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

Marc

The problem is one of volume displacement.  Prepreg flows and fills in 
the space between traces on a copper layer.  So, the final thickness of 
a layer of one or multiple prepreg sheets is variable.  For example, if 
prepreg is placed between two solid ground planes, there is nothing to 
fill, therefore no change in thickness - except for the small amount of 
material that is squeezed out the edges of the board during the press 
process.

If prepreg is placed between a ground plane and a trace layer, then 
there will be no filling on the solid ground plane layer, but there will 
be filling of the space between the traces on the trace layer.  
Thickness will be dependent upon the volume change in material that 
flows in between the traces, and the density of traces in any area of 
the board.  If prepreg is placed between two trace layers, then more of 
the material will flow, decreasing the spacing even further.

Average Er is just that, an average of the two materials.  A trace will 
see an average of the two dielectric constants of the prepreg material 
above and/or below it.  Use a simple average for the volume.

best regards,

Scott

-- 
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com



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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-- 
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC

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