In a message dated 8/18/98 10:20:27 AM Pacific Daylight Time, [log in to unmask]
writes:
<< Does any contract assembly house store leftover solder paste in airtight
jar
at room temperature for next day's use? If so, I have few questions.
(1) Does your paste manufacturer approved such process?
(2) Is the paste remain in good condition and not drying out next day?
Any comments are most welcomed. My assembly house has implemented this
process and I have a serious reservation.
Re,
Ken Patel >>
Hi Ken,
I suppose nobody likes to throw things away, but you need to be careful
about this, you can wind-up costing yourself more in rework than what you're
trying to save in solder paste.
First question I would have is how come they wind-up with so much solder paste
at the end of the day that they have to put it in a jar in the refridgerator?
I can see excess paste being generated if for example, you're building just a
couple of prototypes, because you need to put enough paste on the stencil so
it'll roll for you when you're doing your print stroke, and that's much more
paste than what's actually deposited on the PCB. But what I would do with the
leftover paste from my prototype board, is to go put it on another stencil for
a board that is currently running...your contractor builds more than one
assembly at a time don't they?
Another way to generate excess paste is to put too much down on the stencil in
the first place. You should have a roll of paste in front of the squeegee when
doing your print stroke about as big around as your thumb. I've found that
sometimes operators will dump practically a whole jar on the stencil with the
mistaken thought that by putting more solder paste on the stencil reduces the
number of times that you need to put the paste back in front of the squeegee.
You see, while printing, the paste "snowplows", traveling to the outside edges
of the squeegee, and forms a line of paste parallel with the travel of the
squeegee...like what a real snowplow does. Periodically while running
production, you need to move that paste back in front of the squeegee. (there
are little "wings" you can attach to your squeegee to reduce that, but not
everybody has those). Some operators think that if they put more paste on the
stencil, it cuts down on the number of times they need to move the paste back
in front of the squeegee...but it doesn't, you just wind-up with more paste
out there that you have to move back in front of the squeegee.
One other excess paste generator is using too long of a squeegee for the board
you're printing. The squeegee should be only around .5" - 1.0" wider than the
pattern you're trying to cover...a longer squeegee needs more paste to get the
"rolling" action of the paste you're looking for during the print stroke.
Also, a longer than normal squeegee could "oilcan" your stencil embossing the
shape of the board into the foil.
Now, after all that, I'm not saying that you can't put solder paste back into
the refridgerator and it still be good, because you can...you just need to
keep good control over it. Don't open the jar until the paste has come up to
room temp. Use that paste as quickly as possible after the initial opening of
the jar, solder paste is perishable. The solder paste vendors will all tell
you that it's not good to store and reuse paste, but then again they want to
sell you more paste.
The real focus should be how NOT to generate excess paste, not whether it's
okay to store and reuse it.
-Steve Gregory-
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