Re: Utility of copper pours on four-layer boards
- From: qrk <SpamTrap@xxxxxxxx>
- Date: Thu, 10 Apr 2008 05:09:11 GMT
On Tue, 08 Apr 2008 21:12:54 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:
qrk wrote:
On Mon, 7 Apr 2008 13:48:36 -0600, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:
"Joel Koltner" <zapwireDASHgroups@xxxxxxxxx> wrote in messageAs explained to me 10 years ago, the outer layers are etched, then
news:hYsKj.119342$Gv.95140@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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?
Tends to be much easier to etch(less copper to remove), it looks nicer, and
does have some electrical uses(remember reading about it but forgot the
details). I'm sure there are other reasons too.
plated up to desired copper weight. Thus, having copper pour on the
outer layers requires more plating.
Hmm, doesn't that only apply to thicker copper such as 2oz?
They need to plate the vias somehow.
I wouldn't say pouring the outer layers with ground is the norm. I'll
do it only if there's a good reason to. If you're running high-speed
signals around an FPGA or other HS parts, then a ground pour on the
FPGA mount side may help.
You'll probably see somewhat of a renaissance of copper pour coming, on
account of all those chips that require a heat sinking pad.
.
- References:
- Utility of copper pours on four-layer boards
- From: Joel Koltner
- Re: Utility of copper pours on four-layer boards
- From: Jon Slaughter
- Re: Utility of copper pours on four-layer boards
- From: qrk
- Re: Utility of copper pours on four-layer boards
- From: Joerg
- Utility of copper pours on four-layer boards
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