Hello there Russ,
A 4-mil stencil with .012 apertures sounds fine to me, that's what I've
used on the stencils I've made for the trade shows when we placed Tessera
µBGA's on the little keychains we built (I used to work for Zevatech).
Although I didn't have a step-down (I try not to use step-downs at all,
there's better ways to get additional solder volume for your 50-mil stuff with
a thin stencil than with a step-down).
Also, I didn't need a laser etched stencil either, the stencil I had made
was from a company that uses a different method of chemical etching that
doesn't give you the "hour-glass" cross-section of the aperture that a
standard double-sided chem-etch does, and why electro-polishing is sometimes
specified to smooth that out...the stencils work as good as a laser cut at
about 1/3 the cost. The rest of the keychain had a 128-pin, 15.7-mil TQFP, a
pair of MELF's, a couple of SOT-23's, 1206's, 0805's, 0603's, and 0402's. It
also was in a panel of 5-up. The fab was HASL'ed.
At the shows we used a no-clean (Alpha LR-78) the mesh was a type-4...we
also used the same paste placing flip-chip which had a much smaller opening
than did the µBGA stencil, and the stencil was only 2-mils thick...almost like
aluminum foil! We always used metal squeegee's.
There were times during the set-up for the shows that we did have some
problems like you're having, and it all came down to placement accuracy. We
would use local fiducials for this little puppy, it makes a difference.
Placement force is pretty crucial too, if ya' slam it down and try to push it
into the PCB, you wind up smearing all that good holding force off of your
board...you gotta be g-e-n-t-l-e...just lightly place the part so it barely
sets into the paste. One thing I wish we did have on the panels that would
help a little to get our placement down, is a silkscreen on the board that
would outline the µBGA's body when you were in the ball-park. As it was, we
would run over to the Nicholet booth (after they had their stuff set-up) and
get a panel x-rayed to make sure we were on money...did the same thing for the
flip-chip we placed.
We built thousands of these boards starting at SMI last year, and built
the same thing at this past NEPCON West without a hitch. It did take some
focus to get it set-up and running good in time for the exhibitions, but it
can be done...if we can do it in 3-days setting the machines on padded carpet,
Russ you can do it too!
-Steve Gregory-
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