Hi Jack,
for complex problems there are simple, easy to understand, wrong answers.
The first part of what you describe is done to achieve DC-isolation. You do
this to achieve conformity to safety standards or keep signal traces from
noisy ones.
The via stitching is done to get a low-impedance AC-coupling between planes
on inner and outer layers. That way no part of any plane will act as a
resonator for HF signals. It prevents coupling of AC-signal into the plane.
In short, it is done to keep noise in or out of the PCB, and coupling
between horizontally adjacent planes to a minimum.
If I have to 'flood' the PCB's outer layers with copper or have a number of
GND layers, I stitch them together with a number of vias, keeping the
distance between them short.
Volkmar
-----Original Message-----
From: Jack C. Olson [mailto:[log in to unmask]]
Sent: Thursday, March 16, 2006 4:45 AM
To: [log in to unmask]
Subject: [DC] Walls of Air? or Pillars of Metal?
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 transfomer, 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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