Special feature: Cleaning chemistry vs. mechanical impingement
Wack: It is from 1990, and a number of in- Wack: Agree. Work with an analytical lab fully removing residues. If you speak to a
dustry participants are currently reviewing that can offer additional tests to verify. chemistry supplier, they will argue that you
and updating it in collaboration with the Then I would also suggest climatic stor- should choose the chemistry first, then the
IPC. But it contains some of the concerns age to simulate the long-term climatic machine you should use it in. Can each
I have mentioned above. exposure. of the panelists give their view on how to
develop a cleaning methodology?
Konrad: In my opinion, cleanliness Stach: Averages can be deceiving. New
specifications are relative. For example, fluxes require new solvent, chemistries. Konrad: Chemicals play a vital role in re-
many assemblies fail in the field due to Why not cleanliness tests for new fluxes? moving flux. Getting the chemical solution
electro-migration issues even though they to the fluxed areas requires mechanical
meet military standards of cleanliness. On
Konrad: I don’t understand your question
impingement forces. The rinse process is
the other hand, relatively dirty boards may
(cleanliness test for new fluxes).
the most important process in defluxing.
not fail in the field. Failure predictors are While flux should not be left on a board,
the amount of corrosive residues left on
Stach: We continue to use IPA for cleanli-
wash chemical is worse if left on. The rinse
the board, and the board’s environment.
ness testing exclusively.
cycle cannot utilize the surface tension
The best cleanliness specification is ZERO
Galbraith: I think he means can the manu-
reduction capabilities of a wash chemi-
residues.
facturers not stipulate the types of testing
cal, so mechanical energy is required to
remove it from the board (including under
Wack: You could educate customers to
needed for each flux type?
components).
perform failure analysis tests post cleaning
Wack: I would think that would be hard.
and get a comfort level with the level of
They rely heavily on cleaning chemistry
Wack: We have completed numerous
quality.
experts.
recent studies on this topic with equip-
ment partners, investigating the effect
Stach: Zero residues, by what method—
Question from sherry_stepp: How do you
of surface tension, density and viscosity
visual?
address the flux residues you can’t see,
of different microphase cleaning agents,
as well as surfactant-based materials. We
Wack: Climatic storage is one of those
under low stand-off components?
have found that the cleaning speed of the
additional methods. Costly, but long-term
Wack: Sherry, good point. But also the
cleaning chemistry was very important. In
less costly. I had mentioned flux and rosin
fluxes you can see but are not dissolved,
other words how good can the residues be
test before. Those are statistical tests but
with IPA for example.
solubilized and how fast. This of course
help tremendously.
was also complemented by the mechanical
Konrad: ROSE testing is an immersion
Konrad: Ionic testing determines conduc-
support, meaning various spray designs
process that effectively get under compo-
tive residues (required for electro-migra-
did provide improvements. It is important
nents. No problem there. IPA is the test
tion).
to note however that we found other cases
solution of choice.
where neither additional pumps, pres-
Wack: Sure, Mike, I agree. But for instance
sures or increased number of nozzles, and
Wack: Yes, Mike, but you know as well as I
silver metal becomes silver salt after water
their respective orifice diameters, provided
do that IPA is a very poor solvent.
dissociates on the board. And then it
much of an improvement. In other cases
migrates. The reason water gets on the
Stach: Sherry, We need to dissolve them. If
we have found that the reflow profile for
board is the hygroscopic nature of the flux
they are ionic, they will lower resistivity, or
the respective paste changed the ease of
left behind.
we can remove every device and look!
cleanability significantly. We have seen
that cleaning under 1 mil is not an issue
Stach: I agree ionic residues can be corro-
Wack: Also the nonionic contaminants. in general. We have also seen that in some
sive. How do we deal with an average value
You have a lot of rosin in fluxes, not just cases bigger components are also challeng-
micrograms NaCl/in2?
ionic salts. Rosin and resin. ing. Best is to use a test board, or an actual
board that has a number of different sized
Wack: Well, what do we do when the value
Galbraith: Ok gentlemen, here is my
components present. From our current
is 0 but you can still visually see flux?
next question. Today board densities are
viewpoint, we believe that each board has
increasing, components are getting smaller,
to be specifically tested and evaluated to
Konrad: No tester exists that can test for
and the clearances beneath some of these
determine which parameter is most effec-
all future electro-migration causes. ROSE
components are 1 mm or less. If you
tive. Ask your cleaning service supplier for
tests for the usual suspects. Add visual and
speak to a machine vendor, they will claim
their closest technical center to perform
IC (if necessary).
that impingement force, nozzle design
such cleanability tests.
etc., contributes more towards success-
28 – Global SMT & Packaging – May 2009 www.globalsmt.net
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