TechNetters, (and Brian), I suggest looking into statistical process control techniques for something like this. A standard Shewart chart would be the thing. In fact, I plan on doing it myself in the very near future, thanks to Brian's earlier suggestions on cleanliness testing techniques.
If you take 1 standard deviation as a "control limit" you'll have lots of false alarms. The purpose of SPC is not so much to tell you when to do something, but rather when not to do something, to leave things alone. A while back, a technetter said he'd made a chart, but found points always inside the limits. That's exactly the way things you want things to be. Go to work on something else and don't waste time tinkering with a well-behaved process. Unless it is producing bad stuff, in which case you need a new process.
Lou Hart
----------
From: Brian Ellis[SMTP:[log in to unmask]]
Sent: Saturday, March 25, 2000 8:12 AM
To: [log in to unmask]
Subject: Re: [TN] process control of final cleanliness in a no-clean world
Wade
I agree with the others but would add one minor point. Qualify your
process as "safe" according to your needs. Measure the ionic
contamination with whatever instrument you will on, say, 20 boards from
a similar batch, keeping the test going to near-asymptote (say less that
1% conductivity change in one minute). Note these final values and
calculate the mean and standard deviation (ideally, the distribution
should be a neat bell-curve). For that type of board, run regular tests
in production: if they fall significantly outside the mean +/- SD
(either higher or lower), then your process is probably going off the
rails. Note that a lower reading of contamination may be just as
dangerous as a higher one, because it signifies a change in the process
parameters.
Brian
"Oberle, Wade" wrote:
>
> Dear technetters,
> How do all of you in the no-clean world monitor and control board
> cleanliness. In the days of OA flux, we used an Ionograph or Omegameter to
> monitor our 'cleaning' process. Do some of you still use an ionic
> contamination tester or do you use SIR testing or nothing or what?
>
> Thanks in advance for your advice.
>
> Wade Oberle
>
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