Gary, Assuming that the Ionograph operates on much the same principle as the
Omegameter, I offer this possible answer to your inconsistencies: ARE YOU
SURE THAT YOU HAVE CALCULATED THE "TOTAL" SURFACE AREA CORRECTLY? "TOTAL
SURFACE AREA" MUST INCLUDE NOT ONLY THE SURFACE AREA OF THE BOARD, BUT THE
"TOTAL" SURFACE AREA OF EVERY PART ON THE ASSEMBLY. I RAN TESTS USING THE
SIX BOARD SURFACES AND A GUESSTIMATE OF THE PART SURFACES AND GOT
INCONSISTENT ANSWERS TOO. WHEN THE "TOTAL" SURFACE AREAS OF THE PARTS WAS
REALLY MEASURED AND CALCULATED THE RESULTS BECAME MORE CONSISTENT. It had me
going for a while too.
Hope this thought helps.
Ron Dieselberg
BAE SYSTEMS
Cincinnati Electronics Corp.
-----Original Message-----
From: Gary Camac [mailto:[log in to unmask]]
Sent: Monday, March 27, 2000 16:49
To: [log in to unmask]
Subject: Re: [TN] process control of final cleanliness in a no-clean
world
Good afternoon everyone,
A few years back I used Ionograph testing to try and prove statistical
control of my in-line washing process. I was always within the spec limit,
but I could never show a normal distribution. The data was collected from
tests on all varieties of assemblies: single and double sided through hole,
and single and double sided mixed technology.
Then it dawned on me that I was throwing apples and oranges in the same
crate. Since the topography of these assemblies are different, I concluded
that I needed charts for each assembly type. Hmm....quess again Sherlock.
Once again I could not get a normal distribution.
I could not come up with any assignable causes to explain the lack of a
normal distribution. Since I never had any test outside the spec limit, I
had a hard time justifying putting more of my time in the effort. As you
all know, you have to know when to fish and when to cut bait.
Lou Hart wrote:
> 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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> --
> Brian Ellis
> Protonique SA
> PO Box 78
> CH-1032 Romanel-sur-Lausanne, Switzerland
> Voice: +41 21-648 23 34 Fax: +41 21-648 24 11
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