Howard- your summertime solder beads may be a result of paste slumping from
the higher temps and humidity. More likely though, its something related to
stencil design/placement. Check the archives for "home plate" and see what
you find. Four days is probably a bit too long. Hope it helps.
Bill C
-----Original Message-----
From: Howard Watson [mailto:[log in to unmask]]
Sent: Friday, February 16, 2001 5:17 PM
To: [log in to unmask]
Subject: Re: [TN] Solder paste reuse
Steve,
When you say that the "re-used" solder paste will not perform like it is
advertised to, do you basically mean that the solder defect rate will go up,
or reliability will go down? Specifically, what would be the down side of
re-using solder paste? Could the paste not be scraped off of one stencil
and placed on a stencil for the next build? It seems like it could,
especially since most stencil paste manufacturers are quoting >8 hour
stencil life.
We "re-use" solder paste when changing out the screens for UP TO 4 days,
and I'm not convinced that we are doing the right thing. We use a no-clean
paste and our environmental conditions inside the plant change considerably
from season to season, for example, the winter humidity/temperature is
around 15%RH/75 F, however, in summer it's around 65-75%RH/65-80 F. This is
the down side of living in the desert with evaporative cooling and forced
air heating.
I am relatively new at my company, but what I have seen so far is a
significant occurrence of mid-chip solder beads in the summer, and an almost
nonexistent occurrence in the winter. The solder bead problem occurs
whether the paste is from the "re-use" jar or fresh out of the tube. Other
than solder beads, the post-reflow defect rate is <500 PPM (suspect the main
contributors are part/design related). Since I'm a new kid in SMT, I
appreciate all of the excellent advice from everyone in TechNet! Thanks.
Howard Watson
SMT Mfg. Eng.
AMETEK/Dixson
"Stephen R. Gregory" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
02/15/01 08:22 PM
Please respond to "TechNet E-Mail Forum."; Please respond to SteveZeva
To: [log in to unmask]
cc:
Subject: Re: [TN] Solder paste reuse
In a message dated 2/15/2001 8:46:23 PM Central Standard Time,
[log in to unmask] writes:
<< Solder paste reuse: does anybody has this practice? Obviously, my solder
paste supplier does not recommend it.
Nicolas Ortiz
Process Engineering Group Leader
Conexant Systems >>
Hi Nicolas,
Your supplier is not recommending it to in order to sell you more paste, it
is a valid recommendation.
I'm not quite sure about what you mean by "reuse", but if you mean taking
paste off a stencil, putting it back in a jar after a run, and then keep
using it, that's a bad idea.
The paste will absorb moisture, the volatiles in the paste will evaporate,
basically changing everything about the formulation of the paste = not
performing like it's been advertised to do...I don't know of any
manufacturer
of solder pastes that would recommend to reuse solder paste after it's been
dispensed on a stencil...
I do however, understand your concern about the expense when throwing away
solder paste...it's not cheap, anywhere from $50.00-75.00 for a 700-gram
jar.
But you can recover some of the costs through recycling...Alpha/Fry Metals
will recycle solder paste and give you back the money that the tin/lead
market prices are at the time you turn it in...
Bottom-line, don't "reuse" your solder paste...it'll bite you...
-Steve Gregory-
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