Re: gEDA-user: Parts!

[Joerg <joergsch@xxxxxxxxxxxxxxxxxxxxx>]

Levente Kovacs wrote:
> On Fri, 15 May 2009 13:32:12 -0500
> Bill Gatliff <bgat@xxxxxxxxxxxxxxx> wrote:
> 
>> I think we're in violent agreement, but might be talking about subtly 
>> separate things.
>>
>> A given device e.g. a particular transistor may be available in one
>> or more specific physical _packages_, i.e. TO220, DPAK, etc.  If you
>> had a collection of such devices in different packages, then they'd
>> be different part numbers--- but you'd probably capture the basic
>> name of the transistor device in the description fields of your
>> database i.e. "2N7002", "n-channel", "transistor" so you could later
>> ask for "a transistor in a TO200 package, any vendor".  You'd have
>> two or more rows with that same description (hence the need for a
>> description convention), each of which differed only by the package
>> and part number for the device(s) you bought.
>>
>> There are several PCB layout _footprints_ that are compatible with a 
>> TO220 _package_, but that's information that's tied to a specific
>> part number only through what package the device referred to by that
>> part number came in.  PCB doesn't care that it's a 2N7002, or even
>> that it's in a TO220 package--- it just wants to know what footprint
>> you want, and you want to make sure that the footprint that you tell
>> it to use  will actually work with the package the physical device
>> came in.
>>
>> So your device table needs a package field, which reflects the actual 
>> physical package the device came to you in.  Then you need another
>> table that maps package types to compatible layout footprints.  Then
>> you select from that list of footprints based on the characteristics
>> of your circuit board.
>>
>> (Actually, since Fairchild has their own naming conventions for 
>> packages, which will be different names but the same physical 
>> characteristics as those offered by other vendors, you might want the 
>> schema to be slightly more complicated than the above.  You could
>> even have a schema that fills in some of the package and other
>> information for you based on the header and footer of the part
>> number.  But I digress).
>  
> 
> OK. I understand, but I guess it doesn't worth the trouble to implement such
> complicated stuff. To be honest, I don't see why one would combine some device
> coming in different packages into one part. I've seen some commercial
> implementation of this, but we rarely used it. I have one example in my mind.
> It's the IRF540 hexfet. It is in TO220, but there is a DPAK2 package. I think
> we could live with it if it was two different part, or whatever we call it.
> 
> On the other hand however, I have a few footprint variant so far for certain
> packages. With the current architecture, I don't know how could we implement
> a footprint selector at gschem level. Maybe my dbsym_update could be
> interactive... but it is awkward.
> 

Eagle has a footprint selector. It is nice and useful if you find out 
the board ain't going to fit and then change most resistors from 0804 to 
0603. But other than that it's dangerous. For example in your example 
above the part number must be different. The TO220 version is IRF540NPBF 
while the D2PAK is IRF540NSTRR. If you don't have that properly embedded 
the BOM will be wrong, purchasing buys the wrong parts, the boss gets 
red in the face, and things aren't looking up. In very regulated 
industries you can't even do the swap with passive parts.

[...]

-- 
Regards, Joerg

http://www.analogconsultants.com/



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