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Offline exe

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Flux conductivity (NC-559-AS)
« on: October 12, 2017, 07:42:39 AM »
Hello my friends!

I just bumped into one youtube video claiming the flux I just bought (NC-559-AS from tme.eu, quite expensive!) is garbage because it's conductive (not post link here as it is in Russian).

I didn't believe my eyes, so I made my own experiment, and... Guess what, it is very conductive when just applied! Then I tried to heat it up, and became "superconductive" for a while showing some tens of KOhms on my test PCB. After cooling down it eventually went beyond measurement (which is 220MOhm on my ut-61e). But if I heat the board it starts conducting again. Clean it with IPA was hard, I don't think I was able to fully clean it with a toilet paper...

My question is: is this normal? I'd prefer a non-conductive flux if such exists...
 

Offline dl1640

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Re: Flux conductivity (NC-559-AS)
« Reply #1 on: October 15, 2017, 05:25:23 PM »
in my opinion it is a good action to clean the flux right after the soldering work.
if your flux is labelled water-washable it is possible to brush/wipe off the flux residue using warm water.

it does not mean your flux is rubbish.
 

Offline SeanB

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Re: Flux conductivity (NC-559-AS)
« Reply #2 on: October 15, 2017, 06:36:21 PM »
Flux generally comes with a carrier fluid, generally water and alcohol mix to make it a liquid. Then the water and alcohol evaporates when in preheat, and then eventually the flux itself melts and does it's job, which is being an ionic liquid that reacts with the oxides and dirt in the solder and board to remove them. Thus when applied you will find it is conductive, generally from the water dissolving some constituents, and when active it is again an ionic liquid. These will conduct electricity, and thus the measured resistance and drop when it is active.

No clean fluxes are always recommended to be used where the residual resistance after soldering is not an issue, if you have high impedances, or need low leakage currents, you must use either a water soluble or alcohol soluble flux and remove it after soldering, and then dry the board again. The second heat cycle showed the flux residue was melting again, and becoming an ionic liquid again.

An analogy is that glass is also an ionic liquid when hot, but when cold it is a pretty good insulator, used as the base material in printed circuit boards as glass fibre.
 

Offline blueskull

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Re: Flux conductivity (NC-559-AS)
« Reply #3 on: October 15, 2017, 07:22:18 PM »
No cleaning flux types usually don't need to be washed off unless leakage requirement is super stringent.
However, repetitive reworking may cause carbon deposition, which makes it conductive.
For water soluble types, cleaning is a must since it is conductive and it corrodes copper over time.
I would clean them anyway regardless NC or WS types.
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Offline exe

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Re: Flux conductivity (NC-559-AS)
« Reply #4 on: October 15, 2017, 07:50:15 PM »
Thank you guys for helping.

I decided not to go with this flux as another one I use (edsyn FL-22) maintains high resistance (>220MOhms, the highest resistance I can measure) at all times. Same for cheap non-name flux from AliExpress. This is because when I do hand soldering there may stay some "unactivated" deposits of flux that often not possible to completely remove. Even after intensively rubbing the PCB with toothbrush and lots of IPA the flux was still there. I didn't measure the activation temperature, buy in my power supply the PCB temperature may go quite high. So I prefer to be save.
 

Offline KL27x

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Re: Flux conductivity (NC-559-AS)
« Reply #5 on: October 15, 2017, 08:02:45 PM »
Yes, it is normal. All fluxes are conductive. They are all pretty highly conductive when first applied, and less so after being used.

What you have there is a rosin-based flux, which most electrical fluxes are rosin-based (or synthetic "resin" with similar characteristics.)

Flux works by dissolving metal oxides with acid, and for something to be acidic, it must create H+ ions and negative ions, and ions are conductive.

When you first apply it, the flux is in a solvent "carrier" which keeps it in solution/ionized, hence it is conductive. When you heat it, the conductivity increases, greatly. This is because the very long chain rosin acid (abeitic acid) becomes more mobile. This is the very characteristic which makes for a good flux. That it becomes most active near soldering temp, and less active below say 70C.

Now if you've heated it all the way to soldering temp for several seconds, the carrier solvent should mostly boil away. And what solvent is left should fully evaporate shortly after, within hours, maybe.* And then you're left with the salts leftover from dissolving the metal oxides plus the hardened rosin, which is water insoluble. The hard residue prevents the salts from dissolving in trace moisture from humidity and becoming conductive. But if you get the rosin hot enough to liquefy, it will become acidic and conductive, once again, although not nearly to the same degree at a given temp as it originally was with the solvent present. It relatively cumbersome for these big long chain molecules to move past each other without being dissolved in a lighter solvent, so pure rosin isn't particularly conductive when it is still viscous.

