The two biggies are:
Electrical leakage of a very noisy sort was one of the worst problems.
I had some boards that were intended to be potted. The flux messed with the potting process so they had to clean the PCBs with very nasty stuff to make the potting work right.
There is also an inspection problem. The goo hides things that I'd like to have inspected.
-- -- kensmith@rahul.net forging knowledge
Scrub the worst of the flux off under running tap water with a soft brush. Soak for several hours in hot TAP ***not deionized*** water. Rince with a little deionized water. Soak for several hours in hot, fresh deionized water. Dry in a warm oven overnight.
This procedure was written by a chemist and it has worked brilliantly on very sensitive boards where every pA counts. However, it assumes that the board has no parts that will be bothered by either the heat or the water.
Let me say that most replies to this post were full of spit....
I have seen MORE failures do to difficult cleaning of water-soluable flux in SMT than in No-Clean processes. Some of you sound like you gave up on no-clean 15 years ago. Well, your knowledge is outdated. How do I know? I have seen contamination tests on modern formulations and they are consistently BETTER than the water wash.
The trouble with no-clean comes when people muss with it and don't understand it. DON'T clean it improperly; it will fail. Don't conformal coat it with just anything, use something water-based. Used properly, it works WELL.
John, for your issue, get some isopropyl alcohol as stated in one post. Use it in an ultrasonic cleaner with your boards in it. Some toluene in it, to, will help if needed. This will clean the boards off and get them working (assuming the assembly fits in the available ultrasonic cleaner).
Keep us posted!
I spent a couple of years working on a conductivity meter in Venlo. The tap water in Venlo and Nijmegen has a conductivity of about 300uS/cm. I don't have any reason to believe that North American tap water is much different.
For the record, our de-ionised water measured less than 1uS/cm when first dispensed, but crept up to a couple of uS/cm when left open to the air for any length of time, which I believe to be caused by CO2 from the air dissovling in the water and forming carbonic acid (H2CO3).
Tap water os conductive by virtue of dissolved carbonates and sulphates, mostly calcium and magnesium - the stuff that water softeners convert into chlorides.
-- Bill Sloman, Nijmegen
I use a Hanna 98308 PWT daily to monitor the quality of distilled water and can confirm your observations on CO2. Most dw I get measures 0.6uS to 0.8uS when fresh, and it increases slightly as it absorbs CO2. It may go as high as 1.2uS.
I recently switched to another brand of dw that measures 1.2uS and seems to stay at that value regardless of how long it has been opened. That may be the limit of CO2 absorption in dw.
The tap water here in Midland is off scale on the Hanna, so it is over 199uS/cm. It is very hard water and leaves calcium deposits on containers. It would be unusable for cleaning high-impedance pcb's.
Regards,
Mike Monett
"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...
Water soluble is not nice stuff - it is conductive, will tarnish metal, make solder joints brown if left on for one to several days, and it can be hard to clean. On the other hand, it is easy to thermo profile as it has a wide process window, is very active, which means it will wet oxidized parts better then most other fluxes. It is also easy and cheap to clean, if you can get to the flux under the parts.
To prevent any problems like you describe, we only consider using water soluble for thru - hole and mixed boards in the wave, since the parts are big and chunky (mixed boards would have RMA (to be solvent washed later) or No Clean on the SMT preventing the entrapped areas from getting water soluble in them), and wash off easily, and some though hole parts have lacquer coatings (mainly resistors!) which will wash off with all but the weakest solvents.
To get the water soluble off, we do a pre wash by soaking the boards with tap water for several minutes, change the water (the tap water soaking is only to help make our expensive carbon and DI water filters last much longer), soak in a water soluble neutralizer for a few minutes, and then wash in a *closed loop* DI water washing machine, which is basically a modified stainless steel dish washer. Putting the boards in the washer until the wash sump resistivity reaches over 2.0 meg ohm (compensated for standard temp, and is the standard 1 cm cubed measurement), which is mil spec for ionic contaminants generally ensures the boards are clean. For problem or high impedance boards, we may set the washer to run until 2.5 meg ohm to make sure all the hard to clean areas are clean. The wash temp is around 140 deg F and the water entering the sump is generally 16.x meg ohm. Perfectly clean DI water at standard temp (~20 deg C) is 18.4 meg ohm. DI water very quickly wants to remineralize it's self and will grab CO2, trace gases, parts of the container, etc and quickly become under a few meg ohms within minutes depending on surface area and temp. Over-washing boards with 2+ meg ohm DI for an hour or two will dissolve some of the metals out of the solder and leave an interesting spider web like pattern when viewed under a microscope, so over washing is likely not a good thing, and shows how aggressive DI water is. Putting 1 clean board in the wash, with a single touched up lead with a small dab of water soluble flux will drop the sump resistivity to 0.0xx to 0.1xx meg ohms!
Washing with something that has decent agitation, can monitor the water resistivity, and keeping it cleaning until you at least meet the mil spec of
2.00 meg ohm should get the boards clean, as DI really wants it's ions back. Using hot water should reduce the surface tension and increase the aggressiveness enough to clean under tight SMT parts. You may even be able to get away with washing the boards in an ordinary kitchen dishwasher (keeping in mind ESD, as most kitchen dishwashers are not in a ESD protected area)!Also don't leave the boards wet for a long time - the solder will corrode, and any steel / iron parts, leads and wire may rust. Bake at ~ 90 deg C for at least 1/2 an hour to dry and remove traces of moisture. Note that some solder masks which may not be fully cured may absorb water and turn murky looking - heat generally drives off this moisture returning the mask to it's original appearance.
If this was working when your boards went through, it should have a DI water final rinse, however, some companies just use tap water as DI is much more expensive. You may want to ask them about the resistivity of the water in the final rinse zone.
Large, tight to the board, dense leaded QFP's seem to be the hardest parts to clean, but they do come clean.
As long as it's the wash or pre wash water, that could be OK. If it's the rinse or especially final rinse water, then that's a BIG problem.
Try a hot water wash in a normal dishwasher as mentioned above.
In article , wrote: [...]
It is what stays behind on the PCB after washing that really matters. A little CO2 in the water won't matter much.
-- -- kensmith@rahul.net forging knowledge
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