[sdiy] Eagle footprints

[rsdio at audiobanshee.com]

On Nov 22, 2015, at 3:28 PM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> Hi Tom,
> 
>> I've been caught like this too - but how stupid is this situation?!
>> 
>> Software houses and parts vendors spend time and money getting together the most extensive parts libraries they can, but nobody uses them because they probably have some errors. Instead we redo all the work they already did...so we can get it wrong ourselves?
>> 
>> It's daft, but we all do it. ( Why? Wrong list for psychology questions, I know…)
> 
> The big boys in the CAD market -- Mentor, Cadence, Zuken, etc -- don't
> put much effort into libraries for several reasons:
> - big customers don't buy the product based on the size of libraries,
> they want other things (integration with manufacturing systems, tools
> that speed up board layout, signal integrity analysis, thermal
> analysis, etc), so why spend money on something that won't get you a
> sale?
> - libraries need maintaining, when vendors update footprints, new
> parts come out, old parts are EOLed, etc.
> - big customers have CAD data management teams that work with their
> own QA departments and work to industry-specific standards --
> automotive, aerospace, medical, nuclear, defence, consumer, prosumer,
> high-quality, high-volume, etc.  One library won't suit everyone, and
> the big customers will ignore them anyway.
> 
> Further down the CAD tool foodchain the budgets for creating libraries
> get smaller.  You get what you pay for.
> 
> Finally, you get to the bottom of the stack: free(mium) tools like
> Eagle, DesignSpark, Altium's CircuitMaker.
> Below that the FOSS tools like KiCad, GEDA, etc.
> 
> And then there are companies who specialise in producing parts
> libraries and/or tools.  For example I occassionally use Ultra
> Librarian, which has a free reader/converter and so I can take BXL
> files from TI, Maxim, etc and autogenerate symbols and footprints for
> a wide range of parts direct from vendor data into the format needed
> for the CAD tools I use.  For example:
> http://www.accelerated-designs.com/ultra-librarian/
> https://www.maximintegrated.com/en/design/packaging/cad/
> http://webench.ti.com/cad/
> Other manufacturers that provide BXL files include Analog Devices,
> Microchip, Freescale, and so on.
> 
> Having been sorely bitten by bugs in high-end CAD vendor libraries I
> no longer trust them for anything other than to provide some examples
> for training.

Thanks for the comments, Neil.

One thing that I've noticed is that there doesn't seem to be a definitive landing pattern for surface mount devices. The impression that I get is that it's highly dependent upon the particular fabrication process, the particular stencil thickness, and the particular assembly house process. Although I want to look to the manufacturer to provide a definitive SMD package layout for their parts, it doesn't seem to be 100% reliable.

For example, a thicker solder resist layer might interfere with placement of small parts, as the resist extends over the copper in some places. This is more of an issue with thicker Cu than with thinner. Also, a thick or thin stencil will affect the amount of solder paste, and thus the appropriate size for the stencil openings. I also imagine that different formulations of solder paste may require slightly different stencil design.

My point is that it seems a given library is only really 100% appropriate for a particular manufacturing chain, so the library that works for me might not work for someone else, particularly if a different fab house, stencil shop, and assembler are used.

This is probably less of an issue for pure through-hole designs, which I assume are less picky than surface mount assembly.

Brian