Quote
But if I heat the board it starts conducting again.
This is why I wash rosin residue off of components I expect to operate at high temp. That said, the stuff you have might have more "stuff" added to it to make it more active. Some other acids or whatnot which increase the conductivity a bit.

*Now, I could be making this up, but I believe the gel type fluxes, like this Amtech stuff in particular, are specifically formulated for LONG active time, for soldering BGA's and whatnot. Seems to be something I read in the brochure. I would guess the most efficient way to achieve this would be to use a less volatile solvent and or additives to prevent the solvent from boiling off as fast. So you may find that leaving the board to dry for a day or 3 may result in lower residual conductivity. (Or heating it longer and/or using less of it).

 
« Last Edit: October 15, 2017, 08:26:37 PM by KL27x »
 

Offline exe

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Re: Flux conductivity (NC-559-AS)
« Reply #6 on: October 17, 2017, 06:49:32 AM »
Thank you very much for detailed answer. Now I understand the subject a little bit better.

One question remains: why other fluxes I have don't exhibit this behavior? Like Kester 952-D6 (pen) or Edsyn FL-22? Okay, unlike the edsyn, kester was conductive when I applied it. But after heating it's not conductive anymore.

NC-559 starts conducting at ~80-90C and I literally failed to remove it. It's just there, nothing helps (it has been a few days from now since I applied it). I can record a video if someone is interested.
 

Offline KL27x

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Re: Flux conductivity (NC-559-AS)
« Reply #7 on: October 17, 2017, 07:36:05 AM »
I wonder where you bought it? I have heard Amtech flux is a target of counterfeiters.

(If you haven't already) I would try heating the residue with hot air gun at near soldering temps for some amount of time to see if that changes anything? You said kester was conductive, but now it isn't. Maybe the amtech just needs more cooking time? At room temp of way 74F, rosin residue will be hard and will break like glass if you press on it with the tip of a screwdriver. If it's soft, it is maybe not dried out all the way?

If it is fake, who knows what's in there. But it is possible this flux just has more residual conductivity as a compromise for other characteristics to say said in BGA reflow.

« Last Edit: October 17, 2017, 08:27:54 AM by KL27x »
 

Offline nanofrog

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Re: Flux conductivity (NC-559-AS)
« Reply #8 on: October 17, 2017, 07:38:42 AM »
One question remains: why other fluxes I have don't exhibit this behavior? Like Kester 952-D6 (pen) or Edsyn FL-22? Okay, unlike the edsyn, kester was conductive when I applied it. But after heating it's not conductive anymore.
Keep in mind you're comparing a liquid to a gel/paste, so there's a large difference in the solvent content they contain. The Kester 952 is mostly IPA (85 - 100%, according to the linked SDS). But in the case of the Edsyn FL-22, you've much less at 3 - 10% (Tetrahydroabietyl Alcohol), as well as additional non-conductive carriers (i.e. polymer).

SDS for Edsyn FL-22 (.pdf)
Kester 952 SDS (.pdf)

NC-559 starts conducting at ~80-90C and I literally failed to remove it. It's just there, nothing helps (it has been a few days from now since I applied it). I can record a video if someone is interested.
How hot will your board be during use?

Regarding flux removal, no-clean products that use synthetic/modified resins tend to be more difficult to clean IME.
 

Offline exe

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Re: Flux conductivity (NC-559-AS)
« Reply #9 on: October 17, 2017, 08:17:31 AM »
I wonder where you bought it? I have heard Amtech flux is a target of counterfeiters.

I bought it from tme.eu .

(If you haven't already) I would try heating the residue with hot air gun at near soldering temps for some amount of time to see if that changes anything?

I did that I think more than five times (with a heatgun, 240-300C temperatures, 1-2mins exposure), and tried to clean it with IPA at least three times. The results stable. BTW, here is the original experiment, you can see the "setup": https://photos.app.goo.gl/UCR9WjFDNRoSd6ic2 (sorry for shaky video, it was more for my reference rather than for sharing). Since then I tried to wash the PCB with IPA and toothbrush several times, no luck.

How hot will your board be during use?

Well... I'd say no more than 80C in my upcoming power supply (worst case, more like 40-50 most of the time). But I'm more concerned about soldering precision and high-impedance circuits (I wanted to build a differential probe with j-fet opamps). It's not like I have many of them, just I want to have flux that I can use in all cases without much thinking. Other considerations: any "unactivated" and unwashed flux may compromise the device and testing devices while they are hot after soldering is impossible (it takes some time after soldering for resistance to settle).

