From: Paul M Fly

Mike,

What your engineer is asking for is quite common here! The theory behind it
is that by adding multiple
parallel paths from the capacitor to the plane you are actually lowering
the inductance. This makes
some sense if you remember how inductors behave when wired in parallel,
same equation as resistors
wired in parallel. The total value of the parallel network turns out to be
less than the smallest individual
value.

Engineers here ask us to make those traces running out to the vias as short
as possible. We will bring
the vias in as close to those pads as we can without causing problems in
manufacturing. Making those
traces as short as possible is probably the best you can do to reduce trace
inductance. One other thing
they often ask for is to have the traces between pad and via as wide as
possible, usually that ends up being
equal to the vias capture pad diameter.

Hope this helps,
*********************************
          Paul Fly

Eastman Kodak Company
Engineering Technology Center
901 Elmgrove Rd.
Rochester, New York 14653-5313
Phone: (716) 726-5670
Fax: (716) 726-0275
E-mail: [log in to unmask]
*********************************





"Kuczynski, Michael @ SPACENAV" <Michael.Kuczynski on 12/01/2000 07:35:25
AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to "Kuczynski, Michael @ SPACENAV" <[log in to unmask]>

To:   [log in to unmask]
cc:    (bcc: Paul M Fly/243609/EKC)
Subject:  [TN] Strange cap Layout




I have an engineer who wants to place 2 vias per side on a 0508 capacitor
for decoupling. (I tried to draw figure below)
He says it will help in filtering and if it isn't done, then don't bother
putting any caps on (idle threat)
The fanout trace is .010 / via is an .013 / SMT pad is .035 x .080 /
spacing
between vias is .0625 (center2center)
I think it's horse-hockey and will introdure more inductance, beside taking
up routing channels
I'm looking for any support pro or con on this configuration.

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Michael Kuczynski    Librarian/Sr. Designer
L3 Communications    1-201-393-2122 (Ph)
Space & Navigation   1-201-393-6681 (Fax)
699 Rt. 46 E         Mail Stop PL1/J8
Teterboro,NJ 07680
[log in to unmask]

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