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July 2001

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Subject:
From:
Paul Brionez <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 17 Jul 2001 07:58:34 -0500
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tech wrote:

> I have a 6 layer design with layers 2 and 5 being planes and 3 and 4
> signals. In this particular case , due to overall thickness and outer
> layer impedance requirements we have .010 from the planes to the signals
> and .035 between the signals. 1 Oz. copper and finished lines at .006.
> My impedance software ( Polar CITS25) is giving a number of 65.02 ohms
> but the actual results we're getting are all at the 72+ ohm range.We do
> many impedance designs and have not seen a discrepency like this. The
> .035 dielectric between the signals is 5 pieces of 7628 which is an
> unusually high amount of prepreg for us and the .010 from planes to
> signals are cores. All 140Tg FR4. Can the high thickness (resin content)
> of prepreg be creating the problem ? Sections on the coupons are very
> close to design so we don't appear to have a process issue.
>  Any input appreciated.
> Paul Greene
>
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Paul

    I have seen the material prepreg can vary your impedance a great deal as the glass
content to resin ratio is increased. You need to ask the vendor if they used REG # FR-4 11
7628 material that has a 40 % resin which has a high glass content and a DK of 4.75.

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