Tomorrow I'll also post a few tests for corrosivness I made. The test is incorrect as I don't think heated the PCB enough time after flux application, but still might be better than nothing. I also tested "cold flux" scenario. I applied flux about a month ago, let's see the result tomorrow...
 

Offline KL27x

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Re: Flux conductivity (NC-559-AS)
« Reply #10 on: October 17, 2017, 08:34:41 AM »
I have to say I have not been able to measure conductivity in my MG Chemicals RA flux residue. I have put it on veroboard and shot it with heat gun, in the past. Not sure of the temps, though. I wasn't particularly scientific about it. Just got it nice and melted.

The Amtech might have different characteristics (compromises) for specific purpose; for BGA reflow with lead free solder, or somesuch. Of course, you want to use the best/cheapest flux for the job, and if the Amtech isn't doing anything specially good for you, maybe you should just scratch it off your shopping list. Thanks for sharing your findings.
« Last Edit: October 17, 2017, 08:36:52 AM by KL27x »
 

Offline exe

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Re: Flux conductivity (NC-559-AS)
« Reply #11 on: October 17, 2017, 08:43:49 AM »
One thing I need to mention is that I made "traces" by scoring. So, the FR4 surface is damaged. May this is why I can't fully clean it...
 

Offline KL27x

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Re: Flux conductivity (NC-559-AS)
« Reply #12 on: October 17, 2017, 10:20:20 AM »
I am not sure why there is such a demand for so many different kinds of $$ fluxes. I started out buying the cheapest stuff I can find, MG 835 RA flux, 25.00 for a liter. In most of my applications I do not clean it. I don't do BGA, but it works great for hot air reflow of QFN and whatnot.

I have purchased more than once, only because I share it. A liter probably would last me at least 10 years. I can burn through 10mL in a day, but those days are few and far between.
« Last Edit: October 17, 2017, 10:25:54 AM by KL27x »
 
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Offline nanofrog

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Re: Flux conductivity (NC-559-AS)
« Reply #13 on: October 17, 2017, 10:39:29 AM »
I am not sure why there is such a demand for so many different kinds of $$ fluxes. I started out buying the cheapest stuff I can find, MG 835 RA flux, 25.00 for a liter. In most of my applications I do not clean it. I don't do BGA, but it works great for hot air reflow of QFN and whatnot.

I have purchased more than once, only because I share it. A liter probably would last me at least 10 years. I can burn through 10mL in a day, but those days are few and far between.
IIRC, it resulted from the RoHS legislation in order to cope with the higher soldering temps, as well as keeping cleaning costs in check.
 

Offline dl1640

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Re: Flux conductivity (NC-559-AS)
« Reply #14 on: October 17, 2017, 10:55:10 AM »
CHIP QUIK no-clean type is found such conductivity issue on my bench.
the manufacturer also suggest to clean the flux residue but in most application cleaning is not required they said.
 

Offline nanofrog

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Re: Flux conductivity (NC-559-AS)
« Reply #15 on: October 17, 2017, 11:20:27 AM »
How hot will your board be during use?
Well... I'd say no more than 80C in my upcoming power supply (worst case, more like 40-50 most of the time). But I'm more concerned about soldering precision and high-impedance circuits (I wanted to build a differential probe with j-fet opamps). It's not like I have many of them, just I want to have flux that I can use in all cases without much thinking. Other considerations: any "unactivated" and unwashed flux may compromise the device and testing devices while they are hot after soldering is impossible (it takes some time after soldering for resistance to settle).
80C  :o ... bit toasty there.  ;D FWIW, a former mentor explained that I should keep stuff at 55C or less for safety if it can be touched (temp hot water heaters/boilers are set at from the factory or during installation).

For the cases you've listed, cleaning will be required.

BTW, have you tried scraping as much off as possible with dental picks, flat blade screwdrivers, ... before you grabbed the IPA and the toothbrush?
 

Offline exe

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Re: Flux conductivity (NC-559-AS)
« Reply #16 on: October 17, 2017, 07:17:46 PM »
BTW, have you tried scraping as much off as possible with dental picks, flat blade screwdrivers, ... before you grabbed the IPA and the toothbrush?

I can, but... it feels like replacing flux is easier :). For the sake of experiment I will do this and report here in this thread.

80C is the worst case. This is the temperature of diode bridge of my current power supply under full load. Of course, only small portion of PCB runs that hot...
 

Offline nanofrog

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Re: Flux conductivity (NC-559-AS)
« Reply #17 on: October 17, 2017, 08:21:22 PM »
BTW, have you tried scraping as much off as possible with dental picks, flat blade screwdrivers, ... before you grabbed the IPA and the toothbrush?

