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.
Then comes the really tricky part. For parts on the bottom side, the
general rule (not in the IPC standard) is to either view the parts
from the bottom with the board mirrored about the Y axis with the
same pin one orientation (upper left in the mirrored image) and
rotation counter-clockwise, or to view the bottom from the top with
rotation clockwise ( with the footprint mirrored about the Y axis so
pin one is on the right, in the upper right corner or top) giving the
same results.
All X,Y positions are with respect to the centroid of the part.
I would expect the software can do all of this, but you need to
layout your footprint with this in mind. In Free PCB, they use a
centroid vector to specify the location of the centroid of the part
and the angle of the zero degree rotational position. Not sure how
this is done in gEDA.
As you say, you can deviate from this and the board house will likely
still give you correct boards as long as you are consistent. But
even though the parts on my board were clearly labeled with pin 1, a
board house assembled all of my prototypes with the chips reversed
once. Now I am much more cautious about the XYRS file, almost paranoid... 8-S
Rick
At 10:42 AM 9/27/2010, you wrote:
Just 10 minutes ago I had my 1st talk with my first assembly house.
Guess what! I'm asked to provide rotation data.
In the other mail I'm currently editing, I'm trying to provide definitions on
where X- and Y-axis is on a part, including where X+ is on mechanically
doubly symmetrical polar parts etc.
As of now, I'll probably have to check/provide every angle by hand,
but for future footprints, the definitions have to be absolutely clear.
If there are contradictory standards, we will have to opt for one.
As Rick said, they are able to adapt to any coordinate system, but at
least the designer must know, what he means himself ;-)
Regards, Armin
Rick Collins wrote:
I am curious about the reasoning for picking values of design rules.
I have not found the assembly houses to be very useful for this
sort of info. They seem to be willing to work with whatever they
are sent and will only give feedback when something causes real
trouble for them.
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