The "appropriate time" needs to be established pretty carefully. The
original ionics testing was designed to test the post-solvent cleaning ionic
residues remaining from RMA fluxes prior to conformal coating. You may find this in
the Naval Avionics Center report MRR-3-78. This document is the one that
contains the "equivalence factors" that still seem to give folks problems. Having
been there for all the meetings where the factors were developed, I'll be
glad to answer questions.
Key here is that the test method was developed using a defined residue from
a defined flux type remaining after a defined cleaning method using a defined
test solution (75/25 v/v IPA/deionized water) tied to a defined pass/fail
limit prior to the next process step (conformal coating).
All these parameters were not in place for all the other combinations of
fluxes, pastes, cleaning agents etc. The other point that needs consideration is
the release rate of the flux residue/ionics from the surface of the PWA under
test. NT developed an ionic tester to monitor their new water soluble flux
soldering process, but found that instead of having about 90% of the residue
off the surface and into the test solution for measurement within 10-15 min.,
it took almost 2 hrs. to reach 90% release. Thus stopping the test after
10-15 min. would lead the operator to think the PWAs were clean, when in fact the
residue was just beginning to release from the surface. NT published this
information at an IPC meeting, so it was public info. (BTW, the ionic pass/fail
limit developed empirically by the military was cross-checked & confirmed in
a laboratory study by Bell Labs.
I would suggest SIR measurements on test boards, running PWAs under
simulated accelerated field use testing, plus measuring ionics on 'identical' PWAs
from the same lot. Then you will be able to see just what the appropriate test
time is, regardless of release rate, to use ionics as a manufacturing process
monitoring test.
Also- you may need to run a very large number of test boards to see any
statistical differences if you are using a mild flux or paste. In a previous life
I ran a study picking PWAs off an in-line RMA flux/soldering process,
cleaning them in 4 different cleaning agents prior to ionic measurement- and
finding no statistical differences in the cleaning agents. One has to go to the old
RA type fluxes that contain enough ionic activator to provide a sufficient
challenge to statistically show a difference between various cleaning agents/
processes. I would think the same would hold if you are not cleaning. The
flux is milder than the old RMAs, designed to 'burn off' or be converted to an
inert residue. How much is that determined by air or nitrogen flow, heat
profile, etc. etc.? What about higher temps for lead-free? How do all these
variables impact release rate? A porous solder mask that retains large quantities
of flux or paste can blind side the operator doing the measurement as well. As
Doug would say, this could be a wonderland of "it depends..."
(If there is sufficient interest, I'll re-post the expanded write-up on
equivalence factors genesis from SMT. Pls let me know at email given below to
minimize requests to TN)
Bill Kenyon
Global Centre Consulting
3336 Birmingham Drive
Fort Collins, CO 80526
Tel: 970.207.9586 Cell: 970.980.6373
email: [log in to unmask]
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