Re: Utility of copper pours on four-layer boards

qrk wrote:
On Mon, 7 Apr 2008 13:48:36 -0600, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:

"Joel Koltner" <zapwireDASHgroups@xxxxxxxxx> wrote in message 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.

As explained to me 10 years ago, the outer layers are etched, then
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?


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.

--
Regards, Joerg

http://www.analogconsultants.com/
.

Relevant Pages

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  • Re: Utility of copper pours on four-layer boards
    ... top and bottom layers of a four layer boards (where the inner ... layers are power and ground) comes from. ... copper pour on the top or bottom layers at all since I can't think of any ...
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  • Re: Utility of copper pours on four-layer boards
    ... the top and bottom layers of a four layer boards (where the ... inner layers are power and ground) comes from. ... easier to just tell them not to use a copper pour on the top or bottom ...
    (sci.electronics.design)
  • Re: Utility of copper pours on four-layer boards
    ... flooding the top and bottom layers of a four 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 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, 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. ...
    (sci.electronics.design)
  • Re: Utility of copper pours on four-layer boards
    ... the top and bottom layers of a four layer boards (where the ... inner layers are power and ground) comes from. ... easier to just tell them not to use a copper pour on the top or bottom ...
    (sci.electronics.design)