<<  We are investigating switching from RMA and SA based fluxes to WS fluxes.
 We notice much higher and inconsistent Ohmegameter readings with the WS
 fluxes. Is there a better means of measuring Ionic contaminants when using
 WS flux? What are others using to check how "clean" a WS processed assembly
 is after wash? >>

Charles,
I just can't seem to let an Omegameter question pass by without comment.  All
of the bulk ionic cleanliness testers rely on three factors.
(1)  the residue extracted must make the extract solution more conductive
(2)  the residue in question must be soluble in water or IPA.
(3)  the residue in question must have a solubility rate that allows all of
the residue to be extracted into solution during the time allotted to the
test.

If a material, such as a purely organic material, does not serve to make the
solution more conductive, the residue will not register on the conductivity
cell.  If the residue, such as an insoluble metal salt, does not dissolve in
water or IPA, then it cannot be measured by this test.  If the residue, such
as a baked on rosin flux, does not appreciably dissolve in the (typically) 10
minute test, you won't get an accurate result.

Many users see higher levels of contamination when using the newer generation
of heated / agitated solution testers.  The added heat and agitation of the
sprays increases the rate at which the residues are removed into solution.
 In reality, you probably always had that high a contamination level, but you
did not extract much of the residues during the 10 minute, room temperature
test.

I have always viewed the bulk ionic cleanliness as a gross test, to be viewed
with caution.  It is much like fishing.  You know you have a fish on the
line, you get a vague idea if it is a big fish or a little fish, but you have
no idea what kind of fish it is.  Is it a good fish (walleye) or a bad fish
(carp)?  What if it's a fish you've never seen before?  Is the fish mercury
polluted?  What is the long term effect of such a fish?

Excuse me, where was I?  

For most of our process evaluations, we use a more rigorous analytical method
called ion chromatography.  The extraction procedure is much longer - 60
minutes at 80C, 75/25 IPA/water vs. the room temperature 10 minute
extraction.  The longer time and the hotter temperatures greatly increase the
solubility of the residues.  In 1995, we took part in an ARPA study run by
Dr. Turbini at Georgia Tech.  Part of our work was to look at optimizing the
extraction procedure.  As part of that work, we did ion chromatography on
extractions at: 25C/10 minutes, 41C/10 minutes, and 80C/60 minutes.  The
25C/10 minute extractions showed very little removal.  The 41C/10 minutes
showed more, but still a relatively low amount.  It was not until you started
going longer than 20 minutes and hotter than 50C that you started to see real
residue removal.

The main reason that we like ion chromatography so much is that you can
precisely determine the ionic species present (what kind of fish it is).  You
can tell how much chloride, bromide, weak organic acids, sulfate, etc., you
have.  Over time, you get a fairly good handle on what residues are benign
(weak organic acids) and which residues are bad (halides).  

IC still cannot detect materials which are not electrically conductive, but
the extract solution can be used for other analytical techniques, such as
high pressure liquid chromatography or FT-IR.

I sometimes (jokingly) refer to ionic cleanliness testers as the artificial
reef materials of the future, but I do concede they have their uses.  Like
any piece of test equipment, you have to know the limitations of the system
(no, it can't do everything the salesman said) and what the response is
telling you.  In your situation Charles, you might ask why you are seeing
such wildly fluctuating results in the tester?  The higher levels can usually
be attributable to the greater solubility of these fluxes in your test
solution.  The rest of it can be due to a number of factors:  solubility of
residues in entrapment sites, the dynamics of the solution in entrapment
sites, perhaps demon possession of the conductivity cell (known to happen).
 What we have most often recommended is a correlation study between an ionic
cleanliness tester and ion chromatography.  Once you understand what the
response is telling you, then you can use the tester for their designed
purpose - as a process monitoring tool.

Hope this was helpful.  

Doug Pauls
CSL

Hmmmm, think I'll go fishing .......

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]>   *
* with <subject: subscribe/unsubscribe> and no text in the body.          *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************