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January 1999

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From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 29 Jan 1999 12:36:05 EST
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In a message dated 1/29/99 6:10:33 AM Pacific Standard Time,
[log in to unmask] writes:

<< Paul Klasek raises some interesting points. If you can't change stencils,
 then it may be possible to use a softer squeegee (less than 90durometer)
 with a slightly higher print-pressure setting, to 'scoop out' the amount of
 paste you need. However, if you then try and slowly introduce
 reduced-aperture stencils, you'll have to mix and match print-processes
 across the production floor.

 I think there may be an opportunity for the solder paste suppliers out
 there to come up with exactly what you are talking about: a high solids
 content flux, with a reduced metal loading (i.e. viscosity/rheology stays
 pretty much the same). The problem with this is that hot-slump becomes much
 worse, so it may not actually buy you much reduction in the solderballing -
 I'm prepared to be convinced otherwise. May be a niche market for some of
 those 'polymer pastes' out there?

 Alternatively, inert reflow, giving improved solderability to all metal
 surfaces, may be another way of reducing 'squeezeballs/capillary
 balls/sideballs'. I'd be interested if anyone has any experience with
 this...

 Andy >>

Good Morning Ya'll!

     Say, this is getting interesting! Let me elaborate a little more about
things during that process change...and no Paul, I won't stick by my guns and
say buying new stencils was the ONLY way I could eliminate squeeze balls,
solder beads. But it was the quickest way I could be sure that I wouldn't be
having a problem with them. I needed to decide on something pretty quickly,
'cause during the busy months we needed to build anywhere from 800,000 to a
1,000,000 modules a month come hell or high water.

     You see, back when I joined that company, the date had already been set
when the company was going to go no-clean. In fact, that was something that
was discussed at my interview as being part of my responsibility and why I was
being hired, to take the process from water soluble to no-clean. I told them;
"No problem! I'm looking forward to it!".

     After the interview while I was walking back to my car, I heard this
little voice in my ear, kinda' sounded like Ren (you Ren and Stimpy fans will
appreciate this..hehehe);  "You EEDIOT! WHAT WERE YOU SAYING IN THERE?"

     I'm not trying to malign anybody, but we're talking about a memory
company here. You know; "Hurry-up and build the darn things so we can ship em'
before the prices drop again!" was the battle-cry we heard constantly. So I
had quite a task ahead of me. Not making any excuses, but I didn't have a heck
of a lot of time to experiment much.

     Although I did try what Andy mentioned, which was using plastic
squeegees. I experimented with durometers from 80 to 100, and I could get rid
of practically all of the balls, but not without also causing insufficient
problems with the other parts...stands to reason don't it? I had to use the
high squeegee force to scoop out the passive pads, so I'm gonna be scooping
out the other pads as well, maybe not to the same degree with the narrower
openings, but it doesn't help when most everything else is 50-mil pitch SOJ
footprints, and you've got caps that sit beneath them...they'll scoop out
pretty good too.

    Even thought about lowering the metals content too, but I was in a
catch-22 there. Lower metals content means more flux residues, and that was
one of the things that was a critera when selecting the paste we were going to
use. The paste needed to have fairly little residue, it could have some, but
we didn't want a lot. The residue also had to stay nice and clear through
double reflow as we did a lot of that, and it also had to come out of reflow
with the residue dry and hard, not tacky or sticky because of all the routing
after the panels are built. Another problem with the residues that came up
after we started building product with no-clean, was with the cache' modules
not passing test until after they were cleaned. They use Syncronous DRAM and
seemed to be real sensitive to the residues. It wasn't a HUGE problem, and
didn't happen with a lot of boards, it was just big enough to be noticed. But
I imagine if I had more residue, that might've increased the problem? I don't
know, I'm not much of a test guy...

     One last little curve that went into the equation deciding whether or not
the stencils needed to be replaced or not, was this in-house "custom"
footprint that was incorporated into many of the designs for the decoupling
caps by a layout guy a few years before I got there. What he had done was to
lay out pads that you could put either a 0805 or 1206 on. I imagine it was a
real keen idea back when you didn't know what package cap you could buy, but
with the way the pads were laid-out so a 0805 could reach both pads, a
squeeze-ball was guaranteed when you placed a 1206...I don't even remember how
many assemblies had that footprint, but it was a LOT!

      Believe you me, there was a lot of questioning going on when I told them
that we needed new stencils, and if I had any sorta clue what to do to keep
from buying them, I coulda been a hero. But as it is, I'm just Steve, poor ol'
guy workin' at a start-up...(GRIN)

-Steve Gregory-

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