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The best you can do is make sure you enter all the information into the Allegro Cross Section (Thickness, Dielectric Constant, Loss Tangent) and you should get the impedance numbers some what close (Display > Parasitics). There are other factors, tolerances, processes, etc. that drive the Fabricator Impedance stack-up model which skew the impedance numbers in one direction or another. The more important thing is make sure you run the Impedance stack-up model past multiple Fabrication vendors to make sure you are able to build it anywhere and not isolated to one vendor.
On my last design I was able to enter in all the information into the Allegro Cross Section and the numbers came in about 3 or 5 ohms off compared to the Fabrication Impedance model but never able to get them to be exact - to many factors - so I always generate physical rules to drive the impedance traces widths and spacing based on the Fabricator impedance model.
One take-away here is to make sure you run Impedance model past a couple of actual Fabrication vendors to make sure you are able to build the PCB anywhere. (You may already know this) If the CM is working directly with the Fabrication vendors then ask them to provide the impedance models that came from the Fabrication vendor to support the data they provided. I have seen too many times when sourcing a PCB from another vendor the response comes back that they are unable to hit the impedance numbers.
Hope this helps,Mike Catrambone
Your CM appears to be using HyperLynx or some other tool for a quick study. The fabricators I use give much more detail that when plugged back in to Allegro match much closer. Here some items to consider:
1) dielectric constant depends on glass structure, resin:glass ratio. Neither of you are showing variances in Er when changing thicknesses of dielectric. Not going to be accurate.
2) copper weight? What thickness are you using? What frequency is the impedance measured at? Using what instrument? At what transmission line length? Using a Polar impedance analyzer? VNA? It will vary the results.
3) Pressed thickness does not appear to be accounted for. When I see four mils for example it seems to be a starting thickness. Not a finished thickness.
four) Be sure to get the *exact* glass structure if you want to move this stackup around to other vendors. If you don't call out the exact construction and copper your results will vary more than the 3 ohms you're worried about.
There are more gremlins to attack in this stackup but these are some biggies. Get with the CM's fabricator and get *their* stackup and you can probably dial this in closer.
In reply to redwire:
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