I think that if you haven't measured the dielectric seperation on the
original build vs the new build, you will find a difference in the
seperation caused by the higher Tg material. We fell foul of thinking
that the same glass style with the same resin content, would give the
same pressed thickness when comparing std FR4 with high Tg material. My
belief is (and I am still waiting for an answer from Polyclad) that the
density of the multifunctional material is different, so the resin
content - which is weight of resin / weight of resin plus glass can be
the same but the volume of resin is different.


Dougal Stewart - Product Development Manager
Viasystems Selkirk Ltd,Scotland TD7 5EJ
Tel: +44 1750 21601
Fax: +44 1750 22513
email: dougal.stewart @ viasystems.co.uk

> -----Original Message-----
> From: Joseph E. J. Duclos Jr. [SMTP:[log in to unmask]]
> Sent: Monday, March 09, 1998 11:16 PM
> To:   [log in to unmask]
> Subject:      [TN] IMPEDANCE vs DK/Er vs Frequency
>
> Hello all, an odd question that has come up in some evaluations:
>
> I have a  "known good" impedance calculator formula from IPC-2141 that
> has worked for years on Polyclad 226 material.
>
> I know my circuit widths, dielectrics, stackup and coupon design are
> good.
>
> THE DIFFERENCE tried Polyclad 370 material      Higher Tg (170 vs 140)
>                                                 DK/Er (4.5 on both)
>
> THE PROBLEM -   my formula says 48-52 ohms
>                         coupons read on 2 different TDRs at 59-63 ohms
>                         One has a Tr @ 30ps the other is newer so
> probably a quicker Tr
>
> THE QUESTION(S) 1) Am I in error ?
>                         2) Should I be using a compensation factor ?
>                         3) Do I have incorrect data on the 370
> material ?
>                         4) Have the laws of physics forsaken me for
> some sin against the stray molecular particle ?
>
> Any help would be greatly appreciated, especially on questions 1,2 &
> 3.
>
> JOE SENDZ....
>
>
>
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