Removing Flux from a PCB. What is the best way?

Thread Starter

dcbingaman

Joined Jun 30, 2021
498
I have recently finished soldering all my components to a PCB. I then used spray Isopropyl Alcohol (99.9% pure anhydrous) and sprayed the board multiple times and then used an acid brush to try to get all the flux off the board. But I have found no matter how much I do this there remains a thin film of flux on the board (you can tell the board is slightly sticky). Is there a different chemical I should be using or a different technique? Thanks in advance.
 

Papabravo

Joined Feb 24, 2006
17,300
We used to use MEK (Methyl Ethyl Ketone), but that is probaly one of the chemicals that is banned.
One of our assembly houses used a household dishwasher which seemed to work, but might not if you have unsealed components like electrolytic capacitors.
 

Thread Starter

dcbingaman

Joined Jun 30, 2021
498
We used to use MEK (Methyl Ethyl Ketone), but that is probaly one of the chemicals that is banned.
One of our assembly houses used a household dishwasher which seemed to work, but might not if you have unsealed components like electrolytic capacitors.
Thanks. There is no Electrolytic caps on this board. There is surface mount ceramic caps, surface mount resistors. Through hole diodes (sealed). Surface mount IC's and that is about it. As far as I can tell all the components on this board are sealed.
 

Thread Starter

dcbingaman

Joined Jun 30, 2021
498

Reloadron

Joined Jan 15, 2015
6,218
I have always used for the most part Isopropanol (99%) and an acid brush angle cut short and scrubbed. I have also used MEK but can't even recall last time I saw any. I have also used bronze and nylon brushes shaped like a toothbrush as used for cleaning gun parts.

Ron
 

Papabravo

Joined Feb 24, 2006
17,300
Thanks. There is no Electrolytic caps on this board. There is surface mount ceramic caps, surface mount resistors. Through hole diodes (sealed). Surface mount IC's and that is about it. As far as I can tell all the components on this board are sealed.
If you can afford to run the experiment, try the dishwasher on a single board. I think this might require water soluble flux, my memory is not firm on this.
 

SamR

Joined Mar 19, 2019
3,615
Rosin is not very detergent soluble, so it takes a solvent. Never tried MEK but 99% IPA and gun brushes and Q-tips with a bit of elbow grease does the job for me. Have to repeat a few times to get the gumminess to go away. Never tried it but gasoline or ethanol would do in a pinch but don't think methanol would be wise due to toxicity. The three main industrial solvents we used in wood rosin refining as a rosin solvent were Methyl Isobutyl Ketone (MIBK), Gasoline, and Methanol. None of which I personally would use for cleaning PCBs. For area cleaning of resin spills the area operators would hand spread some caustic soda crystals to digest the rosin and then it could be water washed down. Don't think that would be a good approach to cleaning PCBs either. In the analytical lab to clean glassware used with rosin it was first placed in a hot solvent wash and could then be placed in the lab dishwasher with Alconox detergent. So after doing a good hand cleaning on the board with solvent a further wash should be OK if the parts can take it. Or use rosin free flux?
 

nsaspook

Joined Aug 27, 2009
8,907
Soak the being cleaned side of the board with pure IPA, vigorously scrub with the brush. I don't wipe at first, I use a hot (below 100C) air gun on high flow to blow most of the fluid flux/IPA mixture off the board. Repeat wet/scrub and wipe with IPA saturated lint-free cloths until the sticky residue is gone.
 

Papabravo

Joined Feb 24, 2006
17,300
Rosin is not very detergent soluble, so it takes a solvent. Never tried MEK but 99% IPA and gun brushes and Q-tips with a bit of elbow grease does the job for me. Have to repeat a few times to get the gumminess to go away. Never tried it but gasoline or ethanol would do in a pinch but don't think methanol would be wise due to toxicity. The three main industrial solvents we used in wood rosin refining as a rosin solvent were Methyl Isobutyl Ketone (MIBK), Gasoline, and Methanol. None of which I personally would use for cleaning PCBs. For area cleaning of resin spills the area operators would hand spread some caustic soda crystals to digest the rosin and then it could be water washed down. Don't think that would be a good approach to cleaning PCBs either. In the analytical lab to clean glassware used with rosin it was first placed in a hot solvent wash and could then be placed in the lab dishwasher with Alconox detergent. So after doing a good hand cleaning on the board with solvent a further wash should be OK if the parts can take it. Or use rosin free flux?
We actually don't know what kind of flux is being used. We have mostly SMT components and a few thru hole parts. There are several different ways the board might have been assembled. You might suspect rosin if the board was hand assembled, but they hardly even make rosin core solder anymore since ROHS 2006.
 

