Usually 3-4 times the height of the dielectric which is between the GND and PWR plane. The idea is to keep the radiation lobe of the power plane as tight to the GND plane as possible. You don’t want the PWR plane to radiate outwards from the edge of the PCB. There is no long-winded calculation, just a rule-of-thumb. If you want super-safe and over-kill ? Then pull back more. Then it’s a compromise to maximizing the PWR plane area. PCB’s are a collection of compromise……..that does what its suppose to do…….. very well !!!!

TDK

From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Don Larisey
Sent: Friday, November 28, 2008 8:07 AM
To: [log in to unmask]
Subject: [DC] PWR & GND PLANES

For some time we have received requests to step back the edge of the PWR Planes from the edge of the GND Plane around the perimeter of the board. My question is, by how much? We have a request for 3.00mm pull-back. I forget what Lee Ritchey recommended many many years ago, and of course, that may not be relevant today. Is there a spec for this? Are there any rules?

Regards,

Don

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