gEDA-dev: PCB paste layer and exposed paddles

[Dan McMahill]

David Carr wrote:
> I have the distinct (dis)pleasure of making some PCBs with QFN/MLF packages
> that have exposed copper paddles.  I think I'll need to use stencils to
> properly apply paste for these packages.  It appears that PCB simply uses
> SMT pads (slightly shrunk) as the apertures for the paste layer.  I think
> this is a simple effective design for most packages --- except exposed
> ground paddles.  These require that the paste be broken into smaller
> "blocks" with say 75% coverage of the paddle.  (See Analog AN-772)
> 
> Has anyone on the list dealt with this issue in the past?  If not, I wonder
> if there is any way to distinguish paddle pads from regular ones so that I
> could modify the paste layer code to do the right thing.  I guess we could
> do something like look for square pads with a size > Xmm on a side.  That
> seems a big fragile though.

That might not be too bad as long as we have a way to specify what X is 
and something about how a large pad should be broken up.  With the 
disclaimer that I'm far from being an expert in the assembly process, it 
would seem to me that you'd probably want to break up essentially all 
larger pads.

Does anyone know of particular recommendations from IPC for example 
about this?  If there aren't any published recommendations, does anyone 
have contacts at some board assembly houses who could comment on this issue?

-Dan