[SI-LIST] Re: Impedance Mismatch due to neck down
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: qazi@xxxxxxxxxx
- Date: Thu, 23 Jun 2005 12:27:36 -0700
Dear Qazi,
If you've got room to parallel two 4 or 5 mil traces, why not just stick
with the single 8 mil trace. Really short sections of high impedance line
(say less than 1/10 of the risetime in transit time) probably won't hurt
too much considering that the source or load at the end has a lower
impedance than you would want (at least at high frequencies.)
Regards,
Paul
___________________
Qazi Arif Iqbal wrote:
>Hi Experts,
>The board that I am using is of a stripline setup with 8mil traces (1 oz
>Cu), FR-4 (4.0) and 10 mil dielectric thickness which gives 50ohm
>(relatively low speed). The BGA is tightly pitched and I have to neckdown
>traces around 4-5 mils which gives impedance mismatch (65.4 ohm). Neckdown
>is a must but I also want to avoid impedance mismatch, is there any way of
>doing it? Does doubling help? (looks like this would give a parallel of 2
>65.4ohms = 32.7, worse than what I have).
>
>Thanks in advance,
>Qazi
>
>
>
>
>
>
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--
Paul A. Levin
Senior Principal Engineer
Xyratex, Manhattan Beach
(310) 372-7352 - home & office
(310) 291-8199 - cell
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