From: mpwalsh@eos.ncsu.edu (MICHAEL P WALSH)
Subject: Re: Pins on grid ???
Reply-To: mpwalsh@eos.ncsu.edu (MICHAEL P WALSH)
Organization: Project EOS - North Carolina State University
Date: Sat, 11 Apr 1992 19:38:23 GMT

In article 76 of comp.sys.mentor Al Onderick (aao@icd.ab.com) writes -

>> Is it better to offset pads on SMT parts so the connect points fall on
>> a known grid or is it better just to leave the pad origin at the center
>> of the pad. My design experience with other systems tels me to leave it
>> on grid but the AE says it doesn't matter. Any input will be appreciated.

In article 77 of comp.sys.mentor John Ruf (john@hpsad.sad.hp.com)
follows up with -

>>   The Mentor system will route to offgrid pins. It does it by getting to the
>> nearest grid intersection to the actual pin ID and then making one final 1
>> or 2 segment route to the offgrid point (will make one right angle
bend). The
>> difficulty comes if you want to merge in an odd grid for these
offgrid points
>> with your normal routing grid. There may be many reasons for doing
this. Its
>> most important for fine pitch surface mount parts (pin spacings of 25
mils or
>> less, or some metric spacing). In order to block the least route
paths you want
>> to come straight out of a pad and go to a via to get to inner layers
(in many
>> cases). If the router first jogs to get to the normal route grid, you
will
>> block more routing area than if you create and merge a special grid
in the
>> area of these weird parts (so thats why you would want to use special
grids).
>>   Most parts which are not metric have a pin spacing which falls on
some
>> multiple of a 25 mil pin grid in at least 1 axis. If you will pick a
point
>> in the other axis which is on a 25 mil grid (but may not be the
center of the
>> pad) you will reduce the number of extra grid points you need to
create for
>> offgrid pins. This in turn reduces total routing memory and time and
may
>> improve the pin entry (in some cases).
>>   We have tried it both ways and determined that putting all pins
(except
>> metric ones) on a 25 mil multiple will give us the best routing
overall. The
>> extra time spent in creating offset pads is well worth it. We use a
company-
>> wide library so any efforts spent in making the library work as well
as
>> possible is leveraged many times over.

>>   Sorry that I don't have any quantitative data on the results of
both methods
>> but our overall consensus was that on grid pins can only help us,
never hurts.


I agree for the most part with what John says above however I'd thought
I'd throw
my own two cents in as well.

First of all, the more pins that lie on the routing grid, the more likely that
the auto-router will succeed in an acceptable amount of time.  Off grid
pins take
time to connect.  A routing grid can be generated so that every pin lies
on the
routing grid.  This may be an accepatble solution for smaller designs
but the 
memory required to store a rotuing grid for a large design with lots of
off grid
pins may well be in excess of 100 megabytes!  Keeping the routing grid
inside
available memory is key in keeping route times to a minimum.

The ideal routing grid provides sufficient channels to complete all connections
without providing excess channels such that the auto-router spends lots of time
analyzing nearly identical channels.  The more grid lines, the more
possibilities
that the auto-router must consider.

Moving the pin id inside a pad is tabboo (sp?) at some sites and
standard practice
at others.  Allowing for non-centered pin ids could potentially mean
that it would
require four separate pad stacks (left row, right row, top row, bottom
row) to
build a surface mount geometry where all pins fall on say a 25 mil
routing grid.
To some people this is a big deal, to others it is not.  Using the
standard pin id
at the center of the pad allows a surface mount geomtry to be defined
using a single
pad with different orientations.

In my opinion it is not worth the hassle to modify pin ids UNLESS you are using
fine pitch (10 mil, 15 mil) parts.  Then it may well be worth the time because
the router benefits from any help you can give it.  Another instance
where moving
the pin ids may prove beneficial is when the amount of memory required
to hold
the routing grid exceeds the available memory installed in the
workstation.

Lately I have seen sevarl instances where organizations have defined the origin
of a surface mount component at the geometric center of the part.  In
many cases
this causes all of the pads of the geometry to lie off grid.  By doing
this and
not modifying the pin ids, the success of the route may be hampered.  I
understand
that many sites define the geometric center of the part as the origin
for generating
pick and place data and this ceratinly is a valid reason.  However, if
the off grid
pins are resulting in poor routing completion then maybe the decision as
to where
to locate the origin should be readdressed.

If anyone is interested, I have an algorithm for generating pick and
place data from
a Board Station database regardless of component origin.  This algorithm
runs under
Board Station 7.x now and I plan to port it over to 8.x in the near
future.  Send
me e-mail if you would like more information, as my schedule prevents me
from reading
this group on a regular basis.

--
Michael P. Walsh
Application Engineer (& part time grad student!)

Mentor Graphics Corporation - North Carolina Sales Office
2525 Meridian Pkwy. Suite 260
Research Triangle Park, North Carolina  27713
(919) 544-0200

mpwalsh@eos.ncsu.edu
Department of Computer Engineering, North Carolina State University
Newsgroups: comp.sys.mentor