[PCB_FORUM] Re: GND Clearance Under SMD Pads
- From: "Schwartz, Jerome" <jschwa01@xxxxxxxxxx>
- To: <icu-pcb-forum@xxxxxxxxxxxxx>
- Date: Wed, 13 Oct 2010 10:25:46 -0400
I have seen this requirement, in the data sheets, for series caps in an
Ethernet application.
I use anti-etch with split planes. Maybe you could build into your
footprint
an anti-etch all layer.
Jerry
-----Original Message-----
From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Dave Schaefer
Sent: Wednesday, October 13, 2010 10:21 AM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Re: GND Clearance Under SMD Pads
Would like to see where the requirement came from, I'm quite certain
it's untested/unproven. Without a measurement, it seems you would be
decreasing your decoupling capacitance effectiveness.
Be interesting to get the source of the requirement and cross post to
the experts over on SI List to get their opinions.
>
> From: "Jean Bratton" <jean.bratton@xxxxxxxxxxxxxx>
> Date: 2010/10/13 Wed AM 06:56:02 CDT
> To: <icu-pcb-forum@xxxxxxxxxxxxx>
> Subject: [PCB_FORUM] Re: GND Clearance Under SMD Pads
>
> I've seen this request for AC Blocking caps on high speed nets,
reasons
> as suggested by Gerry, but not decoupling caps...
>
>
>
> Jean Bratton
>
> Senior PCB Designer
>
> Freedom CAD Services, Inc.
>
> Phone: 603-864-1349
>
> Skype: jean.bratton
>
> Email: jean.bratton@xxxxxxxxxxxxxx
<mailto:jean.bratton@xxxxxxxxxxxxxx>
>
> Visit us at http://www.freedomcad.com <http://www.freedomcad.com>
>
>
>
>
>
> From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
> [mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Randy Dawson
> Sent: Wednesday, October 13, 2010 4:43 AM
> To: icu-pcb-forum
> Subject: [PCB_FORUM] Re: GND Clearance Under SMD Pads
>
>
>
> Hi Alexis,
> Unless I am missing something here, you want that decouple cap signal
to
> launch immediately to the GND layer...
>
> No voids underneath in it either. Question them.
>
> Randy
>
>
>
> ________________________________
>
> From: kathy_descoteaux@xxxxxxxxxxx
> To: icu-pcb-forum@xxxxxxxxxxxxx
> Subject: [PCB_FORUM] Re: GND Clearance Under SMD Pads
> Date: Tue, 12 Oct 2010 20:17:36 -0400
>
> Hi Alexis..
>
> Did you try to add a route keepout on the second layer? It should
clear
> out the gnd plane on the second layer if its a dynamic shape. There
may
> be an issue when it's placed on a brd with a different layer name for
> the second layer... a minor update to the footprint and you should be
> good to go. Just thinking... not sure if it will work.
>
> Kathy Descoteaux
>
>
>
>
> ________________________________
>
> Date: Tue, 12 Oct 2010 20:07:19 -0400
> From: ameehan@xxxxxxxxxxxxxx
> To: icu-pcb-forum@xxxxxxxxxxxxx
> Subject: [PCB_FORUM] GND Clearance Under SMD Pads
>
> For a high speed application, we've had a request from engineering to
> create a special footprint for decouplers (0402, 0201) with a GND
> clearance under the pads. The clearances will be the same size as the
> pads, and will just affect the ground plane on layer 2. I'm trying to
> figure the best way to do this. I thought I could just build a special
> padstack, but Allegro (using XL 16.3) won't allow me to build an SMD
> padstack with an anti-pad unless I define a drill, bottom side pad,
etc.
> But I'm not seeing a clean way to build this into the footprint symbol
> either. Any ideas?
> Thanks.
> Alexis Meehan
> Violin Memory Inc.
> Mtn View, CA
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