[PCB_FORUM] Re: GND Clearance Under SMD Pads

  • From: Randy Dawson <rdawson16@xxxxxxxxxxx>
  • To: icu-pcb-forum <icu-pcb-forum@xxxxxxxxxxxxx>
  • Date: Wed, 13 Oct 2010 03:43:12 -0500

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.


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?
Alexis Meehan
Violin Memory Inc.
Mtn View, CA
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