Issue link: https://iconnect007.uberflip.com/i/1520701
MAY 2024 I DESIGN007 MAGAZINE 13 Moyer: In my experience, it has less to do with frequency than insertion loss. It's in the qual- ity of the signal you're trying to deal with. As I said, with the exception of extremely low Dk, we're around the 3.0–3.2 range, whereas some of the Rogers products are in the 2.0–2.1 Dk range. e big distinguisher is whether your design and analysis show that you need a Df lower than about 0.01. Kelley: For epoxy products that are lead-free assembly compatible, they now actually call them high loss, with Df values around 0.020. Traditional FR-4s, referred to as "standard- loss," are around 0.018 to 0.016. Moyer: So, 0.01 is an important range. If you need Dfs better than that, depending on your RF frequency range, then you still have to use spe- cialty materials. But as long as you can tolerate the insertion losses, the Dks of these "new" FR-4 materials are sufficient to handle the majority of what we're doing, and Dk is the main driving factor of the rest of your signal performance. Another big issue on the digital side is that the silicon designers keep shrinking the size of their silicon geometries to the point where, even at low clock frequencies, you have very high harmonic frequency content, and a square wave with a sharp edge rate. At the same time, your digital circuits are still at a clock frequency of hundreds of kilohertz that can and do have multiple gigahertz of RF fre- quency content because of the rise time of the square wave. Designers have no effect on this. It is completely dictated by the geometry of silicon. I have heard that some OEMs are start- ing to see sub-100 picosecond rise times now in their tests. Kelley: In this image of material development (Table 1), Dk and loss levels, you can see the material development trajectory and segments. Going way back to GETEK, all the copper-clad laminate (CCL) manufacturers began develop- ing mid-loss, low-loss, and down to very-low loss formulas, really for digital applications. Table 1: A look at the wide range of PCB laminates available today, from traditional high-loss FR-4 to extreme low-loss chemistries such as PTFE. The lower the loss, the more expensive the material and the more complex the fabrication cycle.