[SI-LIST] Re: Prepreg dielectric constant and thickness: what actual values to use?

Dear Marc,

There is one more effect that has yet to be mentioned: Prepreg thickness 
has
a real thickness tolerance even before it gets sandwiched between other 
layers.

My understanding is that while the glass content of a prepreg layer is 
fairly
well defined, there is more variability in the amount of epoxy. Now, 
following
Scott's comment that the dielectric constant is the weighted average of the
dielectric constants of the two materials, you have must contend with a 
range
of dielectric constants depending on thickness. (In the cases I've 
analyzed,
this thickness effect accounts for most of the Er variation in raw prepreg
specifications.) Prepreg at the higher end of the thickness range will 
have a
lower dielectric constant than prepreg at the lower end of the thickness 
range.

For designs using both prepreg and core layers to get sufficient 
thickness for
microstrip filters, I now simulate the design at the center values and 
at both
extremes. Designs done that way have worked the first time.

Regards,

Paul Levin
(presently between jobs)
_____________________

Marc Battyani wrote:
>  Hi,
> When you specify a stackup the thicknesses of the cores are well defined but
> the for the prepreg layers it's less obvious. So when using one or several
> prepreg sheets do you just use and add the basic thicknesses or is there a
> correction to make?
>
> For the dielectric constant of those materials, when you have 4.3-4.5 @ 1MHz
> and 3.8-4.0 @ 1GHz what value should we use as a basis for the simulations
> for 125MHz HSTL or 200MHz SSTL for instance?
>
> Thanks,
>
> Marc
>
>
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