[SI-LIST] Re: Walls of Air? or Pillars of Metal?
- From: "Clewell, Craig" <Cclewell@xxxxxxxxxxxxx>
- To: "'OLSON_JACK_C@xxxxxxx'" <OLSON_JACK_C@xxxxxxx>
- Date: Thu, 16 Mar 2006 09:53:32 -0500
Jack,
From my personal knowledge large slots in the PCB are not recommended. The
return current needs to go around these slots increasing the loop
inductance. There are also EMI considerations as well. A big hole has more
leakage then a bunch of little holes of the same area.
Dan Swanson dedicated a chapter in his book to via fences (ISBN
1-58053-308-6). The results of his work showed that reducing the distance
to the ground plane was much more effective for improving isolation due to
the field lines terminating on the ground plane more quickly. I believe
that Eric Bogatin also mentions this in his book as well. Dan did find that
if you double the amount of vias and add a shorting strip between the vias
in the "fence" the S12 will be reduced by about 10dB.
Craig
-----Original Message-----
From: Jack C. Olson [mailto:OLSON_JACK_C@xxxxxxx]
Sent: Thursday, March 16, 2006 9:18 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Walls of Air? or Pillars of Metal?
I just posted a question on the IPC
Designer's Council, and then suddenly
remembered si-list, which might be a
better place to ask this question.
So this is my first visit here, and sorry
for the cross-post.
-=-=-=-
I have never thought about this until now,
but I'm confused about something...
Many times I have been asked to put a slot
in planes to isolate different areas of a PCB
from each other, almost as part of the floor-
planning. So in other words a power supply
area might be isolated from a digital processor
area by putting an air gap between them, or
maybe between primary and secondary of a
transformer, or maybe the 3mm clearance
you put under an opto-isolator, right?
on the other hand...
Recently I have been asked to start building
"via fences" between areas, like stitching
ground planes together, and even been asked
to start putting a line of vias all around the
perimeter of the board, too. And flooding ALL
layers with copper and adding MORE vias to
stitch THEM together. In the past I have
used via stitching a lot, but it was for things
like guard traces along RF lines (1.67GHz
cellular signals) and occasionally along the
edges of shields. but not like this.
so...
Today a discussion came up whether to use
air gaps or via stitching, and I had a total brain
freeze. In my mind they both had the same
reason for existence, keeping things from
interfering with each other. But of course in
reality they are the exact opposite of each
other; one method adds conductors and the
other removes them.
Now I can't seem to grasp what I am missing
about this.
There's noise in my head.
So can someone please help me understand
when to choose one over the other, if at all?
(and hopefully the answer will be shorter than
the question?)
Jack
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