Scott Decker wrote:
>
> snip <
>
> 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....(:-}
>
        Impedance is lighter than FR4 so more of it floats to the
        top <g>.

        Playing with these values on my tool I find that describing
        the last trace layer as 1 oz instead of 1/2 would get you about
        that difference.  Can't get IPC's site right now, only the
        archives are responding to me.  Hope there's not a problem w/
        the server...
--

      Jeff Seeger                         Applied CAD Knowledge Inc
      Chief Technical Officer                  Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                978 649 9800

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