[PCB_FORUM] Re: Planes: Negative vs. Positive?

Hi George,

Apologies... I should have chose better wording. 

Ed : )




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[PCB_FORUM] Re: Planes: Negative vs. Positive?






Ed:
 
The positive plane clearance is going to be the DRC values from the pad, 
where negative thermal or clearance flashes are normally calculated from 
the drill size.  We looked at this extensively when we were deciding 
whether to to to positives, and found the swiss cheese effect on the 
positive planes under BGAs was much more noticeable on the gerbers than 
was the equivalent negative planes using thermal and clearance flashes.  I 
don't have the graphics available, nor do I have time to replicate them 
right now, maybe I will do this when I have more time, since this subject 
comes up every six months or so. 
 
For now, I am going to put this into the "Religious War" category, along 
with 90° traces, vi or emacs,  and Windows/Linux/Mac.  We prefer negative 
planes and have no problem with them, others prefer positive planes.
 
It is moot with us anyway.  Until Cadence decides to allow us to define a 
thermal pad shape with a void in it so we can keep our SHF/EHF analog 
signals discontinuity free when we transition from the outside world to 
the board world,  we have to stay with negative planes and old-style 
flashes that could be defined with lines.
 
Until I can show y'all why it's better, I ain't going to be drawn into it 
again :)
 
-- 
George Patrick
Tektronix, Inc.
Central Engineering, EDS Applications Support
P.O. Box 500, M/S 39-512
Beaverton, OR 97077-0001
Å 503-627-5272 (voice)     Æ 503-627-5587 (fax)
http://www.tektronix.com    http://www.pcb-designer.com
 
"Off-Grid and Proud of it!"
-----Original Message-----
From: icu-pcb-forum-bounce@xxxxxxxxxxxxx 
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Ed Caldwell
Sent: Thursday, July 19, 2007 10:07
To: icu-pcb-forum@xxxxxxxxxxxxx
Cc: icu-pcb-forum@xxxxxxxxxxxxx; icu-pcb-forum-bounce@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Re: Planes: Negative vs. Positive?


Hi George, 

I may have misunderstood your statement... but doesn't the opening from 
pin to positive plane follow the DRC settings (or exception clearances)? 
And doesn't the opening in a negative plane ignore the DRC settings and 
simply follow the anti-pad? 

Respectfully! 
Ed 

Ed Caldwell
PCB Designer
USA 678-473-8707
mailto:ed.caldwell@xxxxxxxxxx
EDS http://www.eds-pcb.com



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Positive plane layers have a much larger opening around pins.  This is 
fine if you are doing low to medium speed boards (roughly < 200 MHz clock 
speed), but on high-speed designs the impedance discontinuities created by 
the huge opening around the pins are significantly greater on a positive 
plane than on a negative plane. 
  
We use negative planes for this reason, and we always use Valor to check 
our artworks to make sure we haven't shot ourselves in the foot :) 
  
YMMV 
  
-- 
George Patrick
Tektronix, Inc.
Central Engineering, EDS Applications Support 
P.O. Box 500, M/S 39-512
Beaverton, OR 97077-0001
Å 503-627-5272 (voice)     Æ 503-627-5587 (fax)
http://www.tektronix.com    http://www.pcb-designer.com 
  
"Off-Grid and Proud of it!" 
-----Original Message-----
From: icu-pcb-forum-bounce@xxxxxxxxxxxxx 
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Bob McCreight
Sent: Thursday, July 19, 2007 06:51
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Planes: Negative vs. Positive?

The design team I work with is split one which to use, negative or 
positive planes.
What are the pros and cons of each? What do most people prefer to  work 
with?
We are currently using 15.5.1 but evaluating 16.x for possible future 
migration.
 
 Thanks for your feedback.
 
 Bob McCreight, C.I.D.

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