[PCB_FORUM] Re: GND Clearance Under SMD Pads

I think that Gerry is right, its just to match the impedances.
 
 

Mashak Maqbool 
GE Energy 
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From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Gerry Meier
Sent: Wednesday, October 13, 2010 10:25 AM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Re: GND Clearance Under SMD Pads



I think what they are really trying to accomplish is to reduce the
impedance mismatch caused by the SMD pad itself. Since the pad is larger
than the trace the impedance would be lower, by removing the plane under
the pad you move the reference layer farther away raising the impedance
to get a closer match to the impedance of the traces.

 

Gerry

 

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From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Dave Schaefer
Sent: Tuesday, October 12, 2010 11:21 PM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Re: GND Clearance Under SMD Pads

 

Alexis,

Sounds like Kathy's solution should work, custom footprints for that 1
particular design with specific layers in both the design and library.

Removing copper from the ground plane for better decoupling? Interested
in knowing more about that requirement, first time I've heard of it.

Dave



On 12/10/2010 7:44 PM, ameehan@xxxxxxxxxxxxxx wrote: 

I haven't tried that yet. As you said, I think the layer names will have
to match. Might be the best solution though. Thanks!

-----Original Message----- 
From: Kathleen Descoteaux 
Sent: Oct 12, 2010 5:17 PM 
To: icu-pcb-forum@xxxxxxxxxxxxx 
Subject: [PCB_FORUM] Re: GND Clearance Under SMD Pads 

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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