kicad-developers team mailing list archive

[Heiko Rosemann <heiko.rosemann@xxxxxx>]

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On 06/08/2015 11:36 PM, Chris Pavlina wrote:
> On Mon, Jun 08, 2015 at 11:28:08PM +0200, Heiko Rosemann wrote:
>>> A given component should have exactly ONE available footprint.
>>> If your opamp comes in PDIP-8 and SOIC-8, those are two
>>> different components.
>> 
>> Why? There's a good point for the opposite: Making different PCBs
>> (SMD for series production/no SMD for prototyping/hobbyists or
>> different casings) from a single - consistent - schematic.
>> Otherwise you are going to run into trouble because you'll have
>> multiple schematics of the same system and you need to change
>> something on the logical (schematic) side.
> 
> I explained this already. Look at the pinouts for the different 
> footprints of LT1013. Only a very inexperienced person would claim
> that assuming different footprints have the same pinout is a good
> idea.

Different footprints for the same component are not required to have
the same pinout. You don't have to touch the schematic (there you only
see IN1+, IN1-, OUT1 etc. - no information about actual pinout), when
you change footprints on the PCB without breaking the connectivity as
defined in the schematic.

Simplest use case: Transistor in SOT23. Existing variant connects pin
B in symbol to pad 1 in footprint, pin C in symbol to pad 2 in
footprint, pin E in symbol to pad 3 in footprint. Now you would save a
few vias with a different alignment and your transistor is available
in reverse, so you create a second variant, connect pin B to pad 3,
pin C to pad 2, pin E to pad 1. Don't need to copy symbols, don't need
to copy footprints to match pin and pad numbers - and when you route
the PCB, you right-click on the footprint and say "change variant",
done. In the kicad-model this use case leads to SOT23-123, SOT23-321,
SOT23-BCE, SOT23-ECB etc. cluttering the footprint list. It's there
already...

Again, I'm explaining how Eagle does it (though it doesn't go all the
way - if you have different footprints with different pad numbers,
like a PDIP40 and a TQFP44, you need to create two components) and
where I have actually used it. I'm not saying "that's how kicad needs
to do it" (not my place to say this, with no code contributions from
myself...), but I do believe the options should be clear.

BR, Heiko
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