Re: gEDA-user: Handling odd solder paste shapes

[Scott E <scott.e.post@xxxxxxxxx>]

Dave N6NZ wrote:
> 
>>
>> I've thought about this problem off-and-on but never sat down to code up 
>> a patch.  My latest thinking is to extend the footprint definition with 
>> an optional "paste" command along the lines of:
>>
>> paste("material", x1, y1, x2, y2, picoliters);
>>
>> Which says: Apply a stripe of <material> from x1, y1 to x2, y2, with 
>> total volume of <picoliters>.  This handles solder paste, mounting 
>> adhesive, potting epoxy, etc
> 

Personally, I would prefer simple polygons for paste and adhesive.  My
background (and the reason I opened the thread) is I work in the central
manufacturing engineering group for a large electronics supplier (we have
over 200 lines worldwide).  My group owns design guidelines and we define
the footprints for all new packages.  We always layout test boards when new
package outlines come along.  For example, when MLF/QFN packages showed up,
my group built a test board to figure out the best footprint for thermal
transfer, manufacturability, and reliability.

I wouldn't want paste volume defined in the design because in most cases
it's much more important to control the boundaries of the paste deposit. 
For example, with chip resistors I need a home plate shaped aperture to
control solder balling and the shape is the same whether I'm using a 4 or 8
mil thick stencil.  One of the few places where I care about volume is
pin-in-paste reflow.  In that case, a manufacturing engineer assigned to the
product will pick the stencil thickness up front during board design and the
paste pattern (which will be printed out onto soldermask) will be defined to
give the right volume without creating shorts or solder balls.  Again, it's
really important to control the shape so being about to have polygons for
the solder paste layer, and soldermask as part of the footprint is key.  The
same is true for soldermask - sometimes we have to do goofy things with
soldermask patterns for manufacturing reasons.

Same goes for chip mounting adhesive.  If I'm screen printing glue I want to
control the boundaries of the glue deposit and I'll choose a stencil
thickness to give me the height I want.  Volume isn't important for wave
solder side glue - height is.  If I'm dispensing glue then I only need to
know the type of component (0201, 0402, etc) and the centroid.  The software
that programs the glue machine will map the part type to what volume is
needed.

Regards
-- 
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