Re: Utility of copper pours on four-layer boards
- From: John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 07 Apr 2008 12:39:49 -0700
On Mon, 7 Apr 2008 10:36:11 -0700, "Joel Koltner"
<zapwireDASHgroups@xxxxxxxxx> wrote:
I'm trying to figure out where the not-so-uncommon practice of flooding the
top and bottom layers of a four (or greater) layer boards (where the inner
layers are power and ground) comes from. I'm often having to get our techs to
widen the clearance between the copper pours and controlled impedance traces
because they like to set the clearance from the copper pour to the other
signal nets so tightly (e.g., 6mils) that they end up turning microstrip lines
into coplanar waveguides and significantly altering their impedances. It
occurred to me that rather than continually dealing with this (they always use
copper pours by default), it might be easier to just tell them not to use a
copper pour on the top or bottom layers at all since I can't think of any
particularly compelling reasons to do so in the first place (and they say
they're doing it by default because it's "common practice" and "their personal
preference"). The benefits of a top or bottom layer ground pour on a
four-layer board that I can think of are...
-- Certainly you could use it as a nice bit of heatsinking if you have power
components elsewhere that connect to the plane layers
-- At low enough frequencies, you probably get slightly better shielding from
outside interferers or for crosstalk. At high enough frequencies, this can of
course come back and actually create greater interference if you haved "nailed
down" (viaed) the copper pour to ground at regular intervals (due to
resonances, e.g., an isolated copper pour with no grounding at all looking
like a pretty good patch antenna).
Am I missing something here?
---Joel
I can't think of any reason to always pour surface layers. Tell them
to quit doing that.
I disagree that it's "common practice." I rarely see this, and never
do it.
John
.
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