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Re: gEDA-user: new footprint guidelines

Rick Collins wrote:

Where I want to get us, is being a consistent customer, for whom they
no longer need to think about step b).
From what I can tell, they don't bother with the two steps. The machine picks the part from the feeder and before placing it, the operator verifies it is oriented correctly. Done once for a given feeder and a given side of your board, the rest of the parts from that feeder should be good.
If for whatever reason the designer used 2 different footprints for the same part occuring several times on a board, if the footprints are position/rotation inconsisten...
This is 100% reliable and not really a lot of time on their part. Even if they do the steps you are talking about, they will do the step I have outlined. They aren't engineers and they don't think like engineers. They don't want to figure out what things don't work, they just want to make them work. Their way is much easier in the long run I am sure.
The guy talking to me has 'MSc.' before his name ;-) No idea how much he's involved with the 
actual operation of the machines.
Even austrian farmers try to figure out and avoid reasons for problems - no cowboy mentality? 
See above / please check yourself.


I don't have PCB, so I can't check.
in that case you have to believe me or others, that with the internal coordinate system 
of 'pcb' X+ is to the right and Y+ is down.
The assembly house I'm talking to, offers to provide standard parts. I imagine, they use a combination of machine vision and having resolved step a) from 
above "once and forever" with their part suppliers.


When you say "parts", do you  mean footprint data for the CAD packages?
No, I mean this assembly house holds some standard parts in store (0603, 0805 resitors, caps,... of common values) and will sell them to you if you like - can save you and them 
some hassle.
The bottom line is, ask your assembler what they want. Don't assume anything. 
I will.

[snip]

About the .xy-file I'll have to read, how the footprint coordinates
and placement in the board influence the actual values. I think it
will be a bit tricky to check the footprints, since pcb doesn't show
the true coordinates but computes an offset on the fly to make all
screen coordinates positive - this is a bad idea for working on .fp-files.
That doesn't make a lot of sense to me, but I'm not sure why it is bad. 
To check, whether all the footprints I use conform to IPC-7351(B), esp.
if the centroid is at (0, 0) of footprint it would be easiest, to just load them into the design program. But pcb is cheating on you: the footprint-definition 
describes say a 2-pad part with pad-centers at (-2.0mm, 0mm), (2.0mm, 0mm)
and centroid at (0mm, 0mm). When loading the footprint definition (that's the .fp-file) 
on it's own, pcb will do some guesswork to squeeze everything in it's
positive coordinate quadrant and compute an offset (failing occasionally btw. 
leaving parts of text and lines in nirvana).
Then it will tell you that above pad-centers are at (0.7mm, 1.5mm), (4.7mm, 1.5mm) and center mark at (2.7mm, 1.5mm). The same applies if the definition had been (2000, -100), (2004, -100) and (2002, -100) - there's no way to tell the numbers 
in the definition by looking at the GUI.

And what I'm trying to figure out atm, to verify the data to be sent to the
assembly house is, how the footprint definition, the guess work and the
actual placement get munched into the XYRS file.
I use 0,0 as the lower left corner of the board and my fab drawing gives coordinates of the fiducial marks on the board along with major drill holes (like mounting points). So all coordinates on the board are positive. Why would you want it different? I don't know what a .fp file is.
I don't want it different for the board (which would require reversing Y-orientation in pcb), 
but for a loaded footprint definition.
BTW, all part coordinates should be wrt the centroid of the part, not pin 1. Some CAD packages used to use pin 1, but it is standard practice to use the package centroid now.
Centroid always appeared natural to me - it's the best position for physical rotation axis as well .
What sort of checking of the footprints do you want to do? You should use a Gerber viewer to verify the Gerber files. Nothing inside the CAD system matters if the Gerber files aren't right. What would be great is a viewer that understands the part shapes and positions the parts according to the XYRS file on top of the Gerber file images so you can verify alignment and orientation.
Since the assembly house won't have/use my footprint definitions and I don't want to make a drawing of each and every part, if a standard clearly states, how it looks, I have to check, whether the CAD-internal definitions conform to the standard. If no standard existed at all and I really have to make drawings with whatever tool, I still have to confirm the CAD-internal definition is identical to the drawings. 
Good luck!

Thanks, Armin


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