Scott,

the IPC impedance calculator doesn't consider the effect of the solder mask on the outer layers.
Usually the real impedances you get will be about 75 to 10% less than those you get from the calculator.

regards,

Peter

-----Ursprüngliche Nachricht-----
Von:    Scott Decker [SMTP:[log in to unmask]]
Gesendet am:    Freitag, 5. Februar 1999 18:27
An:     [log in to unmask]
Betreff:        [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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