Issue link: https://iconnect007.uberflip.com/i/1359517
APRIL 2021 I DESIGN007 MAGAZINE 55 on who or what tools were involved in a given design, there are manufacturing parameters that affect both impedance and signal loss that many design teams can improve upon. At pro- gressively higher speeds, critical factors for sig- nal integrity now include not only impedance, but loss, copper roughness, and glass-weave skew. Indeed, everything that happens in the process of physically building a PCB affects signal quality in a negative way and the details need to be accounted for across not just one PCB stackup, but across stackups from every PCB fabricator involved with a design. Factors That Impact Impedance EDA tools, including those made by my company, can be super useful, if you know what they are really modeling. Even here, the results are only as good as the data being put in. e primary parameters that affect imped- ance are shown in Table 1, including their rela- tive contributions. If you work this list from top to bottom, starting with getting a better handle on dielec- tric thicknesses and working your way down, you'll be better equipped to dial in your nomi- nal parameters with enough margin to know that you're not going to fall off the cliff. And yet, every week I see engineering teams strain- ing to perform complex analyses using data- sets with questionable merit, and without fully grasping the parameters and priorities (1 through 7) noted above. For example, I've never seen detailed, per- layer percent-copper values from actual PCB layouts being used by PCB fabricators to com- pute pressed prepreg thicknesses. And number 4 in Table 1 can be particularly problematic. I've searched far and wide amongst hardware OEMs, PCB manufacturers, and laminate man- ufacturers, and I would submit that dielectric constants are the least understood parameters in PCB fabrication. Trace width (#5) is closely tied to fabricator etching processes. Studies have shown that average PCB suppliers typi- cally maintain roughly 0.25 mils of etchback for half-ounce copper and 0.5 mils of etchback for 1-oz. copper, respectively. Advanced PCB manufacturers can bring these numbers to 0.17 mils for half-ounce copper and 0.45 mils for 1-ounce copper. Knowing what your fabri- cator can maintain and modeling it in simula- tions can increase impedance accuracy by sev- eral ohms. Copper thickness (#6), is another parameter that impacts impedance. I oen see engineers, Table 1: Factors influencing impedance. (Data courtesy of Happy Holden. [1] )