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where footprints come from (was Re: Re: Internal PCB Units?)


--- In kicad-devel@xxxxxxxxxxxxxxx, Werner Almesberger <werner@...> wrote:

> > But I agree that some sort of them may be usable. For example is would
> > be handy to store footprint generayion algorithms in library instead of
> > tons of drawings of all these SOICs, SSOPs, MSOPs, QSOPs, MLFs and BGAs.

My Real Life (TM) experience: I'm using the IPC land patterns for my designs (not hobby stuff, production quality with 1000pcs batches, so small/medium scale).

IPC has published stuff about mostly everything in electronic construction but I'm talking about only PCB patterns here. The pattern viewer/calculatoris free to download (a .NET monstrosity), to directly generate footprints for cad systems you have to pay :P

You also have to pay for the specifications (is a consortium thing) so I'veno idea about the calculations behind this... just say that a 0805 patternfor a capacitor is different than the one for a resistor. The calculator asks for package height so I think there some structural reason to this (thecapacitor is bigger :D)

> Let me explain. There at least the following methods how people make
> their new modules today:
> - with KiCad's module editor

Usually for strange stuff like switches and battery holders.

> - with some homebrewn scripts, in a number of different languages
> (I've seen Perl and Python mentioned, there may be more)

Shell scripts :D I've done the whole Phoenix Combicon series with these.

> - in some other EDA system, then convert to KiCad

On the net you can find eagle libs converted. I don't like eagle so I didn't even looked at them...

> E.g., imagine you pick some BGA from the module library just to
> find that one of its dimensions is slightly off. Let's say the BGA
> was originally made with quicklib.php

I actually trusted some of the original kicad packages more than a years ago... the SMA (or was it SMB?) was missing the solder paste :D Luckily our fab checked them (and greatly insulted me...)

> To sum this up, if the format we use for sharing footprints is less
> expressive than the preferred format for defining them, this can
> easily create a major and possibly surprising burden for end-users
> wishing to make even small changes.

Not sure about this but have you looked into gencam? IIRC it's freely available as documentation and maybe it contains a useful representation for what we need.

ALSO, a little aside, but just to demonstrate that at the end everyone justneed to do his own patterns:

1) the IPC SOT23 seems to be, in fact, too small to guarantee proper mechanical support. Not know if it is true but my fab facility supposedly knows better than me. Enlarged the pads following their specs, just to be sure... BTW the boards in the test run with the smaller pads never had trouble withdetaching SOTs, anyway :P

2) OTOH the IPC SOT.65 (aka MSOP) uses pads with 7mils clearance. Sadly ourprocess for 70um copper requires 8mils clearance to be safe... had to trimthe SOT pads :(

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