Re: gEDA-user: PCB paste layer, revisited.

[Dave N6NZ <n6nz@xxxxxxxx>]

joeft wrote:

>>> For sanity's sake, let's let a footprint define its pad stacks for its
>>> own pads, rather than trying to maintain a global cache of padstacks.
>>>
>>> Then, a BGA footprint would have one "here's what my pads look like"
>>> and 431 copies of "put one here".
>>>   
>>
>> Yup.  Right answer.
>>
>> A footprint needs to be entirely self-contained.
>>  
>>
> I can't agree.  There is no reason to have a pad stack (and drill and 
> keepout and paste and...) defined in every footprint that uses a certain 
> size pad or pin definition.  How many places do we need to have copies 
> of a pad suitable for a .5mm pitch QFP when every such QFP needs the 
> same pad geometry?
> One reason to have the pad stacks separately defined and simply 
> instantiated in the footprint is to account for a process or fab vendor 
> change.  So you want to convert your "library" to RoHS-compliant 
> soldering process and the recommended paste patterns need to change.  
> I'd much rather edit a pad stack definition once than go through every 
> footprint that uses it to make the change.
> 
I certainly see your point. The question this design decision hinges on 
is what unit of information do we want to be able to share.

If a footprint is entirely self-contained, then the sharing of footprint 
files is easier.  If a footprint relies on external base definitions, 
then library maintenance is easier *after* you have made a 
self-consistent library, but bringing in an individual footprint coded 
by someone else would require considerable homework.

Now, perhaps with appropriate coding conventions, we could have the best 
of both.  If we could agree on a common set of pad names, or a pad 
naming convention, then a footprint could often (one hopes) be coded 
against well-known pad names.  But then we would have essentially two 
libraries.  Opportunities for inconsistency would multiply.

Frankly, I rarely code footprints anyway.  I code footprint generators. 
   (Or use DJ's.) I would never be in the position of editing every 
footprint to update a pad definition.  I would tweak a generator and 
rebuild the footprint library.

-dave


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