[SI-LIST] R: Impedance Mismatch due to neck down

I agree with Paul, a slow signal would not notice the 1-2 cm long trace
impedance mismatch.
But you could think to make 5 mils traces ewerywere too. 65 Ohm is not so
high but there are some simple ways to reduce it further.
If you choose a dieletric thickness around  0.150mm you'll obtain an
impedance of 50 ohm.
Isn't er=4 too low? with an er=4.3-4.4 the impedance reduces from 65 to
60ohm. 
With a 8 mil trace and er=4.4 -> Zo=47ohm. Is it what you would like to
obtain?
By
        Giovanni
-----Messaggio originale-----
Da: Qazi Arif Iqbal [mailto:qazi@xxxxxxxxxx]
Inviato: giovedì 23 giugno 2005 19.59
A: si-list@xxxxxxxxxxxxx
Oggetto: [SI-LIST] Impedance Mismatch due to neck down


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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