Ed asks:
I have a client that is building extremely high impedance low voltage
assemblies (counting electrons from a photomultiplier type circuit) that
recently are failing test due to "leakage" They are running water soluble
flux with an SMT type aqueous cleaner fed by closed loop D.I. water
1Meg/cm3 or better. Dynamic Ion testing shows very good results. They ran
a culture and found very high bacteria counts, well beyond fed drinking
water standards.Subsequent cleaning at the end user site (conditions
unknown at this time) seems to resolve the failures. I suggested UV
sterilization (not too expensive/risky) and also to verify "under
soldermask" cleanliness as well as "localized" contamination sites which
"average" into the overall assembly.
My question is if anyone has had any electrical failures due to bacteria
in the rinse water and what solution(s) were successful. Any other
suggestions or suspicions would be appreciated.
Doug responds:
In a previous existance, the lab DI water unit became infected with
bacteria and algae and it was a royal pain in the tucus. These factors not
only caused the resins to stop performing their function (deionizing), but
they also affected the conductivity cell such that the problem was masked.
The unit never got up to 18 megohm-cm like it was supposed to and kept
cycling in the 10-15 range.
So, it is quite possible that the in-line conductivity cell in the closed
loop cleaning system has become similarly affected and you don't know it.
Bacteria and algae should not be able to survive in good DI water, so it is
a further indication that the system ain't working. We had to take our
system apart, clean everything with bleach, flush the hell out of it, throw
out the DI cartridges and start with new, and clean the conductivity cell
(which wasn't easy). After that, we put an in-line UV source and kept a
close eye on it (so to speak).
You don't say whether or not the aqueous cleaner used any detergent or
saponifier. As a close loop system, I suspect not. It is my experience
that the higher you go in impedance, the more sensitive to contamination
the assembly becomes, and for ultra-high impedance applications, DI water
cleaning alone will not do it (I don't care how cleanable the flux
manufacturer says the flux is). For this application, you might consider a
presoak of 5 minutes or so, 140F of a suitable saponifier material (I can
give a few suggestions off-line), to aid in cleaning the residues. I would
bet that if you check the details of the secondary cleaning (end user),
there is a chemical aid involved.
Doug Pauls
Rockwell Collins
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