[SI-LIST] Re: Ground Pour in Signal Layers
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Chris Padilla (cpad)" <cpad@xxxxxxxxx>, ivorlist@xxxxxxxxxxx, Pradeep.RSA@xxxxxxxxxxxx
- Date: Thu, 23 Feb 2006 09:31:04 -0800
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
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Ivor Bowden
> Sent: Thursday, February 23, 2006 8:38 AM
> To: Pradeep.RSA@xxxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Ground Pour in Signal Layers
>
> Pradeep,
> For what its worth, we routinely ground flood single signal layers that
> are immediately adjacent to power layers to reduce ground impedance and
> improve thermal conductivity, but we don't flood signal layers that are
> paired with other signal layers to avoid impedance discontinuities.
> Ivor Bowden
>
> on 2/23/2006 12:04 AM Pradeep RSA wrote:
> > Hi,
> > does a Ground Pour in the Signal layers in a multilayer PCB=20
> > (with sufficient internal ground planes) really helps?
> > =20
> > Thanks in advance
> > Pradeep
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