Hi John,
        First I would like to confirm What Dan Brandler said about doing the
back calculation using Imperical data. Almost always when we do this we get a
lower DK than the supplier recommends. You must realize that the Laminate
suppliers publish a "blanket" DK value that is supposed to be an average of all
their constructions. The DK value for a specific laminate is dependent on the
resin to Glass ratio. So you see this can be quite variable depending on the
specific laminate used. From the Board standpoint, the lamination Process is
also a variable that depends on how much resin is retained in the cured prepreg
vs. the glass content. I caution all who do the Impedance modeling to be aware
of this. The best thing you can do is do the back calculation of the model to
see what the actual DK value is. Also remember this is also a function of the
frequency at which the board is supposed to operate at. The values from most
models provides the Characteristic Impedance at 1 Megahertz.

I hope this is of some help,

Regards,
Les
>  From: John Bownass <[log in to unmask]>, on 2/23/99 11:30 AM:
>  Colleagues,
>
>  I wonder if you can throw any light onto a problem I have with a
>  specific impedance controlled board where the prediction does not match
>  the actual measured results.
>
>  The board is 12 layer FR4 with controlled impedance on layers 3, 4, 6,
>  7, 9 and 10.  Layers 2, 5, 8 and 11 being the planes.  So there are two
>  signal layers between each pair of planes, all impedances should be 48
>  ohms (+/-10%).
>
>  The problem is that the measured impedances from a finished board do not
>  agree with the prediction based on the actual measurements from a cross
>  section of the measured board.  I give 2 examples:
>
>  All dielectric spacings are copper to copper, all figures in microns:
>
>  Layer 3: track width avg 126, spacing to layer 2 = 135, to layer 5 =
>  260. Thickness = 16.
>  Layer 6: track width avg 121, spacing to layer 5 = 100, to layer 8 =
>  270. Thickness = 16.
>
>  L3 measured impedance = 57.3 avg, and calculated from above figures =
>  50.6
>  L6 measured impedance = 54.3 avg, and calculated from above figures =
>  46.2
>
>  Dielectric constant of 4.6 used.
>
>  Of course the Er can be out but it could not explain such a large
>  difference.
>
>  The offset stripline model used works within normal tolerances for all
>  our other work but for some reason not this particular job.  Any ideas?
>
>
>  Has anyone a model that predicts nearer to the actual results?
>
>  Regards
>
>  John Bownass
>
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