Well Scott, it looks like The IPC has a problem, I tried it and got the same
answer. It's funny because I tried some other balanced numbers and obtained
balanced Impedance values. There does seem to be a flaw in their model
calculations.

How about it Hugo, is someone looking in to the model?

Regards,
Les

>  From: Scott Decker <[log in to unmask]>, on 2/5/99 9:26 AM:
>  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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