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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