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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