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Re: gEDA-user: new footprint guidelines
Hi,
Rick Collins wrote:
At 04:53 PM 9/28/2010, you wrote:
For all those, that follow the discussion from here or vaguely
remember some other rotations:
Rick Collins wrote:
I had to go through all this some time ago and recently I wanted to
iron out all the difficulties so that the assembly house could use
my XYRS file (location and rotation data) directly without alteration.
That ended up being a fool's errand, but I did learn a few things.
IPC has a standard for this which everyone "seems" to use. For two
pin symmetrical parts, pin one is to the left. For IC type parts,
pin one is in the upper left quadrant for parts with pin one in a
corner or for parts where pin one is in the center of a side pin one
is on the upper most side. This is the zero degree rotation point
for the part. All rotations are counter-clockwise from this position.
on 2010-08-15 Rick wrote in thread 'Specification of Rotations for
Auto Assembly':
"I just found something that changes what I thought I knew. I have a
PDF of an IPC magazine from 2005 where they are touting a "leap
forward" in land pattern generation. An illustration showing pin 1
in the upper left for SOT components is what I used as my reference.
That and the post in the FreePCB forum of a normally very reliable
source. But I found a copy of IPC-7351 and it clearly says that for
SOT and most other IC parts, the original rotation is with pin 1 in
the LOWER left. That is what FreePCB does in the library editor by
default. "
This isn't Ricks fault: reading the 2005 IPC-7351 I can confirm this,
while the 2009 IPC-7351B says,
that pin 1 is in the upper left corner ;-)
Shall I comment on this ? I'll just use upper left...
I'm not sure what you are saying. Did you have a point you wanted to
make?
The point I wanted to make is, that there's nothing wrong with our
memories but
that the 2009 version of IPC-7351 contradicts the 2005 version (probably
2003 as I see now),
maybe in order to conform to EIAJ/ANSI 481C. So this conformance should
be veryfied.
I went through a very lengthy search for a rational basis for picking
a standard. Virtually no one seemed to actually know the source of
the standard they used or what anyone else was using. It seems like
the board fab and assembly business is full of cowboys who just want
to make the current project work rather than to figure out a system
that would help everyone. In the end I found that the incorrect
IPC-7351 that I found was an initial short form version from 2003,
limited to naming conventions and a brief listing of pin 1
orientations, not a full spec. I had also found some other materials
that had wrong information attributed to IPC-7351, but not official
(dated in 2003). The officially released standard came out after, in
February 2005, with the pin 1 orientation of all ICs either in the top
left or the top. Without knowing the whys, I can see that IPC-7351
seems to be what is more supported than anything else. IPC claims
that IPC-7351 matches EIAJ/ANSI 481C. I have not found an official
copy of IPC-7351 that shows any other orientation than what was
stated. If you have an official copy of the released IPC-7351 spec
that says pin 1 of ICs is anywhere other than top or top left, I would
like to see it.
Regretably I do not have any official version (bought in paper directly
from the relevant standads body)
but only pdf-files from the internet, that show the different names
IPC-7351 and IPC-7351B and
the respective release dates of 2005 and 2009. Neither do I have an
EIAJ/ANSI 481C paper.
The latest reference I found now is:
http://landpatterns.ipc.org/IPC-7351BNamingConvention.pdf
The old version I may have been looking at is 2003, not 2005:
http://www.pcbstandards.com/forums/attachment.php?attachmentid=501&d=1064619067
about EIAJ/ANSI I found only:
http://www.smtnet.com/library/files/upload/The-Universal-PCB-Design-Grid-System.pdf
http://www.thefreelibrary.com/The+future+of+CAD+libraries%3A+will+IPC-7351+be+adopted+globally%3F+Take...-a0129548051
All pdf's I have do not specify any coordinate axe direction, so one is
free to choose
and it's not relevant for rotation as long as the CAD-system has a fixed
"top" side for the design.
Real boards of course tumble around in space with 6 degrees of freedom
as do parts
so here the crazy busines with coordinate systems goes on, since the fab
may have
no intrinsic way to tell where top was in the design.
(I'm used to question coordiante systems, since mechanical (3d) cad will
orient
your model on the screen any way you like.)
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