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Re: gEDA-user: new footprint guidelines
At 07:00 PM 9/29/2010, you wrote:
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.
This is just a 3 page list of the land pattern naming
conventions. This is not really the standard at all. The standard
document is 92 pages long (the 2005 version) and has five pages of
very detailed drawings of each part type showing pin 1
orientations. I can't find a copy of the newer revisions that I
don't have to pay $100 for. The point is that when this was
published in 2003, it was not a standard yet as far as I can
tell. The original revision of the full standard came out in Feb
2005 and has no suffix. The 2007 and 2009 standards (A and B
respectively) should have the same detailed drawings of parts and
land patterns showing pin 1 orientations. I can send you this doc if
you would like.
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.
If you have the positive Y axis down, your rotations will be reversed
from the standard... I think. Actually I have no idea how to
consider this. That may be a standard used for a "display system",
but nothing to do with PCB layout. As you say, go into your CAD
program and move the cursor around. I am pretty confident it will
give coordinates with the positive X to the right and positive Y
up. That is how all the Gerber file viewing programs do it as well
as the PCB layout packages I have seen. Are you telling me PCB does
it differently? Will it really give you a Y coordinate for a part
that increases as the part is moved to the bottom of the screen?
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.
Of course. You have to give them assembly information to show how to
orient the board. Even then the XYRS file is not enough for
them. They have to figure out how to rotate the part from the feeder
to match your orientation. They don't trust standards. I'm sure
that is from experience.
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.
So what does PCB do?
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".
Trouble is that the machine doesn't know how the parts are oriented
in the feeders. Rather than trust that the "system" works if they
get each piece right, they manually run through an sample of each
component type to make sure it is placed on the board right. That is
all they care about and you only do this once for a given
board. They call this "setup" and charge a couple of hundred dollars
for it. Not enough of a charge to worry about and it gives them a
warm fuzzy feeling that they aren't screwing up.
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.
I'm sure they can use all the help they can get. Don't you provide
assembly drawings with TOP and BOTTOM in them? I've never had an
assembler not ask for a good drawing. As long as they know which
side is top and if the component locations are shown on a good
drawing, they can handle everything else the way they are accustomed
to doing it. I ask them what they want, not try to figure out what
they need. That is what I was doing when I got so frustrated with the
goofy standards.
In theory, practice and theory are the same. In practice, they
differ considerably.
Rick
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