Issue link: https://iconnect007.uberflip.com/i/1327102
48 DESIGN007 MAGAZINE I JANUARY 2021 gigahertz, but you could put in a 30-mm disc of a core laminate, and it would give you back all the characteristics of that material. You could program the computer to do these measurements from 10 megahertz to 10 giga- hertz, and from zero to 120° C, and from 30% to 75% relative humidity, and it would give you back a three-dimensional measurement. And the one thing about all FR-4s is that nothing is ever flat, because it changes with frequency, it changes with temperature, it changes with humidity. So, the first thing was that there's no such thing as one Dk for your board. If you're going to do it exactly, you have to run a range of what you think the actual environment is that it's going to operate in. It was so important that I put a separate chapter in the Printed Circuit Handbook about characterizing materials because, the more expensive the material, the more stable and less variation you're going to get. If you're sticking with FR-4, it's a very good mechanical platform, but electrically, it's all over the map. Hargin: Yes, but I think what the fabricators are doing is simpler than what you just described. They're not looking at temperature or humidity or fall results versus summer results, etc. They have a number, and it's at one gigahertz. Your signal may be at 10 gigahertz, but the Dk they'll use is most often at one gigahertz. And they just plug and chug, and here's your stackup. Holden: Everybody is working with a different set of data books that they created themselves. Hargin: Correct. And they did it based on back- ing Dk out of an impedance equation. They do an impedance measurement and then they solve for Dk in the impedance equation, plug in the impedance that they saw, and try to pull their own home-grown Dk number out of it. To me that sounds primitive; it's not the way I would do it. Shaughnessy: Is it a matter of the OEMs just claiming ownership of the design, the stackup, everything? I keep going back to what you were saying about the OEMs punting to the fabricator who, as you say, doesn't know all the stakeholders nor has all the info that the OEMs do. Hargin: Let's say I'm a fabricator front-end design guy, and I'm designing stackups. I might do five or more stackups in a day serv- ing different customers. One of those stackups is the one I'm doing for you, the OEM. But you're doing it the opposite way. You have one stackup that you're doing, let's say, with three different fabricators. In the end, who needs to own the divergent results? It's the OEM that owns it. And, if they own it, they own the result. They need to take more ownership of how the process is done. If one fabricator is using a Dk of 3.75 and another one is using 3.6, and the laminate ven- dor has in their tables 3.65, well, you need to understand the frequency of the assumption. Okay, the fabricator is assuming one gigahertz, but what's my signal frequency? And engineers on the design team need to take more owner- ship of the process. Which laminate numbers would I use? My way of looking at it is I'd mea- sure them myself. Not just on impedance cou- pons, but if I'm using a material on a consistent basis, I would bring in some of that laminate and measure the raw laminate myself. Methodologies exist to do that. Our Z-field product does that. And you can remove the uncertainties. You could say, "Well, look, I've measured it myself from zero to 20 gigahertz. Here is the Dk and Df profile for this particular laminate construction." It's all about remov- ing the uncertainty in my mind—unless you're making some boards in Penang, Malaysia, and other boards in a drier climate, and humidity If you're going to do it exactly, you have to run a range of what you think the actual environment is that it's going to operate in.