Screaming Circuits: Who's Resonsible For The Footprint?


Who's Resonsible For The Footprint?

I recently wrote a bit about CAD library parts for QFNs and a reader posed an interesting question in response to it:

"Okay, that makes sense.. But why don't manufacturers of the part put out their footprints and schematic symbols for their parts in some common format?"

QFN Freescale eagle copper layer That is a very good question. When you purchase or download a CAD package, it will typically come equipped with an established library. Those libraries vary greatly in coverage and quality though, and, of course, they only cover the parts available at the time of release - and only a subset of the most popular, at that.

Who's job is it to make workable library components and or new ones? Ultimately, it usually falls to someone in the organization doing the circuit design. Some people will pay a third party to make the parts. NXP has made the complete library for their chips for the PCB123 CAD package. Some independent companies make it their business to create and maintain libraries. But, really - who should be responsible?

A components manufacturer needs to document a new chip anyway, but they'd have to make a library part for a dozen or so CAD packages for each part variation, depending on how much coverage they want. The CAD companies would need to create maybe a hundred thousand or a million library parts, depending on how much coverage they want, but the CAD package is useless without the library. The designer would need to create just the library components needed for the specific design, but that could easily double to design time. I guess that's why we have what we have - a combination of the above.

Still, the big question is: why isn't their a standard format for the libraries? That would make everyone's life easier. So, CAD folks; why no standard?

Duane Benson


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Whoops, forgot my footnote.

[1] - Gerber (RS-274) files do the job and all, but it's a command language for a photoplotter, it doesn't describe the board I want manufactured.

When I ship my design to the board house, why should I have to send them 8 files that describe my board and tell them which file is for what? Why should I have to tell them that I want .062" FR4 with a lead free solder finish? Why can't I have a single file with the vector art and all the manufacturing data needed to produce the board?

Yeah, gerber is a standard for board manufacturing (and even for that, only because "it's what we have", not because it's very _good_[1]), but it doesn't address the footprint/cream/schematic issues.

In researching this (since I was the one that left that comment you're quoting), I found that some places actually _are_ trying to do this.

National Semiconductor, for example:

..And Microchip:

They make libraries available in a format that a program called "Ultra Librarian" can read, which can then output into a number of different CAD library formats.

I still wish it were a more "open" format, and that it included 3D models of the part as well, but it's a start in the right direction.

The Gerber format is more or less a standard, but unfortunately, it doesn't address the footprint / schematic symbol creation problem. Nor does it address the problem of components that need specialized land patterns or paste layers.

There is sort of a standard available in gerbers, right? Although it would be a pain to import into some CAD packages, at least having manufacturer-recommended gerbers for all the suggested layers would let the designer double check their design.

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