Don’t Overconstrain Your Board Materials

Estimated reading time: 2 minutes

When we started planning this month’s issue, which centers on the use of traditional laminates in certain high-frequency PCBs, we knew we had to interview Kris Moyer and Ed Kelley together. Kris teaches advanced PCB design classes for IPC, and Ed is the former CTO of Isola and founder of Four Peaks Innovation.

What ensued was a wide-ranging discussion on the evolution of “standard” PCB laminates and the recent trend by some OEMs to use these materials in high-frequency and even RF boards. If you’re interested in avoiding the use of expensive, hard-to-manufacture specialty laminates for your high-frequency boards, you won’t want to miss this.

Andy Shaughnessy: Kris, in previous discussions, you said that some designers are now using standard materials for some high-speed designs, and even RF boards. Notice that I’m trying to avoid saying “FR-4.” What do designers need to know if they are designing a high-speed RF board and are considering using a standard laminate instead of laminates designed for higher speeds?

Kris Moyer: The two big things, whether you're coming from the engineering side or the designer side, are awareness of Dk (relative permittivity), and Df (dissipation factor). We talk about Dk and Df all the time in my classes. The classic FR-4 is not a genuine spec; FR-4 is just a flame-resistance rating for this flame-retardant material. But we are talking about epoxy resins and woven glass-reinforced bases, which is what everybody assumes they're getting when they say “FR-4.”

Historically, we would have Dk in that segment on the order of 4 to 4.5. The Df is on the order of 0.015. They are pretty lossy materials, which was fine for analog and in the early days of digital. But the formulations of the resins 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 better. Even at a Dk of 3.5, an epoxy base doesn't have all the processing limitations of PTFE. There is a big segment of consumer-grade products that we don’t call RF, such as Wi-Fi, Bluetooth, and ZigBee. Those product 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.

To read this entire article, which appeared in the May 2024 issue of Design007 Magazine, click here.