Re: gEDA-user: new footprint guidelines

[Gabriel Paubert <paubert@xxxxxxx>]

On Mon, Oct 04, 2010 at 12:09:22AM +0200, Armin Faltl wrote:
> Rick Collins wrote:
>> At 08:24 AM 10/3/2010, you wrote:
>>> Rick Collins wrote:
>>>> I really have no idea how things work in the gEDA/PCB world.  With  
>>>> FreePCB the library has a default orientation for parts and there 
>>>> is a centroid vector to allow the pin 1 orientation to be set  
>>>> compatibly with the Gerber files.  If you use someone else's design 
>>>> you need to verify that their library parts were done correctly or  
>>>> you need to use the same footprints which are a part of the layout  
>>>> and so are available.  There is no reason to screw up something as  
>>>> simple as this.
>>> How the Gerber file looks depends on the footprint definition. Once  
>>> one knows *exactly*
>>> a) how the transformations work
>>> b) that all libraries/generators(/custom made footprints) conform to  
>>> a sensible standard
>>> checking is as superfluous as with screw diameters and pitches and  
>>> before that point
>>> I don't believe it's simple enough.
>>
>> I really don't know what you are talking about.  The footprint will  
>> show up on your layout in some orientation.  That is the orientation  
>> it will have on the board in the Gerber files.  How will the  
>> "transformations" affect that?  What you see is what you get.
> Yes, what I see is what I get. And to see it, I have to read the source  
> code of the CAD
> system, unless it's stated somewhere more accessible - like in a  
> standard ;-)
> E.g., where is the centroid of a 3-leged part? Is it:
> a) the center of the bounding box of the pads
> b) the center of the bounding box of the pad centers
> c) the center of gravity of the pad centers (each weight 1)
> d) the center of gravity of the pad areas
> e) (0, 0) in the footprint definition file (or a designated vector inthere)

Take as an example a SOT89 transistor like NXP's BCX52-16 (I've 
just used one in a recent design) and look at the recommended footprints: 

Package page: http://www.nxp.com/#/page/content=[f=/packages/SOT89.xml]
Package drawing: http://www.nxp.com/documents/outline_drawing/sot089_po.pdf
Reflow footprint: http://www.nxp.com/documents/reflow_soldering/sot089_fr.pdf

I'm not sure what point a pick and place machine would like to
use as centroid, probably the crossing of the two axes in the
last drawing. This apparently rules out options b, c, and d, but
seems to work with option a.
 
I'm guessing here, but pick and place machine have to orientate the 
part very fast, so it is important that they pick the component from
a principal axis of inertia. It is not always easy to determine where
the axis lies when the component is asymmetric, which is frequent with
power components.

For another example, look a DPAK or D2PAK components (SOT404, SOT428, etc).
I'm not even sure that they option a) would work, but it might be 
a good default, provided you can override it.

	Regards,
	Gabriel


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