Issue link: https://iconnect007.uberflip.com/i/1520701
12 DESIGN007 MAGAZINE I MAY 2024 have continued to get better. We're now seeing epoxy-based resins that have a Dk down in the 3.2 to 3.3 range. Df is still not there yet, but it's getting bet- ter. Even at a Dk of 3.5, an epoxy base doesn't have all the processing limita- tions of PTFE. ere is a big segment of consumer-grade products that we don't call RF, such as Wi-Fi, Blue- tooth, and ZigBee. ose prod- uct lines are in the beginning, lower-frequency spectrum of RF, and we can absolutely use these FR-4 products to build those. Now, if we're not trying to get super low loss, we still have to worry about insertion loss issues. If we don't have an insertion loss issue and we are just in the higher frequency range, then absolutely we can get the required performance out of the dielectrics in the more traditional epoxy-based resins. Another thing designers need to know about are the improvements in standard epoxy or acrylic or polyimide-based resins as we move on the spectrum toward full RF. Ed Kelley: I agree with everything you've said. If you talk to 10 people, you will probably get at least five different definitions of FR-4, and four of them may not be correct. For example, if you go to UL and want your material to be classified as an FR-4, there has to be a certain amount of epoxy. Historically, most FR-4 products were virtually all epoxy. To get better Dk and Df val- ues, companies started blending epoxy with other resins—PPO, PPE, SMA, and a few oth- ers, to improve electrical performance. You get to a point where, if there are enough of these other resins relative to the epoxy content, it is no longer really FR-4, but everybody still refers to these products as "FR-4." I've spoken about the historical divide between FR-4 PCB materials used for digital applications vs. what was being used in the RF world. As the frequencies of digital applica- tions have increased, people have started blending these other resins with epoxy and ultimately switched to the major- ity non-epoxy resins to get electrical performance. ose Dk and Df val- ues approach the same performance as the Rogers and RF products in general. Marcy LaRont: Are we talking about a whole new classifi- cation of materials, or will they continue to be called "FR-4-like" as a catch-all? Kelle y: Again, it depends on the definition you're using. ere is FR-4, and non-FR-4, or non-ANSI. It is a significant issue with UL because the general class of FR-4 has grown so large, now including both halogen-containing and halogen-free materi- als. Now UL includes "FR-4.0 and FR-4.1" to distinguish between these. Historically, there was no distinguishing of materials by relative thermal index (RTI), which many people con- fuse with a maximum operating temperature (MOT), which is a PCB-level designation. UL has introduced FR-15.0 and FR-15.1. Without getting into the weeds, these represent hal- ogen-containing and halogen-free materials with higher RTI ratings. e hybrid and elec- tric vehicle markets drove much of this. Appli- cations now require higher operating tempera- tures, which creates all sorts of interesting con- versations about wanting to use FR-4, but hav- ing an operating temperature that exceeds the original definition of FR-4. e FR-15.0 and FR-15.1 designations now indicate materials that meet an RTI of 150°C, or the same maxi- mum operating temperature when referring to a PCB. UL now has these four categories, but people still use FR-4 as a catch-all. Shaughnessy: What would the cut-off be for using FR-4? Would it be rise time or signal speed? At what point do you absolutely have to use the high-speed or RF materials? Kris Moyer