[SI-LIST] Re: Ground Pour in Signal Layers
- From: "Haller, Robert" <rhaller@xxxxxxxxxxxxx>
- To: <leeritchey@xxxxxxxxxxxxx>, "Chris Padilla (cpad)" <cpad@xxxxxxxxx>, <ivorlist@xxxxxxxxxxx>, <Pradeep.RSA@xxxxxxxxxxxx>
- Date: Thu, 23 Feb 2006 14:50:46 -0500
Lee, Chris
I worked in a board shop many years ago. My understanding of the
original intention of adding thieving to signal layers was to promote
consistent etching (or plating) and as a result even distribution of
etch widths. The chemical process of etching inner layers is in general
more consistent if there is an even distribution of copper across the
panel (subtractive or additive processes - etching or plating both
benefits).=20
There are numerous subtleties like etching/plating varies from center to
edge of the panel and what specific type of additive or subtractive
processes, chemicals, electro plating, ... you are using.
=20
As folks have pointed out you must be very careful with spacing to
critical signals and adjacent layer stripline pairs.=20
We had some interesting arguments at the dinner table when
"Auto-thieving" was first introduces at Digital - my brother is a
Chemical Engineer, my mom is a Produce-ability (Manufacturing) Engineer
and I am an EE .....
Regards,
Bob Haller
Enterasys Networks=20
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf of Lee Ritchey
Sent: Thursday, February 23, 2006 12:31 PM
To: Chris Padilla (cpad); ivorlist@xxxxxxxxxxx; Pradeep.RSA@xxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ground Pour in Signal Layers
Chris,
Of the three reasons listed below, only number 2 is of value. Shield
islands don't really do that. They may induce cross talk. Fabricators
don't need fill in signal layers to prevent warpage. I'm not sure where
that one got started. Also, doesn't help control impedance, but rather,
can make it drop if the fill is too close.
Don't mean to offend anyone with these answers, but the record needs to
be
set straight.
> [Original Message]
> From: Chris Padilla (cpad) <cpad@xxxxxxxxx>
> To: <ivorlist@xxxxxxxxxxx>; <Pradeep.RSA@xxxxxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 2/23/2006 8:53:40 AM
> Subject: [SI-LIST] Re: Ground Pour in Signal Layers
>
> "Ground" pour can serve several purposes:
>
> (1) Create shielded islands
> (2) Create some extra interplane capacitance (not sure how useful it
is
> for charge storage as it is most likely located electrically far from
> the chip needing the current)
> (3) Balance out a layer to avoid warpage issues and to ensure better
> trace dimension (impedance) control
>
> I think (3) is the big one: Fab shops often request the ability to
add
> "thieving" in areas of sparse metal. Some folks just go ahead and add
> the metal pour and often sink several "ground" vias through it to
> connect it up to something. I forget the details why, but the fab
shops
> have better control of trace width if they don't have to etch away a
lot
> of metal.
>
> As in all cases, such items can cause problems if not carefully
handled
> and thought through.
>
> Chris Padilla
> Cisco Systems
> San Jose, CA
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