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

Hi all,

(been watching this thread for a while and want to step in...)

The thing that's not being discussed is that negative planes void are
created by an antipad diameter, which is independently defined from pad
diameter. That they could be compared or interchanged easily is not
realistic. (IMHO) 

Example; It is entirely possible to create an antipad that is near or
identical in diameter to a pad which could provide adequate clearance in
a negative plane (not that you would do it in some cases), but in order
to the identical voids around a pad using positive plane, you would have
to set the DRC to zero, or some infinitesimally small number, OR you
would have to alter the inner layer padstack diameter for plane layers
only. Also, you could do it on a shape by shape basis (I wouldn't).

It is likely that antipad diameter is not always equal to FHS + some
fixed number in most designers' padstack libraries.

It is a completely different design methodology that would require
review of your padstack library, spacing constraint setup, and other
design parameters in order to have an effective implementation.  

Regards,
Andrew

-----Original Message-----
From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Austin Franklin
Sent: Friday, July 20, 2007 4:44 AM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Re: Planes: Negative vs. Positive?

Hi Robert/George,

> I fully agree with George that negative planes give better results 
> than positive ones in terms of Swiss cheese effect under BGAs.

Shouldn't you be able to setup a positive plane to give the same
results?

Regards,

Austin

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