cleaning hi-z high-z high impedance areas - acetone additive

[klem]

searched and oddly no threads with this as a title, hence the redundant phrases.

i've read in other threads about cleaning the high z areas with various flux removers, just isopropyl, or even acetone. IME, in line with others' here, is that the flux removers leave significant sticky residue (in spite of cleaning with toothbrush and multiple rinse cleanings, etc.), and isopropyl doesn't seem to leave the residue, but also requires a lot more cleaning iterations and brushing to break down the flux. i'm now interested in trying acetone, but only want a small quantity of it, just a few oz, and most drug stores seem to have added (a drinking deterrent?) denatonium benzoate. no idea what amount is included in the Walgreen's "100% Acetone" bottles...

can anyone comment on whether or not DB is problematic in this application? plan is to do a cleaning with isoprop after the acetone eats away the flux and hopefully nothing else, then finally a conformal coating and be done with it. if any seasoned folks have better methods they'd like to share, i'm all ears.

 

[Khron]

If anything, might be worth breaking / scraping off the bigger chunks of flux, and just wipe away what's left with the isopropanol (instead of using that for the heavy lifting as well). Surely there's not that vast an area to work through (1-2 joints per mic?), right?

 

[kingkorg]

FWIW i stopped using toothbrush, as the material itself for some reason can sometimes start dissolving, especially with acetone. For the longest time i thought acetone makes the mess, turned out for me it was the toothbrush in combo with flux and acetone.

 

[klem]

If anything, might be worth breaking / scraping off the bigger chunks of flux, and just wipe away what's left with the isopropanol (instead of using that for the heavy lifting as well). Surely there's not that vast an area to work through (1-2 joints per mic?), right?

I do just that, as there's something mildly satisfying about mechanically breaking off the chips of flux! However, that's only effective for the easily accessible areas. So I'm trying to find ways to best get rid of the nook and cranny type flux parts, but also the more transparent stuff like any oils from fingers etc. I wish I had the tech to be able to visually see such contaminants so I wouldn't go overboard in cleaning, I really have no idea when it's as clean as possible for noise purposes, as I don't really know what the noise floor 'should' be (and really don't want to go back and forth with testing anyhow with half-assed cleaning measures, better to do it right the first time me thinks).

FWIW i stopped using toothbrush, as the material itself for some reason can sometimes start dissolving, especially with acetone. For the longest time i thought acetone makes the mess, turned out for me it was the toothbrush in combo with flux and acetone.

Good to know! Are there other cheap-ish tools anyone can recommend that are compatible with acetone (and the reactions it has with flux and isoprop)?

 

[jonnyeye]

Are you using something to absorb the residues? Isopropyl is great at dissolving flux but not good at taking it away when it evaporates, you need to wash or otherwise capture the residues. What I've been taught is to use a brush, preferably ESD-safe, to flood and wipe the area with isopropyl, then place a lint free wipe (Kim-wipes are standard, I've used dry lens tissue and I've heard that coffee filters work well too) over the area and brush to wick the residues into the wipe. Repeat as necessary. This should work with acetone as well, and may work even better, but I'd suggest you try it first with isoproypyl.

 

[klem]

Given the toothbrush non-compatibility, anyone care to share what tools they are using for detail cleaning work, with bristles that are compatible with acetone (and the reactions it has with flux and isoprop)?

I recently bought a hog-hair brush from Mouser, but it's wayyyy too big for these small areas.

 

[Khron]

Tip of a semi-sharp pair of tweezers to break off the flux, then a small wad of cotton wool drenched in isopropanol for cleaning up what's left. Arguably a bit labor intensive, but i'm not manufacturing mics, so there's that too...

 

[soliloqueen]

also requires a lot more cleaning iterations and brushing to break down the flux.

at what percentage? most drugstore isopropyl alcohol isn't strong enough to do this well

 

[klem]

@Khron
labor no big deal, just a hobby here. the only problem with cotton wool i can foresee is the same issue i have with q-tips / cotton ball on end of a stick - they inevitably get pulled apart by a solder joint or other snag, leaving strands behind. It always feels like 2 steps forward one back with that method. are you not having that problem? I realize it's somewhat contingent on how you leave the through-hole leads or cleaning with direction of cotton wound, trying to get better with my joints to have good mechanical connection but not be a PITA come cleaning...

@soliloqueen
99%. however, i've found 70% to have its use as well due to slower evaporation. (thankfully no longer living in a steam-room of a state.)