I can, but... it feels like replacing flux is easier :). For the sake of experiment I will do this and report here in this thread.

80C is the worst case. This is the temperature of diode bridge of my current power supply under full load. Of course, only small portion of PCB runs that hot...
The mechanical removal aspect will help speed up the cleaning process with any rosin or no-clean formulation IME. Especially the thicker deposits between small pitch IC pins.  :phew:

Why not get a bridge in say a D-34 package and mount it on a heat sink?
 

Offline stj

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Re: Flux conductivity (NC-559-AS)
« Reply #18 on: October 17, 2017, 08:35:52 PM »
flux absorbes moisture,
so it should always be cleaned off.
 

Offline dl1640

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Re: Flux conductivity (NC-559-AS)
« Reply #19 on: October 18, 2017, 01:18:05 PM »
we really dont know what make up that flux, the activators and additives etc.
they're all chemical stuff, i really would like to clean the target area before and after the soldring/desoldering.
 

Offline floobydust

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Re: Flux conductivity (NC-559-AS)
« Reply #20 on: October 18, 2017, 01:33:07 PM »
In production, experienced CD4000 DIP package bonding-wire failures.
A lot of electron scanning microscope of failures. Blaming ESD, vibration, reflow heat etc.
Ended up being corrosion. TI blamed the "aggressive flux" ingress entering their DIP package and corroding the bonding wires. Then said their revised DIP package doesn't seal the greatest against moisture and flux. It was quite the debacle.

Point is, you want the mildest flux even though an aggressive flux works better, for solderability.
 

Offline Awesome14

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Re: Flux conductivity (NC-559-AS)
« Reply #21 on: October 18, 2017, 02:39:33 PM »
I use 99.9% isopropyl alcohol to wash the board if I want the flux removed. Most boards can also be washed in a dishwasher. Fluke Corporation actually recommends a dishwasher, if the boards are sufficiently dirty to require washing, for the boards in some of it's products; the 8842 comes to mind.

The pure alcohol is usually quite inexpensive, dries quickly, and leaves no residue. Just don't dip anything with flux on it into the bottle. Pour a bit out into a small container. Dip your cleaning tool in it. Then pour out the remainder when you're done. 
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Offline exe

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Re: Flux conductivity (NC-559-AS)
« Reply #22 on: October 19, 2017, 07:41:00 AM »
BTW, have you tried scraping as much off as possible with dental picks, flat blade screwdrivers, ... before you grabbed the IPA and the toothbrush?

No, but scalpel helped.. But that was difficult.

Why not get a bridge in say a D-34 package and mount it on a heat sink?

Space constraints, it's a tiny PSU. Although, there is nothing around that bridge that would be sensitive to leakage or temperature. Yet, one year later uncleaned flux started to corrode wires (it's a proto-board). Fortunately, I noticed this and tried to clean with IPA and toothbrush, but not all places are accessible due to relatively dense mounting.

I really would love to use only non-conductive non-corrosive flux, but it's not that easy to get data on this. So I conducted my own one-month experiment. I left six different fluxes on a copper clad. Please see the picture. There are two strips, both with same fluxes, but the bottom was heated with a heatgun. Not for long because fluxes became liquid and tried to mix with each other. They also started to dry out, which is not how it works when I do repairs or prototyping.

The interesting part is NC-559-AS-TF is only corrosive before heating. While FL-22 vice versa, safe until you heat it.

PS do you think colophony is a good coating for PCBs? I'd like to coat to prevent oxidation. That's what this flux suggest to do: https://www.tme.eu/en/details/10_200/fluxes/kontakt-chemie/74509-003/ . Here is an excerpt from datasheet:

Quote
KONTAKT CHEMIE Flux SK 10 forms an evenly spreading, transparent protective film. Softsolderable metals (e.g. copper, tin, lead, brass, steel) are protected against corrosion. Solderability is maintained, the protective film does not need to be removed prior to soldering, since it also acts as a highly effective flux

PPS suggestions for cheap and good fluxes are welcome. But I'm going to test them before putting into use.
 

Offline nanofrog

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Re: Flux conductivity (NC-559-AS)
« Reply #23 on: October 19, 2017, 09:12:06 AM »
flux absorbes moisture,
so it should always be cleaned off.
With water soluble (OA), definitely (must be cleaned off, no exceptions).

But I'd have to disagree with this as a general case, as neither any rosin or no-clean* flux could be left on the boards without mandatory cleaning (does need to be cleaned off under specific circumstances, such as high impedance circuits).

* Whether it be modified rosin or purely synthetic resin based.

