Issue link: https://iconnect007.uberflip.com/i/1327102
JANUARY 2021 I DESIGN007 MAGAZINE 47 I get in these discussions every week, often every day: Which Dk numbers can we trust? One fabricator says it's this, and another fab- ricator says it's that, and the laminate ven- dor says it's another thing. Which values can design teams trust? I'd like to hear Happy's opinion on this. But the common mythology is that those dif- ferences are due to individual fabricator pro- cesses, and that's not true. It's fundamentally illogical. Happy Holden: It's interesting that you talk about field solvers. In your expertise, is it the different way that people have put their field solvers together, or is it the independent vari- ables like thickness and Dk that would result in fabricators coming back with a different stackup? Not so much whose field solver it is but that each one of them has their own num- bers for thickness and Dk that may or may not be close to what it really is. Hargin: Look at it in terms of degrees of free- dom, which is usually used in modeling motion. You can have two degrees of freedom; you can have three degrees of freedom. Here we have at least four, if not five, degrees of freedom. We use HyperLynx field solver, but others may use different field solvers. There's one set of Maxwell's equations, but field solver A and field solver B might use slightly different meshing techniques. Now, do the field solvers usually agree? Yes, they do. But let's say that the field solver is one degree of freedom. The second one is this: What parameters are being fed to the field solver? Are they the same, or are they slightly different? The third is the operator as a degree of freedom: Person A versus Person B at two different fabricators. They're not sitting side by side, comparing their work. That doesn't happen until the stackup gets back to the OEM. So, that's a third degree of freedom. They could be using different Dks and many times they are—a fourth degree of freedom. Then the fifth degree: They could be using dif- ferent percent copper values, and therefore dif- ferent prepreg thicknesses. So you can get all that variation. That's five degrees of freedom that we've identified that can lead to divergent stackup results, right? Let's talk about Dk. This week, I was asked, "Which Dk do we use? Do we use the laminate vendor's Dk? My fabricators use slightly differ- ent Dk values." Some fabricators will send a stackup back to you and they won't even tell you the Dk they used. They'll just tell you trace widths and dielec- tric height. And sometimes they'll include their calculated impedance values. Sometimes, if you give them a 50-ohm or 100-ohm target, they'll just say, "Oh, yeah, it's 100 ohms." As if everything was exactly 100 ohms right on the button. And they won't give you the Dk numbers. So you get all of this variation, and that's why I say you need to fence those cattle in. I feel bad because I'm referring to people as cattle, but it's just a metaphor. Shaughnessy: Designers have been called much worse. Hargin: You need to have a process by which you rein this whole thing in. If you take a big fabricator like TTM that's worldwide, they'll use different Dk numbers at different sites. I believe what people do is they take their test coupons from boards, and they backward- engineer the Dk from using the IPC imped- ance equations. Is that special sauce? No, I don't think that's the special sauce; I think it's some guy using Excel. Is that due to their spe- cial processing? No, I think it's a guy sitting in front of Excel, typing in equations, and pulling a number out of it, right? Shaughnessy: Right. It's a matter of communi- cation, but that's just part of it. Holden: At HP, we were in the business of mak- ing test equipment, and one of the challenges with test equipment is it has to be about 10 times better than what it's trying to measure. Being very accurate on these things was a hall- mark of Hewlett-Packard excellence. Now, the second thing was that we made dielectric test- ing machines. The first generation went to 1.8