Text item: 

The effective dielectric constant is:

X*log(DK glass) + (1-X)*log(DK resin)= log (DK effective)
where X is the fraction of glass. 

I think the DK of glass is ~6.2 and the DK of resin is ~3.4, but I am not 
positive.

So the dielectric constant does change based on the resin content. It is a 
constant because it has no units. It is a ratio of material to air.

Steve Joy
Intel Corp.
Stephen_Joy @ccm.hf.intel.com


> Bob Vanech writes:
>   Avner--
>          If I am wrong, I hope someone out there will make me smarter, but
>    the dielectric "CONSTANT" is just that--constant for that type
of material.
>    IMOP, when calculating impedance of microstrip or stripline,
the dielectric
>    constant of a material doesn't change, less the material's unique
tolerance
>    range REF: IPC-D-275 Table 3-2. Therefore, unless proven wrong,
the constant
>    DOES NOT change, but the impedance value WILL vary depending on which
>    formula you are using depending on you pwb construction.

Bob,

I only wished it was this easy. If it was I wouldn't have a weekly arguement
about controlled impedence.

Let me first express my opinion. The dielectric constant of material
WILL change
dependant on a few things, 1) resin content of the material (for FR4), 2) the
frequency at which the signal is going through the material, and 3) which fab
house you are using.

To make my point, let me ask a question to the user's community. "What is the
dielectric constant of FR4, Tg 170 degrees?" I'll gladly publish the results
that come back (IF ANY).

I only put #3 above because I can't seem to get a "true" value for the dielec-
tric constant from our three main fab houses. No real problem because we allow
them to modify our trace widths and stackup to give us back our impedence
required, BUT with speeds in excess of 1.2 Ghz now, we are asking for much more
precise information. Our circuits will just not work correctly with the "fidge"
factor any longer.

Waiting to hear any response.

Mitch Morey
Sr. PCB Designer
TRW, Inc.
Redondo Beach, CA 90278
Phone: (310)814-5765
Fax:   (310)812-4949
EMAIL: [log in to unmask]

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Subject: FAB: Re: Controlled Impedance - Dielectric Constant- Comment
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