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

Hi Marc,

The actual finished thickness of the prepreg layer will depend on 
several factors: resin-glass ratio of the
prepreg, how much copper (trace) you have left on the signal layer, how 
many and how big (size and
depth) holes the resin need to fill on the adjacent plane layers.  
Unless you are familiar with these
details, you may want to leave this part to the PCB fabricator and just 
specify the important electrical
parameters that the traces need to meet.

Regarding the dielectric constant value changing with frequency, in this 
frequency range, it is due to
dielectric loss.  Bigger drop in dielectric constant means higher loss 
tangent and vice versa.  As it was
discussed in detail earlier on this list, the frequency dependent 
dielectric constant and loss tangent
(together with the frequency dependent conductive losses) make it 
somewhat irrelevant to ask what
value to use: ideally you need to do the frequency dependent simulation 
over the entire frequency
range of interest.  In less demanding applications you can get away with 
simple approximations,
such as for instance using the dielectric constant value just at the 
fundamental bit-rate frequency.

In the tools section of www.electrical-integrity.com you will find a 
simple spreadsheet giving
you the causal model of frequency dependent Dk and Df.  Since you know 
Dk at two frequencies,
it implicitly specifies Df as well.  You can plug in for instance the 
1MHz Dk value, set the
frequency in the spreadsheet to 1MHz and adjust manually Df until you 
get the Dk value at 1GHz
matching your 3.8-4.0 range.

Regards,

Istvan Novak
SUN Microsystems

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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