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November 1997

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Subject:
From:
Doug Pauls <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Mon, 17 Nov 1997 08:05:55 -0500
Content-Type:
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Ken,
I have read a number of other responses to your question and agree with most
of the responses, especially Graham

Anyone who desires to use an ionic contamination tester should first read an
EMPF report RR000013, titled "An In-Depth Look at Ionic Cleanliness Testing".
 This report covers all the major models available at that time (1993, I
think) and looked at them for repeatability and accuracy.  That report can be
obtained from EMPF at 317-655-3673.  They may also have a request form
available through their web page (www.empf.org).

That report generally found that the units had a severe lack of
repeatability, both from machine to machine, or even consecutive serial
numbers of the same model.  Another conclusion was that the equivalence
factors, used to compare results between the different kinds of testers was
largely fiction (my paraphrase).

These testers can have a place, that for which they were designed - process
monitoring.  They were not designed as analytical equipment.  They were not
designed with water soluble fluxes or low solids fluxes in mind.  They should
not be used to determine if a process is acceptable or unacceptable.

Graham Collins response regarding the blind use of these instruments by
people who never seek to understand what the measures and responses indicate,
was right on.  I could not agree more.  I have lost count of the number of my
customers who have problems with corrosion, metal migration and electrical
leakage, yet claim it could not possibly be ionic contamination because their
ionic cleanliness tester showed levels under the 10 microgram limit.

If  you desire to implement such a device into your manufacturing scheme,
then I would recommend that you do some studies to benchmark your response
using ion chromatography and your instrument.  Ion chromatography can give
you a better indication of acceptable or non-acceptable (because it is ion
specific) in a competant analysts hands.  This information can be used to
help set a realistic process control mean and limits about the mean.  Forget
the historical ionic cleanliness pass-fail limits, unless you are still doing
high solids RMA processes.  If you have an "8" on Monday, Tuesday, Wednesday,
but a "15" on Thursday, something changed and you better go find out what it
was.

While I see great abuses of these machines and would like to see them go away
(eventually), they are one of the few process control tools available to
assembly personnel that work in real-time.  But you need to understand what
they can and cannot do or you are wasting your time and signing on for
ulcers.  Of course, if you like ulcers.......

I agree with Graham Collins remarks about the periodicity of testing.  If you
have a process that is well under control, then SPC methodologies indicate
you can decrease the frequency of sampling.  I would not eliminate it totally
though.  To do so would mean that you assume the following:

the laminate never changes
the solder mask did not change lot to lot (the vendors would NEVER do that to
me)
the solder mask cure process is always perfect and gives me perfectly cured
the boards from my fabricators always come in squeeky clean
nothing else changes in my process

There are some huge pitfalls waiting for you if you make these assumptions.
 Changes in ionic cleanliness readings can be a way to catch problems that
occurred upstream in your process.

Hope this was helpful.

Doug Pauls
CSL

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