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42 DESIGN007 MAGAZINE I JANUARY 2025 Modeling and Measurement e downward shi in geometries means there will be increased impacts of trace resistance, but it is also time to take a fresh look at inductance, as this, too, is affected as trace separations come down. Additionally, because materials are thin, we may need to look at the impact of inductance as we model narrower plane separations. ese are areas for future writing. For now—as has happened with previous downward shis in geometries—addi- tional factors that people previously ignored come into play. For example, it's now possible for the copper trace to be embedded in the PCB's surface layer rather than sitting on the top layer of prepreg. is allows for ultra-flat surfaces and facilitates the attachment of active devices. UHDI: Two Tracks, Two Levels of Investment UHDI is a generic term for PCBs with line widths under 75 microns. ere are two main branches. e first is from 75 to 25 microns, which is expensive but reasonably viable for PCB fabricators to invest in. is still uses conven- tional (albeit very thin) glass-reinforced prepregs and slightly modified chemistry and equipment. However, step below 25 microns, and UHDI enters a different world. It requires specialized machinery, new factories with cleanroom capa- bility, and a host of new chemical processes. e clue to when someone is discussing sub-25 micron is the use of build-up films. People oen use the term ABF® to refer to Ajinomoto Build- up Film, as Japan's Ajinomoto effectively domi- nates the market for the thin, unreinforced film used in building sub-25-micron PCB traces. Ajinomoto developed ABF as a side product related to the production of umami seasoning in the food industry. ABF is a film-type insu- lator developed with amino acid technology. I find it fascinating to dig into the roots of inno- vation and how creative products sometimes cross unrelated industries. Adaptation: Taking up This Technology In the U.S., this technology is primarily being deployed in facilities with government support to buffer the significant investment in plants and systems. Time will tell if the initial capability becomes more widespread as the industry and supply chain find innovative ways to reduce the adoption costs. What's Next? It will be exciting to see whether this tech- nology becomes mainstream. e challenge for suppliers of equipment and measuring and modeling tools will be to discover whether working at these smaller geometries on con- ventional materials will expose us to physical effects that, until now, we have been able to sweep aside. DESIGN007 Martyn Gaudion is managing director of Polar Instruments Ltd. To read past columns, click here. Martyn is the author of The Printed Circuit Designer's Guide to… Secrets of High-Speed PCBs, Parts 1 and 2.