This is pretty much a how long is a piece of string question, the answer will
be different from place to place according to the complexity of the assemblies
being soldered, inspection criteria and so on.
Theoretically  and ideally to determine the actual number for your setup you
should record bath analysis against fault rate and time/number of boards
processed. At some point you will get a fault rate that is unacceptable and
then you can see what the analysis is that gives you the problem. Then in
future you could take 90% the anaylsis to give a margin, and relate that to
time/boards passed so you can forecast pretty well when corrective action will
next have to be taken.
You could also plug in the other process variables into your graph and have a
nice statistical process control setup.
Alternatively most of the solder suppliers publish guidelines and you could
use those as your end points.
If you do go down this route I suggest you might want to take samples more
frequently than you analyse. That way you can go backwards in time by
analysing samples retrospectively if you get  problems. I seem to think either
the UK SMART Group or EPS has made up a little kit on this including a
spreadsheet on disk which plots out graphs and things. If you are interested
let me know and I can forward contact details.

Mike Fenner
...................


> [log in to unmask] writes:
>
>  << Fellow Technetters:
>        In your experiences, at what point will one start to see differences
> in
>   solubility in wave soldering.  I monitor our solder composition and am
>  starting
>   to see my Tin deplete as time goes by.  I'm not seeing any immediate
> defects,
>   but am wondering what is the alarm point.  None of the other contaminates
> are
>   out of spec.  I just notice my Tin composition depleting and the balance
is
>  the
>   Pb.  Any thoughts?   I can give technical expectations from looking at the
>   Eutectic, but am looking for the experience point of view.
>
>   Jason Smith
>   Lexmark Electronics
>   Process Materials Engineer >>
>

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