials of the PCB.

The outcome of the Elcosint work set a meaningful background to Wickham’s discussion of the results of the Tamessa project. Some interesting observations had been made on conductor cross-sections after long-term 250°C ageing; the copper metal was being progressively converted to oxide. The Tamessa project aimed to create a lead-free manufacturing process based upon standard SMT technology with organic substrates and produce complete electronic systems capable of continuous operation in high-ambient-temperature environments. Substrates and coatings had been developed that specifically inhibited the ingress of oxygen and in high-temperature reliability testing gave results far superior to those achieved with polyimide substrates and coatings and offered a cost-effective alternative to the use of ceramic materials in high-temperature applications.

Work was continuing to understand potential performance enhancements associated with high-temperature coatings and included a review of design rules, improvements in attachment reliability, and the development of a test vehicle to determine the rate of oxidation of inner layers using electrical resistance measurements.

The final presentation was an eye-opener from Garry Millington, CEO of Devtank Ltd, entitled “The Technological Challenges of Designing and Making PCBs for Space.” An RF engineer with 30 years’ experience, the last five of which had been with major contractors in the space sector, Millington identified two main categories within the sector: big space and commercial space. Big space was associated with science missions to outer space, typical satellites in geostationary earth orbit at a height of 22,000 miles, making one rotation per day at a speed of 7500 mph. Commercial space related to satellites in low earth orbit, 200– 1000 miles high, typically making 10–11 rotations per day at speeds around 17,500 mph. The International Space Station fit this category.

A huge commercial project in prospect was the OneWeb constellation of over 800 satellites to provide global broadband internet service to individual customers. And thousands of launches were planned of CubeSats, NanoSats, and PicoSats—very small satellites weighing between 1 and 25 kg—from U.K. spaceports. So, there would be a demand for printed circuits and assemblies suitable for the space environment but at a relatively low cost, creating massive opportunities for the U.K. PCB industry. “In the big space business, you get paid for the paperwork, not the product! This is more commercial stuff.”

But what specific requirements would PCBs and PCB assemblies have to meet for space applications? Millington discussed the mechanical stresses, shocks, and vibrations associated with the launch profile, and the subsequent in-service stresses of vacuum, radiation, and thermal and power cycling.

Two critical items of documentation the PCB fabricator was required to provide were the declared materials list (DML) and the declared process list (DPL). “You need to understand your materials and processes if you want to work with the agencies. You will go through a lot of pain before you get on the qualified suppliers list, but it’s worth the effort once you’re there!”

Outgassing, the release of gases entrapped during manufacture or evaporation of residual volatile material under the vacuum conditions experienced in space, was a major issue—particularly the potential damage it could cause to optical instruments on the payload. Millington showed illustrations of vacuum chamber testing and referenced NASA specifications for materials such as plugging resins. All of these materials were required to be declared on the DML with supporting data sheet information.

Millington listed several other factors that could affect PCB performance in space, particularly the robustness of layer-to-layer interconnections over mission durations. He believed matched CTEs to be essential to avoid failures during temperature cycling. It was important that the dielectric constant of the laminate was stable with temperature and long-term ageing, and breakdown voltages were affected by vacuum. Thermal conductivity was key to heat removal from electronic systems in space because, obviously, there was no air to enable convection cooling. Tin whiskers could not be tolerated, so leaded solders were mandatory together with conformal coating where appropriate.

There were ongoing technology challenges as frequencies continued to rise and structures decreased in size with increases in complexity and power density, requiring more integration and improved heat dissipation. Many of these challenges were similar to those faced in designing 5G infrastructure, and the PCB was no longer just a means for carrying and providing power to components—it was itself becoming a component. Projects were underway to determine the suitability of embedding components in PCBs for space applications and integrating thermal interface materials in the PCB for heat management.

Serial manufacturing of commercial satellites was becoming a reality, using automotive-style production techniques. Millington ended his presentation with a video of a dedicated factory already producing more than two OneWeb satellites daily.

Professor Andy Cobley made closing remarks, thanking the speakers for sharing their experiences, delegates for their attention, and GSPK Circuits for supporting the event, and a special thanks to Bill Wilkie for pulling it all together. Cobley was delighted to take the opportunity to present Victoria Blaisdell, group managing director at Holders Technology, with a certificate to welcome Holders as the Institute’s newest corporate member.

I have been recording ICT events for approaching a quarter of a century, and I never fail to be impressed by the community spirit that exists within our Institute. Although these seminars are offered a technical learning experience primarily, they are much more than that—they are an opportunity for members to get together, network, and communicate on a friendly and informal basis. And, although there may be fewer of us these days, we are still proud to belong to a strong and thriving industry.

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