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Re: gEDA-user: new footprint guidelines
Rick Collins wrote:
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.
Do you have a copy of the 2003 version of IPC-7351? I think there was
no 2003 version of the spec. If I understand the little bit I can
find on this it was only released in 2005.
Apparently not - only the URL below with the 2003 note in the footer.
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
These are the naming conventions only and do not explain anything,
they just list some basics. The full spec from Feb 2005 is 92 pages.
This rev has no suffix letter. There has been a rev A in Feb 2007 and
and a rev B in 2010. I don't believe any of the released revisions
change any footprints that have been published, but rather add new
footprints.
On the last page of the pcbstandards.com-URL there is a 16-item list titled
"Component Zero Rotations Pin 1 Location:". It describles pin 1 of a e.g.
SOIC, SOP, TSOP, etc. as at bottom left.
This is no longer the case in the 2009 and 2010 versions.
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
I don't find any info on EIAJ/ANSI 481C in the first reference. The
second is the same article I found the referrence in. Tom Hausherr
had his article published in a number of publications in the Feb 2005
time frame. Or at least the same article shows up in a number of places.
Sorry, there isn't really any info, the standard is just mentioned I think.
There are some sites claiming to provide EIA-481-D-2008 for download.
They all require registration and since they look a bit like warez
distributors I didn't.
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.)
All PCB CAD systems I've seen display a layout with the positive X to
the left and positive Y to the top.
Till now I thought for 'pcb' X+ is to the right and Y+ is down - and I
still do.
This is/was the convention for scan-line oriented 2D graphics display
systems.
(0, 0) is in top left, X is right, Y is down.
How you orient your board in this frame is always up to you.
sure - and I will give the assembly house at least part of my gerbers.
But with an XYRS file and the raw board alone, they will have to
guess/test, how I did it.
I doubt that any PCB CAD system will change the XY axis frame from
that standard orientation, at least without making that obvious.
Because you made me think twice, I just tested what pcb does with some
cursor movements,
looking at the coordinate counters. That's what makes the axes obvious
for me.
The parts are all designed to be soldered on a board, so they don't
have complete freedom to be "tumbled" unless you aren't planning to
assemble them. The fab house will know the top of the board is the
side you tell them on the assembly or fabrication drawing. Usually it
is not one of the long skinny sides, e.g. 0.062" wide. Are you
over-thinking this part?
Yes and No. The number of practical orientations a board and part can
have are very limited,
but to check them, until now a human will be involved. True automation
readines requires
that you can feed the file into the machine, the machine knows, where
it's fixtures are
and therefore will correctly transform design positions to machine
positions without
manual intervention. The operator just has to follow the rule, that the
(0, 0) marking
on the board (to be invented) "has to be at the fixture with the red dot".
To help everyone involved, I include 'TOP' and 'BOT' in my copyright
notices, written
in copper. My current board isn't square, but then I could state, that
the baseline
of the copyright is parallel to X-axis in the XYRS file.
Armin
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