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94 DESIGN007 MAGAZINE I AUGUST 2019 of fiberglass in rigid. I think that helps a lot in how they would visualize a stackup. Creeden: There are many different materials out there and different types of circuitry get- ting more into high-speed and RF. All you have to do is say the buzzword "5G" and everyone's ears perk up. Steve, can you give me a brief overview of some of the materials based on what Chris just said? Bowles: Coming directly from DuPont, we are still marketing heavily in our existing commer- cial materials and aren't fabricating or devel- oping anything new specific to that, except for Pyralux® AG which is a lower cost option for some applications. The Pyralux family and the AP materials are the industry gold standard for reliability and performance. The HT, and the TK are still one of the best materials for high-speed and high-frequency applications. Creeden: And earlier, we were talking about low-loss materials. Those appeal to an RF en- gineer. Hunrath: Flex materials don't have skew be- cause they don't have fiberglass, so they're uni- form in their dielectric properties. On the AP, TK, and HT classes, HT adhesives have good performance numbers. And knowing the rigid materials and how they stack up there, they're on the higher end of the rigid material spec- trum in terms of loss in dielectric constant. Creeden: It's quite a challenge when an RF en- gineer wants to send an RF signal across a flex board, and they're concerned about emissions. You can add a 45-degree ground plane to still retain some flexibility, and perhaps make it in a hatch pattern; that's one way of coaxing these signals. Hunrath: Yes. There is data available on using those cross-hatch patterns. Even the geom- etry of the cross-hatch patterns can be opti- mized to get more performance, and that's an area where DuPont can help. DuPont has some mestically, in multiple processes and engineer- ing positions. Creeden: Next, can you tell us about yourself, Chris? Chris Hunrath: I've work- ed both on the fabrica- tion side and on the sup- ply side of the PCB in- dustry. I started with multilayers back in 1983 and never looked back. I have been working with the supply side about two-thirds of my career and with Insulectro for the past 18 years. I work a lot with our custom - ers, especially on the flex circuit side, by help- ing them select the right materials and with the processes in the fabrication of flexible circuit boards. Creeden: Although the percentage varies for most designers, the ratio of how many flex cir- cuits to rigid circuits is often low. Therefore, a relearning curve is often a concern. The infor- mation about the flex design, manufacture pro- cess and correct material selection may not be readily available to them. How could a design- er start making some of those decisions, and when should they make those decisions? Hunrath: That's a great question. If you were to look at flex circuits compared to rigid PCBs at a high level, I like to use the analogy that in flex circuits, the Kapton® material—which most people are familiar with—is the replace- ment for fiberglass. In other words, if you were to look at rigid PCB stackups, you have cop- per and prepreg or glass-reinforced layers. In flex, it's Kapton on an adhesive with the cop- per. In the flex world, Kapton is the base mate- rial, and then you add copper and adhesives to it to make the various kinds of stack ups. There are lots of other important factors in doing a stackup in terms of where the bend axis is as well as the neutral axis, etc. But if they keep that concept, the Kapton is the flex equivalent Chris Hunrath

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