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26 DESIGN007 MAGAZINE I SEPTEMBER 2020 Interconnect impedance is a trade-off between the variables, including trace width, trace (cop- per) thickness, dielectric thickness, and dielec- tric constant. If you need to include differential impedance, trace clearance also comes into play. For minimum crosstalk, the coupling must also be considered. Plus, one needs to bear in mind the exact materials that are stocked by your pre- ferred PCB fabricator. Determining the correct configuration to achieve the desired impedance is not as simple as clicking an impedance goal- seeking button. But rather, one should weigh up all the pros and cons of changing each vari- able and make an informed decision. Impedance is the key factor that controls the stability of a design; it is the core issue of the signal integrity methodology. The substrate is the most important component of the assem- bly and needs to be planned correctly to main- tain consistent impedance across layers, avoid unintentional signal coupling, and reduce electromagnetic emissions. In this month's column, I continue with Part 6 of my stackup planning series and look at the correct process of stackup impedance planning and the conse- quences of bad decisions. Goal seeking is a function that is used to find an unknown value from a set of known values. If you know the answer, then the algorithm will fix all but one variable and hone in on the desired result. But keep in mind that there is no AI involved here. The software does not take all the possible variables into account as the designer would. PCB fabricators know their production pro- cess extremely well, and their manufacturing Stackup Planning, Part 6: Impedance Variables Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA

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