Mike, I've run into trouble by assuming the board house
would equalize dielectric thickness in the layer stack.
Some do, some don't.

I now add a fab note to "equalize dielectric thickness +/- 10%"
to get an approximately known dielectric thickness. Or you
can specify the prepreg thickness between critical copper
layers, even in a proto.

Brian
Foothill Services LLC

>i have another question.  this is iffy, but i'm a heathen
>designer anyway.  this is just a proto board, and although i'd
>like controlled impedance, i don't want to pay for it.  SO, 
>what i am thinking is that if i get an 8-layer board at the
>standard 62 mil thickness, that they are going to be just about
>forced to give me between 6 to 8 mils between layers, without
>me ever having to spec it.  does this sound right? 
>
>thank you, miker
> 

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