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 --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------