gEDA-dev: PCB paste layer and exposed paddles

[David Carr]

Attached is a patch that I wrote that gives PCB the ability to generate 
non 100% paste filled areas on selected pads.  (screenshot of generated 
pads with 50% coverage also attached)

The patch specifies a minimum "solder paste island" size and then 
proceeds to calculate the number and size of the solder paste islands 
required achieve the desired coverage.  I have tested the code a bit, 
but there could of course be some hidden bugs in there.  If it matters, 
I use the GTK hid (lesstiff not tested).

This particular patch has a couple of flaws in that I hard coded the 
percent solder paste coverage and the minimum island size in the file, 
and also that I used/abused the "nopaste" flag to mark pads that want 
the island treatment.  I did the latter because I don't yet understand 
all of the nuances of adding a new flag(s) to the PCB file.

To use the patch:
Apply to draw.c
Build.
Open a PCB layout.
Select the pad to which you want apply reduced solder paste coverage.
Use the :ChangePaste(Selected) command to set the "nopaste" flag.
You can verify the above operation with the "Ctrl-R" report command on 
the pad--- look for the "no paste" flag.
Export the gerber files --- you should see the solder islands on the 
paste layer.

Cheers,
David Carr (celebrating 23 years today)


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.
>
> Hmmm,
> -DC
>
>
>
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