Mitch Morey wrote:
>
> Hi Jack,
>
> You must be using the Cadence tools for your analysis (hee hee). It
> defaults to the same values. I have been using 4.3-4.5 for my
> controlled impedance measurements and have been quite accurate
> with deteremining trace widths and layer separation that our fab
> house could confirm.
>
> I have used an impedance calculator from IUD in the past that uses
> ranges that you have talked about, but for standard FR4 material I
> still revert back to 4.3-4.5
>
> BTW, which INTEL do you work at, if you don't mind me asking? I've done
> some work for INTEL, Hillsboro and they always seemed to knowthese
> values.
>
> Good luck,
>
> Mitch Morey
> Sr. PCB Designer
> ComStream Corp
>
> >>> "Olson, Jack C" <[log in to unmask]> 03/03/98 09:21am >>>
> WARNING: this is a very unexciting question...
>
> My software does some basic signal integrity analysis,
> and I just noticed it defaults the dielectric constant for
> regular FR4 to 4.7
>
> I was under the impression that most FR4 is at 4.2-4.3
> these days (more resin? thinner cores? can't remember...)
>
> Even a brief response will be appreciated!
> Jack
>
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Hi Jack, Jana and Mitch
There has been quite a range of dielectric constants (Dk) used for GF
like materials. In general, most e-glass fabric reinforced, epoxy resin
laminated printed board base materials have had published Dk's ranging
from about 4.3-5. As we know, printed board base material is
"electrically" (and physically) a "mixed" dielectric system, consisting
of E-glass, which has a Dk of about 6; and epoxy, which has a Dk of
about 3.3-3.5.
The to determine the "effective" Dke of a mixed dielectric, you need to
use the "volume" percent (not the more frequently used "weight")
portions of each dielectric to determine the Dk.
The general formula is: ln Dke = wt%lnDk1 + wt%lnDk2 + .. + wt%lnDkn
In the past, most Dk measurements were performed at <= 1 MHz, and the
reported Dk's were about 4.7-4.9. At the time, measurements at higher
frequencies, >200 MHz indicated Dke dropped to 4.2-4.3. Recently,
however, some newer Dk testing has indicated that the "drop-off" at
higher frequencies isn't true and that the Dke remains about 4.6-4.8.
For the numbers you need for serious designs, contact your printed board
manufacturer, or the mateial supplier, and for serious designs you may
need to qualify each lot of mateial and adjust the design.
Hope this helps,
Ralph
--
Ralph Hersey
Ralph Hersey & Associates
3885 Mills Way
Livermore, CA 94550-3319
PHN: 510.454.9805
FAX: 510.454.9805
e-mail: [log in to unmask]
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