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[daywalkerdha]

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[Ian.M]

Try presoaking in a covered tray of IPA to soften the flux residue, and reducing the time in, and temperature and concentration of the solution in, the ultrasonic cleaner.  If IPA doesn't work, try proprietary solvent flux cleaners.

If that doesn't work, you may need to change fluxes as so-called 'No-Clean' fluxes are notoriously difficult to remove when you actually do need to clean them!  If you aren't doing volume production, a ROL0 flux used sparingly usually gives similar acceptable residue if fully heated to its activation temperature and *NOT* cleaned, but will be much easier to clean when you need a pristine board.

[Ian.M]

You'd have to change your solder as well to avoid an unholy mix of resin and water washable flux residue which could well be more difficult to remove and could easily leave corrosive trace residue.

Water washable fluxes generally aren't worth it for hobby, service trade and low volume production uses, because of their highly corrosive nature, the risk of future corrosion damage due to trace residue under larger ICs etc. is significant.  OTOH volume production can afford to sample boards and strip chips to check for residue to 'dial in' their wash-line for a particular design, paid for by the cost savings due to VOC reduction.

Also you may not legally be able to dispose of wash water contaminated with a H400 hazard rated substance to the sewerage system . . .

[geggi1]

You can use isopropanol in the ultrasonic cleaner.
I used to work in a subsea workshop many years ago we had glass jars with different liquids like isopropanol, WD40 and thinners that we would immerse the parts in before the jar went in to the ultrasoning.
As far as i know its also possible to use metal containers for the cleaning.

[jmelson]

I've seen that there is a water soluble version of the flux i use, might that be worth a shot? I would also much prefer to just use distilled water without any additional chemicals. Easier to dispose of after etc..

YES!!!  I have started using WS flux on anything that requires cleaning.  In general, just holding under a water faucet for a few seconds leads to a
totally clean board, but I brush for a few seconds with a toothbrush just to be sure.  WAYYY better than struggling with sticky flux that leaves crusty white residue no matter how much you scrub with solvent.

Jon

[IconicPCB]

Try turpentine.

[S. Petrukhin]

Isn't the ultrasound power too high? Maybe it's like destroying a piece of solder that's smeared all over the board?

[IanJ]

That white residue looks like the solution/ultrasonic cleaner quite simply isn't doing it's job.  I have been there..........

I have tried quite a few different cleaning solutions over the years in my ultrasonic cleaner.
The best yet is ELECTROLUBE SWAS05L Cleaner, Safewash Super, PCBs, Electrical, Electronic Components, Tub, 5 L.
I get no residue at all using various flux types.
No need to mix in IPA (I would advise against that anyways).
Run it at 50degC, and for around 4 minutes.

SWAS05L should have a blue tinge to it when first used, but if you leave it in the ultrasonic tank it may get greener over time, probably a type of algae growing, but you can arrest this by decanting the solution into a sealed container after every use.....lasts much longer.

Another thing you should do is operate the ultrasonic cleaner for 10 minutes at heat with no Pcb in it in order to degas the solution which helps the decavitation work better. It's the decavitation which actually removes the flux and residue from the Pcb's and if there are air bubbles etc then this inhibits decavitation.
Some ultrasonic cleaners have a degas mode.

Use the basket supplied with your ultrasonic cleaner, keep the Pcb's off the sides/bottom of the tank. Too close to the bottom inhibits decavitation.

Make sure you fill your tank to the recommended level. Not enough in the tank inhibits decavitation.

Ian.

[eb4fbz]

I've been there. After cleaning SMT+TH boards in ultrasound IPA bath to remove hand soldered TH connectors residues, those white marks appeared all over the board. Over time, some corrosion was evident on connectors and leads stained with this residue.

I was told IPA was not appropiate to wash a board with no-clean flux residues from the SMT assembly. That white residue being corrosive salt leftovers.

Never use no-clean flux if you pretend to wash your boards.

[Kean]

Here is an old-ish article that talks about similar white residue https://www.edn.com/flux-residues-can-cause-corrosion-on-pcb-assemblies/

I started having this problem some years ago with PCBs coming back from a new assembly house a client used.  Apart from an aesthetic issue, the residue was apparently sufficiently conductive enough (or hygroscopic) to cause problems in some of our high impedance designs. 

The white residue can't easily be removed by IPA, as it is the wrong solvent.  Water washing may also not work by itself depending on the type/mix of fluxes used.  I found IPA could work with extensive scrubbing, but that is hard if not impossible with lots of small passives and/or fine pitch devices.  Something like Ultrasolve (Electrolube ULS) worked better than IPA, but was still not great and incompatible with some parts.

After another recommendation here back in 2015 I started using Safewash Super (Electrolube SWAS) as mentionend by IanJ.  It may seem expensive compared to other cleaners, but I found it works really well for me and is well worth it.

I filled up an small ultrasonic cleaner with it (don't recall if I diluted it), and usually run it for 180-300 seconds depending on how many PCBs I load up.  SWAS is a saponifier so the solution and PCBs will feel very soapy, and you need to rinse the PCBs well after cleaning.  I initially use tap water and then rinse again in deionised water.  I then dry the PCBs in a low temp oven.  I don't have particularly large production runs, and generally only put my smaller PCBs through this cleaning process.

Interesting what IanJ mentioned... I have had the same batch of SWAS sitting in a dedicated ultrasonic cleaner for several years, and although it has turned a deep green colour it still appears to work fine.  I should really start a new batch considering it comes in a 5L container and it has a limited (4 year?) shelf life.  I avoid using heat with it as I recall the data sheet says it decomposes at lowish temperatures, I think 45C max is recommended.  Also it is a skin irritant, so best to wear gloves.  Read the TDS & MSDS!

Note that some components (particularly electromechanical) should not be immersed in liquid, or may be damaged by ultrasonic exposure.  This may not be evident immediately, but lead to premature failure.  Check the component manufacturer data sheets & published advice.

[IanJ]

Hi,

I have dropped Electrolube an email asking about the green colour in SWAS.....be nice to know!

The SWAS datasheet says: "Stage  1 -Cleaning: Typically 3-5  minutes  with agitation.  Safewash  is  designed  to  be  used  at  ambient temperature, though if required this can be increased up to 45°C."................albeit I seem to remember discussing or reading the problem of decomposition somewhere before.......perhaps that was when above 45degC.....I can't remember.

Ian.

For ref:-
https://res.cloudinary.com/electrolube/images/v1603136061/SWA_y5u1pb_7078fa35c/SWA_y5u1pb_7078fa35c.pdf
https://uk.beta-layout.com/download/rk/RK-10079_41.pdf

[Kean]

The MSDS mentions decomposition, but no indication of temperatures involved in section 9.

Thermal decomposition or combustion products may include the following substances: Harmful gases or vapours.

That is in the section 5.2 "Firefighting measures" though, so probably not relevant to normal ultrasonic cleaner temps 

Section 10.6 "Stability" states

Does not decompose when used and stored as recommended.

Section 7.2 "Storage" states

Do not expose to temperatures exceeding 50°C/122°F.

So I'd suggest avoiding using heating mode in the ultrasonic cleaner, even for degassing.  I've never used it and get good results.