Scott & the DC,
I too have found a similar problem with the Impedance Calculator in Pads
PowerPCB ver.2.1. I've noticed that the calculated value DOES NOT change
when you change the thickness of the copper. It does not matter what the
layer stackup is or what board type, 4L, 6L etc ...
Another possible problem is that the final value is about 5-6 ohms higher
than the calculators on the IPC & AUTOMATA web sites. So much for your 10%
budget. It's amazing how much time is needed/wasted to check & re-check
something as simple as this. Especially when you have to account for added
copper plating thickness' and reduced track width due to the etching
process.
The question is .. What do you believe and if you do are you right ?
If anyone knows the right answer to this one inquiring minds want to know.
PS. Off on a tangent - Does anyone know how to remove coloured pencil marks
off this small digitizing light table. It's really a pain in the neck. :)
And by the way its -2 C and sunny here in Toronto.
Regards, Greg.
Gregory E. Bordash,
Team Leader, PCB CAD Group
ATI Technologies Inc.,
33 Commerce Valley Drive East,
Thornhill, Ontario, Canada, L3T 7N6
Phone:(905) 882-2600 ext: 8370, Fax: (905) 882-9339
Email: [log in to unmask]
> ----------
> From: Scott Decker[SMTP:[log in to unmask]]
> Reply To: DesignerCouncil E-Mail Forum.;Scott Decker
> Sent: Friday, February 05, 1999 12:27 PM
> To: [log in to unmask]
> Subject: [DC] ? Impedance Calculator at IPC's WEB site.
>
> Hi All
> I have a question / problem....
> I recently used the Impedance calculator on IPC WEB site and
> found a very interesting problem. On one occasion a few weeks
> ago, I used the calculator to do a quick stackup for a board I'm
> working on. The results were fine and were used as a baseline
> for the FAB shop to find me the values they use. I was just showing
> another designer here this neat calculator and changed the values
> I used to the values that the FAB shop gave me. Guess What....
> The calculator came back with an un-balanced impedance value!
> This is a very strange problem. I put in a balanced stackup
> and the values for impedance were different from the top to the
> bottom by 5 ohms. Sure you say, that's the 10% but, why are the
> numbers different? Maybe some of you out their can try these numbers.
> First. 6 Layer Board.
> .5 oz on both outside layers and 1 oz on all inside signals and planes.
> .005" traces on all layers
> Dielectric thickness as follows:
> 6.5
> 10
> 18
> 10
> 6.5
> Er value of 4.2
> OK now you have the numbers, the impedance should be around 72 ohms.
> But Wait! The values I got were 77 72 72 72...... What Gives?
>
> Any Comments???
> I already herd that it's because the UV rays from the sun were hitting
> the top of the board more, but I don't think that's true....(:-}
>
> --
> Scott Decker
> Senior PCB Designer
>
> AKA: PadMasterson
> E-Mail: [log in to unmask]
> -----------------------------------------------------------------
>
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