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Delay Before Cleaning Partial Assemblies
Board Talk
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TranscriptPhil Welcome to Board Talk, this is Phil Zarrow and Jim Hall, the Assembly Brothers, coming to you from high atop Mount Rialto. And what's today's dilemma? Jim This is a cleaning question from J.C. "We're assembling circuit boards with a water soluble flux, but have run out of some parts." J.C. is the first person in electronic assembly to ever run out of parts while assembling a board. "Is it acceptable to leave these partially assembled boards for 3 to 4 days, then put on the rest of the parts and then finally wash them?" Phil In a word, no. You have to think in terms of organically activated water soluble fluxes as basically acid fluxes. This stuff is very highly reactive. The benefit is of course you can clean it with water or with deionized water. But the idea is that they are water-soluble. The downside is you have to clean them, and the wisdom is clean them as soon as possible. Let me give you some anecdotal information. Years ago, there was a client we knew in the Boston area who had ENIG boards, and they were soldering with the OA flux and their inline cleaner bombed out. And so they were waiting for the repairman to come and the boards sat over the weekend. What they found Monday morning was that in some instances the OA flux residue had actually eaten through the gold. That's how nasty this stuff is. And another situation with another company where it was the same type of situation. They were assembling boards on Friday and the old quitting bell rang and there were a bunch of boards that hadn't been cleaned. Again, with water-soluble fluxes and in this case on HASL boards, and lo and behold, come Monday, there was dendritic growth. Where did that come from? I believe the bias voltage in this case came from some testing they had done. But it then the alarm went out, were the other boards sent to the customer clean or not. Scary stuff. The rule of thumb is clean as soon as possible and make sure everybody knows that they are being cleaned. Jim On a much less dramatic level, it's also easier to clean them the sooner you clean them, you leave OA fluxes, harden and dry out for another couple of days and it makes them that much more difficult to clean even if you didn't have any of the horrible effects that my brother has alluded to. Phil So the long-term solution here would be- you know, in this case of the parts shortages is if you're going to solder those boards with the OA flux, missing a few parts, clean them as soon as possible, and then when those other parts come in, you can -- depending on whether you're wave or hand-soldering, whether you're going to use an OA flux and clean them again, or you're going to use a no-clean flux, again depending on the application, everything else and not cleaned, so you know, be careful. But we feel your pain about missing components, welcome to the club we call electronic assembly. It's happened before, it will happen again. Hopefully you weren't waiting for an answer before you got our wonderful wisdom on this, but clean them as soon as possible. And on that note, whether using a OA flux or a no-clean or an RMA or even an RA flux, whatever you do, don't solder like my brother. Jim And don't solder like my brother. |
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