SamR

Joined Mar 19, 2019
3,615
When he said the board was slightly sticky I assumed it was rosin flux. Which I still use in lead solders and on occasion to make a module look good, I clean off. Non rosin fluxes I can usually wipe up with a moist fingertip and paper towel.
 

nsaspook

Joined Aug 27, 2009
8,907
We actually don't know what kind of flux is being used. We have mostly SMT components and a few thru hole parts. There are several different ways the board might have been assembled. You might suspect rosin if the board was hand assembled, but they hardly even make rosin core solder anymore since ROHS 2006.
I would suspect that most hand assembled boards would use a traditional rosin core solder on some of the SMD parts and most of the connectors. Even if 'no clean' solder and hot air is used for fine-lead chips most everything would use a iron.
 

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dl324

Joined Mar 30, 2015
13,535
Is there a different chemical I should be using or a different technique?
Try dipping in a container of 99% isopropyl alcohol and scrubbing with a short bristle brush. I use a horsehair acid brush with the bristles cut short down to 1/8" or so. Sprayed or brushed on 99% isopropyl alcohol evaporates too fast, so you get the sticky film residue.

I haven't resorted to dipping yet. I've had pretty good results by applying 70% isopropyl alcohol with a brush and scrubbing to get most of the flux residue off and then finishing with 99%. I use a lint free wipe (Kimwipes) rolled up in some heat shrink to wipe off residue. The heat shrink gives me something to hang on to and I choose a diameter small enough to avoid snagging a lot on leads. If I want it to be cleaner, I use a commercial flux removal spray to remove any remaining residue.
 
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Papabravo

Joined Feb 24, 2006
17,300
I would suspect that most hand assembled boards would use a traditional rosin core solder on some of the SMD parts and most of the connectors. Even if 'no clean' solder and hot air is used for fine-lead chips most everything would use a iron.
The techs at the last place I worked, that assembled products, did hand assembly with solid (no core) lead-free solder. The clear flux was applied to the SMT component from a squeeze bottle, and then the solder was applied. No rosin, no mess. That was 2007-2008 and I though everyone did it that way when eutectic (Sn-Pb) solder became a rarity.
 

nsaspook

Joined Aug 27, 2009
8,907
The techs at the last place I worked, that assembled products, did hand assembly with solid (no core) lead-free solder. The clear flux was applied to the SMT component from a squeeze bottle, and then the solder was applied. No rosin, no mess. That was 2007-2008 and I though everyone did it that way when eutectic (Sn-Pb) solder became a rarity.
Old school here, I still use thin rosin-core solder and a sharp tip to hand solder SMD parts on DIY prototypes and systems.
IMG_20190414_075927985.jpgIMG_20190505_084103698_HDR.jpg
Eutectic (Sn-Pb) solder is not a rarity in the industrial repair and manufacturing field.
The electronic ROHS is mainly about manufacturing commercial products. The industrial products I build use lead electronic solder because I want them to last and leaded products are much easier for hand soldering joint quality. In the last year I had a long discussion with the local OHA about our engineering soldering practices during a general inspection. I told them in no uncertain terms we would continue to use lead-based solder for repairs and for building systems for internal industrial use.

Back to cleaning.

Really pour on the IPA, the board needs to be wet.
PXL_20211006_224804119.jpg

Use the brush on the wet solvent then remove the mixture before it dries too much.
PXL_20211006_224819979.jpg

Back side residue cleaned
PXL_20211006_223211636.jpg

PXL_20211006_223621842.jpg

Fully cleaned and tested boards.
PXL_20211006_225313315.jpgPXL_20211006_230550589.jpg
 
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Lo_volt

Joined Apr 3, 2014
233
91% isopropyl alcohol is strong enough to dissolve rosin core flux and cheap enough (most drug stores have it available) to flush the board after scrubbing. This should remove the film that is left after scrubbing.

Rosin core flux is the traditional solder flux that has been in use for decades. It is not water soluble nor can you clean it using water based cleaners.

Organic, or water-soluble, or activated, or acid core fluxes are water soluble and can be cleaned in a dishwasher. I don't recommend using your kitchen dishwasher but several board assemblers that I've seen have used them in a manufacturing environment. These fluxes MUST be thoroughly cleaned and rinsed as leftover flux can attack the copper on the board and open traces or destroy solder joints.

MEK is a pretty strong solvent. If you use it (I don't recommend for PC boards) be sure to ventilate properly or use it outside.
 

nsaspook

Joined Aug 27, 2009
8,907
Most of the problems I see with sticky rosin core flux residue is from a lack of good wetting with IPA to dilute the dissolved flux. A thin IPA film with just transfer the mess to the board substrate. Keep the surface very wet and dripping for the first IPA scrub application and while rinsing.
 

SamR

Joined Mar 19, 2019
3,615
Which is why I like Q-tips. Kinda like mopping up the board. Keep a small squeeze bottle of 99% IPA with 10-gauge tip handy in a carousel with my small tools within reach on the bench. Apply it to the swab and go to it mopping up the mess. Add IPA as needed.
 
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