Re: gEDA-user: Open Source mechanical CAD on the horizon

[Dave N6NZ <n6nz@xxxxxxxx>]

On Mar 10, 2010, at 4:24 AM, Peter Clifton wrote:

> 
> Questions though..
> 
> What to do with a manually defined paste layer if the user fiddles with
> the size of the copper pad / solder mask? (Assuming that eventually
> becomes more flexible to edit).
? I don't think I understand the question.  Are you asking what to do if the user is editing pads and masks inside of a placed footprint inside PCB?  I think the answer is, if the user wants to change the solder mask, he fiddles that, too.

> 
> Solder mask aperture is important as well as pad size, since the stencil
> opening probably ought never include areas which are solder-masked. It
> is possible (although I'm not sure how useful) to set a partially masked
> pad - perhaps as some kind of heat-sink for a transistor, with a defined
> mask opening.

There is such a thing as a "mask defined pad" where the copper is larger than the mask aperture, and the mask aperture is what defines the effective size of the pad.  In that case, the stencil should be the smaller of pad or mask aperture.  I've never built a board that way, FWIW.

Heat sinks are an interesting case.  First off, the pad might be much larger than the device tab.  The whole thing will be unmasked. But it probably wants a pattern of a few dots of solder on it, not a giant puddle.

The problems I am trying to solve in my particular case are:

1. In some cases, heuristics have no chance what so ever of deriving the mask that I want from the pad layer.  So I want to specify it precisely in a footprint file and not have any tool do any underhanded automatic tweaks to it, ever.

2. My personal technology for homebrew stencils has limitations that require modifications to the current paste layer that are not easily programmed into an automated tool.  Again, I just want to say it once in a footprint file, and no tool should ever try to out guess that.

-dave




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