kicad-developers team mailing list archive

[Lorenzo Marcantonio <l.marcantonio@xxxxxxxxxxxx>]

On Mon, 14 Jun 2010, Werner Almesberger wrote:

> Lorenzo Marcantonio wrote:
> > The playground should be a simple bounding box, but manually defined
> > (since, for example, BGA requires a lot of tooling space for inspection
> > and replacement, while a simple MLCC only needs space for tweezers)
>
> What would constitute an invasion of the "playground" ? Another
> component's playground ? Part of the package of another component ?
> A pad of another component or a trace ?

Only another playground... its the "bounding tile" of the component,
needed for testing/replacing it. The standards for example gives about
1mm for BGA since you need space for inspection and the reworking nozzle
(there is a special optical probe for looking *under* the chip, to avoid
x-raying the board).

Oh, BTW the official term is 'courtyard' :D

IPC-7351A define it as:

"The smallest rectangular area that provides a minimum electrical and
mechanical clearance (courtyard excess) around the compined component
body and land pattern boundaries."

To put thing simply you first compute the 'electrical' courtyard
(bounding box of the pads *with* their isolation clearance), then you
compute the mechanical one (the body plus some 'courtyard excess', which
is tabulated for component type), then pick the rectangle containing
*both* of these... in other word it's not something you can compute
automatically without access to the full IPC standard (the free land
pattern calculator gives it, anyway). So I would expect a button like
the 'component origin' to select the two corners of the courtyard
(ideally it would be on yet another layer).

> Examples for each:
> - test equipment attached to a component (clips, etc.)

These are mostly *over* but for a turret test point it would be useful.

> - rework

Mostly this

> - soldering accuracy, creepage/clearance distance

Not exactly... pad clearance are for these things. The playground is
mostly a mechanical, not electrical thing. OTOH the pad clearance
contribute to it, so, yes, in a way.

> It's a very good point, though. I never liked the amount of
> handwaving that goes into drawing component outlines because of
> just this kind of issues.

You you read the standard there is no handwaving... you pick the maximum
material condition (i.e. the biggest that a component can get out of the
factory) and this would be the silkpad (some people instead use the
nominal size for silkpad, but without the courtyard it could be risky).
The courtyard simply encloses all the manufacturing constraint for
a given fabrication level (I always use nominal, BTW, since it gives 99%
yield anyway, in our experience).

The trouble is during component placement... it's easy to put a 0603
under a big TQFP (which is of course physically impossible... but 
socketed PGA are an exception) :D. A courtyard based DRC would signal
this (and the autoplacer could use it too, altough I've *never* seen it
doing a decent work).

The IPC standards suggest a clearance of 0,05mm (the standard grid pitch)
about playgrounds but even a simple intersection check would be useful
(OTOH we already have the pad-pad collision routines, so it's almost
free).

--
Lorenzo Marcantonio
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