[PCB_FORUM] Re: 15.1 dynamic shapes and negative planes
- From: "Musetti, Carl" <cmusetti@xxxxxxxxxxxxxxxx>
- To: <icu-pcb-forum@xxxxxxxxxxxxx>
- Date: Fri, 6 Aug 2004 15:55:52 -0400
Thanks Ed, but I'm still not convinced that negative shapes are friendly or
easier to manage, I question the need to clean up voids on edges on pos vs. neg
if is need for pos than why not negative. Personally I just draw the shape I
want on the layer and have program dynamically void for me. 99% of time it does
a good enough job that I never have to touch it. I model my DRC vi to via and
vi to pin clearance so that there is always ample room to get sufficient copper
pour that the fabricator can fabricate to and the plane can handle the driving
current. The biggest problem I have seen through out the years and I call it
abuse not use is to many people get lazy with setting up their constraints and
use the minimum clearance in a design though out out the entire design on every
feature, I worked for a board fabricator and worked closely with assembly
process at Motorola Boynton beach and found that you just can't do that and get
what you thought you were going to get. Though now a days with Dual IR
processing and solder mask tenting over vias over both sides it is true that it
is more forgiving than it used to be.
Thanks for the info though, but I am very happy with positive shapes, but it is
nice to see that it was implemented in an attempt to make everyone happy and
satisfy all users usages of the tool I think it is one of the slickest pieces
of software to be implemented into allegro in a long time! Great job by you and
the developers!
Now to get to work on that padstack editor HaHa
See you September
Carl
-----Original Message-----
From: Ed Hickey [mailto:ehickey@xxxxxxxxxxx]
Sent: Friday, August 06, 2004 3:12 PM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] 15.1 dynamic shapes and negative planes
From the Shape's best practices doc that Barbara and I co-authored.
Negative Planes - Dynamic or Static Fill?
It will be advantageous to use Dynamic Filled Shapes on internal power and
ground plane layers, especially on boards with split planes. Prior to 15.0, the
intersection between the split planes known as 'Anti-Etch' would create 'false'
DRC conditions when encroached by obstacles like vias or pins. In addition,
when using the 'Slide' function to move a via across the Anti-Etch area, the
via being seen as a DRC condition would not move fluently across but would
rather jump erratically resulting in undesirable results.
Negative dynamic shapes are useful because you eliminate the need to void pads
that are close to edges, or fix up chopped edges where pads were voided but
have moved. The dynamics of the negative shape only operate on the edges which
make split planes much friendlier.
You do not have to change shapes to dynamic, you can leave them static.
If you have a pre-15.0 board with copper pour areas that were customized
by manually adding void areas, then it is not advisable to change those
shapes to dynamic unless you manually re-add the void(s) in the new dynamic
shape. This is true for negative and positive.
The reason for this is that Allegro, with static shapes, cannot differentiate
between voids created via the autovoid process and those the user created. Once
a shape is dynamic, Allegro can track user added voids versus autovoid created
voids.
The conversion from static to dynamic was provided to aid those customers who
wanted to convert pre-15.0 builds to use dynamic shapes. Additional manual work
may be required in the conversion process to add user voids and to smooth the
boundary. If the voids you have are based on route keepouts, dynamics will
generate the new voids for you and they would then move if keepouts changed
later.
14.2 False DRC condition 15.0 Dynamic
Planes
I'm not sure about this ED hickey or some one from Cadence may need to answer
but I don't believe dynamic shapes are intended to be used on negative layers.
I don't use negative planes anymore since dynamic shapes came, I don't give a
hoot how big the Gerber file is I would rather see my plane and DRC as is it
really exists. Dynamic shapes has obsoleted the use of expense tools such as
Valor and ADI in my case, I can now use a a free Gerber viewer just to take a
look at my data going to the fabricator just to make sure it is what I had in
allegro. So far I haven't seen any problems.
Ed Hickey
Allegro Technical Marketing Manager
Cadence Design Systems
Chelmsford, MA 01824
978-262-6545
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