[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
Re: gEDA-user: new footprint guidelines
Rick Collins wrote:
This is just a 3 page list of the land pattern naming conventions.
This is not really the standard at all.
[snip]
I can send you this doc if you would like.
Yes, please
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?
It's dawning on me, that we are talking of different things:
the Gerber export probably (didn't check) has X+ -> right, Y+ -> top as
you say -
I was talking about the internal and screen coordinate sytem of "our" pcb.
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.
An assembly house with some experience with a customer can mentally split
this rotation into two operations:
a) rotate from the feeder to a (house internal) standard orientation
b) rotate from the standard orientation to board placement
Where I want to get us, is being a consistent customer, for whom they
no longer need to think about step b).
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?
See above / please check yourself.
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.
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.
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.
Tbh, this is the first time I ever talked to an assemby house. I want to
provide them
data as good and consistent as I can, right from the start. To show them the
compoent locations I will provide the silk, solder-stop and component
layer as gerbers
accompanied by the XYRS file for placement, the paste-mask as starter
for their own. Do you think that's enough / too much?
They also want a reference of the XYRS to partnumbers&sources, so they can
provide an offer with their own components (ev. cheaper because tape&reel
instead of cut tape) and need it anyway, if they use my provided material.
So I have to merge my part list (open office calc) with the xy-file from
pcb.
That's why I think a "part" attribute known to gschem and pcb makes sense.
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.
Armin
_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user