 

[waltzingbear]

Cheap bristle 1” paint brush and cut off all but about half inch.
I get mine from Ace Hardware, the disposable kind.
Works great with acetone

 

[k brown]

You can also get natural-bristle (animal hair) toothbrushes (usually at places like heath-food stores, or natural-foods markets), which are impervious to solvents.

 

[klem]

thanks @waltzingbear and @k brown! will pick some up soon and try them out.

 

[Tubetec]

Use a bamboo skewer to scrape encrusted residue , then get in there with a q-tip with iso to disolve the rest ,dry up and clean with a fresh q-tip ,
nail varnish remover is for girls .

 

[industrialarts]

I have a machinist scribe (2 actually) a tool that I got introduced to when I worked for Neotek about a million years ago. It is the single most useful item in my tool kit and it works great for scraping off flux. After getting rid of large pieces, I also use the alcohol and toothbrush method. In addition, I have a metal brush like those used by pumbers for really tough residue. I am liberal with my use of alcohol and if I want to reflow the solder mask (which makes it shiny again) I'll use a naptha based product.

 

[klem]

interesting, i bet a scribe could be great for floating joints or on turrets! don't suppose you have a name/model for the ones you like best?

 

[dai h.]

hi, some reading material I found:

(section from close to the middle of the page titled "Properly cleaning the board"--the 3 part set of articles is also pretty interesting)

https://www.edn.com/design-femtoamp...leakage-part-3-low-current-design-techniques/
((section) 3.0 Cleaning)

https://www.ti.com/lit/ug/snou004/snou004.pdf?ts=1683793465783
https://www.eevblog.com/forum/testg...rrent-femtompere-electrometer-keithley-617/5/

 

[klem]

expanding scope a bit - i have regularly used kester 44 rosin flux solder for most through-hole components, but when it comes to final parts like trimmers, polystyrenes, maybe hi-z area, it would be nice to have an alternative solder.
no clean or water soluble flux? i've used neither in my 20+ years inhaling the stuff. no clean seems a bit more ideal for not getting water near unsealed pots etc, but i can work around it. thanks

 

[Андрій Чубака]

I have good results with flux for bga soldering and isopropyl alcohol clearing after it. This flux is conductive bit and fatty but clean on soldering and very useful. After soldering just need clean it 2-3 times in with good amount isopropyl alcohol and brush, and after dry pcb.

This flux more easy for cleaning with isopropyl alcohol than rosin flux so final results is good also even if this flux bit conductive and very fatty.

Possible additionally use a cleaner for cylinder carburetors in order to blow out the remnants of flux from under individual parts, for example, if it is some chip which it is difficult to wash the flux completely, but in general it is quite possible to work with this flux in this case because it washes off well.

 

[papadajoke]

Chemist here. Isopropyl alcohol, or isopropanol (IPA) 90%, seems to work well with removing solder flux. I use it with a new microfiber. But, I also found that gently scrubbing the pcb joints using 90% IPA and a small toothbrush starts dissolving the larger flux deposits before swooping in with the microfiber to finish. Avoid solvents containing acetone, methyl ethyl ketone or ethyl acetate, as they can possibly dissolve some inks, plastics, paints, and lacquered surfaces.

 

[Tubetec]

Acetone or nail polish remover could damage the FRP board ,
Maybe the one time its use might be appropriate is to help soften a conformal coating on a PCB , not by dunking the entire thing in the solvent ,but by applying a tiny drop ,when and where its needed .
A small brass wire brush is usefull at times , mainly to clean the metal on component lead outs , Id definately avoid its use on the PCB as it might leave scratches or tiny traces of metal behind , same advice goes for cleaning the sparkplug in your car , never use a wire brush as the metal it leaves behind can damage the porcelain insulation .
An old toothbrush is very effective for getting into nooks and crannys but does require flooding the board and wasting a lot of alcohol .

I find a bamboo skewer perfect for scratching away deposits or blobs of flux , the wood is a little softer than the pcb material ,so does no damage to the undelying surface , I remove as much as I can by scraping first , I then use a qtip moistened with iso to swab the areas , I try to avoid spreading it around , so I wipe with the qtip , rotate it 90 degrees , wipe again , rotate etc .
Its very easy to flood the area with iso ,disolve all the flux and end up with a microthin layer covering the entire board when it all dries out again .