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

R: [SI-LIST] Impedance Mismatch due to neck downThanks guys for the
responses.
One reason for which we do not want to have 4-5 mil traces all the way is
the resistive loss.
5 mils within the DUT and then gradual increase to 8 mils looks like a
pretty good solution.
Rgds,
Qazi Iqbal
  -----Original Message-----
  From: Guasti Giovanni [mailto:Giovanni.Guasti@xxxxxxxxxx]
  Sent: Friday, June 24, 2005 12:41 AM
  To: 'qazi@xxxxxxxxxx'; si-list@xxxxxxxxxxxxx
  Subject: R: [SI-LIST] 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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