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Subject:
From:
Lou Hart <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 1 Jan 1996 00:08:30 -0500
Content-Type:
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text/plain (265 lines)
John, FYI  from TechNet.  Deals with cleanliness testing when components are on board.  Lou

-----Original Message-----
From:   Brian Ellis [SMTP:[log in to unmask]]
Sent:   Tuesday, March 28, 2000 7:17 AM
To:     [log in to unmask]
Subject:        Re: [TN] process control of final cleanliness in a no-clean world

Ron

First thing: Gary and I, in my previous answer, are assuming that we are
talking about a single type of PCB. Secondly, we are talking about
"no-clean" assemblies where 95% of the ionics come from the flux itself
(typically) and thirdly, most of the flux residues will remain on the
board surface and most certainly will not climb onto the top surface of
all the components or even onto the sides.

Your argument is therefore a non-sequitur. If you have n ug/cm2 eq. NaCl
of ionics on the board itself and calculated to the board surface and
this is at what you consider the level of safety, you will be lulled
into a false sense of security by finding you have n/3 or n/4 ug/cm2 eq.
NaCl because your total area with components is 3 or 4 times greater
than without (as can happen). Your board will certainly fail just as
readily.

This business of component area is a total red herring (see my book for
a full discussion) and was introduced initially in MIL-P-28809A as a sop
to assemblers who found that with the specified RMA to MIL-F-14256
fluxes and CFC-113 azeotropes, it was impossible to meet the
requirements of the original 28809 which specified board area alone.
Similarly, in the UK, the initial project of DEF-SPEC 00/10-3 specified
board area only, but as a sop to complainers, they added that this may
be voluntarily weighted to an extra 10% max. to compensate for the
contaminants that do reach the components. About 20 years ago, I took
some cleaned assemblies, snipped off the components and measured the
board and the components separately per cm2 of the board alone area. I
forget the actual figures but I do remember that they averaged about 90%
of the contaminants were on the board and 10% on the components (I think
this was the rationale that the UK Ministry of Defence in Bromley, Kent,
used for their 10% weighting, as I was working very closely with them,
at that time).

So my advice has always been, forget the components totally because any
error in so doing will be fail-safe i.e. the resultant apparent
contamination will be slightly higher in doing so.

Brian

PS BTW, the original 28809A used the word "estimate" of the component
area. The Contaminometer had an automatic means of estimating it, if the
value was required, which was accurate to +/- 10% with ordinary-sized
components, somewhat more with very large ones. I think this feature
still exists in the Multicore version of these instruments, but, as I am
no longer involved with them, I cannot tell for sure.

"Dieselberg, Ron" wrote:
>
> 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
> > E-mail: [log in to unmask]
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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
E-mail: [log in to unmask]
URL: Technical and consultancy divisions:
       http://www.protonique.com
     Web services division:
       http://www.protonique.com/webserv

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