Issue link: https://iconnect007.uberflip.com/i/1327102
50 DESIGN007 MAGAZINE I JANUARY 2021 is a factor, like Happy was highlighting. Unless that's happening, there shouldn't be a strong reason that one location would have systemati- cally different Dk numbers than another one. To be honest, philosophically, I don't under- stand why design teams will spend dozens of hours laying out a circuit board and own- ing the CAD layout, then maybe dozens more hours using up to $100,000 worth of EDA tools, to design and simulate designs. Why don't the same people that are putting that kind of invest- ment into the board layout and SI and PI simu- lation process pay more attention to the spinal cord of their design—which is the stackup— and why do they delegate that to the fabrica- tors? That's a design-philosophy question. Shaughnessy: You mention TTM. Julie Ellis at TTM recently said, "How can any designer pos- sibly know all of this stuff that I've been work- ing with for 30 years, and I'm still learning?" Hargin: I actually talked to Julie a week or two ago, and we were talking about pressed pre- preg thickness calculations and percent copper. One of the gaps in the design flow is that for a lot of stackups, the fabricators are calculating pressed prepreg dielectric thicknesses based on round numbers. For example, for the percent copper on a specific layer, they might say that, signal layers are 35% copper, or they might say 40% copper, or 50%, whatever they calculated. The fabricator then might use those percent- ages all day for signal layers, and they might have a number like 90% for plane layers. But your design has a different percent copper on the various signal layers. Some CAD tools know that. Valor NPI knows that and those numbers could be used in the stackup design flow to refine prepreg dielectric thicknesses and therefore get more accurate impedance num- bers. I find that people in this space who I deal with every week get confused about manufac- turing process variation. Let's say impedance can be plus or minus 10%, okay? And that's a common standard; some people target plus or minus 5% and pay a bit more with a more expensive fabricator for tighter control. But that manufacturing variation includes copper thickness variation, Dk variation, percent resin variation, and a thickness variation; all these things that add up to impedance variation. But you don't want to give away some of that plus or minus 5% or 10% on your nomi- nal values. If you don't center your nominal, and you're off on the nominal, that's a terri- ble design practice. And people get confused about that. They think, "Oh, it's only a 1% difference. Oh, it's only two, two and a half percent different." But those percentages are eating into your tolerance-tolerance band. So, there's the manufacturing tolerance and then there's the engineering tolerance. Shaughnessy: It sounds like they're making decisions based on these numbers that may or may not be accurate. I mean, every little trade- off you do can affect something down the line. And like you said, you don't know what the numbers are for each for a prototype shop, much less for overseas. So it sounds like a lot of it is kind of a crapshoot. Hargin: I would say that an informal descrip- tion of the design team's job is to reduce uncertainty in their designs. If you have first- pass prototype success, and production is all done on schedule, you've reduced uncertainty down to an optimal level. I'm not really saying it's a crapshoot; what I'm saying is, it's our job as designers, engineers and design teams to reduce uncertainty wherever we can in our design flow. Just because I didn't get bit by something on my last design doesn't mean that I'm not going to get bit by the same issue on my next design. Glass-weave skew is an example of that. Just because it worked yesterday doesn't mean it's going to work tomorrow. That's how my son thinks when he's driving to and from col- lege. He says, "I don't get tickets." Well, until he got a speeding ticket for going 20 over the speed limit. Anyway, I think we need to reduce uncertainty. That's why I use the metaphor of getting the cattle back into your barn and keep- ing track of more things than you were keep- ing track of yesterday. Because when speeds increase, the margin of error decreases.