The pure alcohol is usually quite inexpensive, dries quickly, and leaves no residue. Just don't dip anything with flux on it into the bottle. Pour a bit out into a small container. Dip your cleaning tool in it. Then pour out the remainder when you're done.
For IPA, the highest mixture I have on-hand is 91% which can be found very easily from local sources.

That said however, I found something even better IME and is even less expensive than 99.9% IPA IME.   :o  Specifically, Klean Strip brand denatured alcohol from Home Depot which is a mix of ethanol and methanol which won't leave a residue either (not the Green variant*, which will leave a residue). Cans use red on the label rather than green, and is only ~$16 for a 1 gallon can.

FWIW, I also keep acetone, xylene, and MEK on-hand as well to strengthen the denatured or use on their own for really stubborn deposits or other difficult stuff (i.e. adhesive residue after removing labels).

This is what the can you'd want looks like:



MSDS for ^ (.pdf)

* Half the cost of the regular denatured alcohol though, and you could clean the residue by other means such as IPA (or possibly even distilled water?  :-//).
 

Offline nanofrog

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Re: Flux conductivity (NC-559-AS)
« Reply #24 on: October 19, 2017, 10:26:52 AM »
BTW, have you tried scraping as much off as possible with dental picks, flat blade screwdrivers, ... before you grabbed the IPA and the toothbrush?

No, but scalpel helped.. But that was difficult.
I'd suggest getting some other tools that would be easier to use IME, and you don't need to spend a lot of money either.  :-+

Dental or electronic probe/pick/scribers in particular, but a wax carving set, nylon spudger, a soldering aid set, and toothpicks can be useful too. And not just for cleaning.  >:D

In my case, I've the following:
  • Moody 55-0296 25mil 5-Piece Precision Probe Set
  • Moody 55-1784 10mil Precision Probe Set, 4-Pieces (useful for getting between pins on small pitch IC's; say .65mm & smaller)
  • Beau Tech SH-121 Soldering Aid set (includes a SH-20C, SH-20D, SH-20G, and a nylon spudger, SH-80)
  • A couple of dental picks with slight spatula ends (got these used)
  • Wax Carving set*
  • Orange sticks (wood versions of a nylon spudger, and are disposable)
  • Various brushes (goat, horse, and hog hair for natural fiber based versions, and ESD compliant synthetic bristles as well)
Ebay may have better prices for ^, so worth a look IMHO.

* The spatula ends on the carving set are good for mixing epoxy, the knife ends for applying it, and the pointy ends for flux removal.

Space constraints, it's a tiny PSU. Although, there is nothing around that bridge that would be sensitive to leakage or temperature. Yet, one year later uncleaned flux started to corrode wires (it's a proto-board). Fortunately, I noticed this and tried to clean with IPA and toothbrush, but not all places are accessible due to relatively dense mounting.
Sounds like you didn't fully remove the solvent/s allowing the resin to properly contain the activators.  :( You can run into the same issues with improperly cleaned flux as well (activators still remain, and nothing to contain/seal them from corroding traces, pins, and even bonding wire on a package that's not fully sealed).

Regarding the diodes, are they SMD or thru-hole?

I ask, as if they're thru-hole you can raise them off of the board by using a U or V shape bend in the legs between the diode body and PCB. They also help by reducing the mechanical stress to the PCB that results from thermal cycling. In fact, there are even specialty pliers that do this in a single operation. Otherwise, get a pair of needle nose or flat nose pliers and something round, say a #1 or #2 Phillips screwdriver to make them on occasion.

BTW, the U or V shape is aimed toward the diode body for horizontal mounting (or any other PTH passive). In tight spaces, you can mount them in a vertical position, with the U or V shape on one leg only, and is aimed toward the other hole. Do note that the horizontal method offers better thermal and strain relief. PnP machines don't typically do vertical mounting well (or at all on the lower cost machines IME).


I really would love to use only non-conductive non-corrosive flux, but it's not that easy to get data on this. So I conducted my own one-month experiment. I left six different fluxes on a copper clad. Please see the picture. There are two strips, both with same fluxes, but the bottom was heated with a heatgun. Not for long because fluxes became liquid and tried to mix with each other. They also started to dry out, which is not how it works when I do repairs or prototyping.

The interesting part is NC-559-AS-TF is only corrosive before heating. While FL-22 vice versa, safe until you heat it.

PS do you think colophony is a good coating for PCBs? I'd like to coat to prevent oxidation. That's what this flux suggest to do:

PPS suggestions for cheap and good fluxes are welcome. But I'm going to test them before putting into use.
Colophony is rosin.

I've not tested for conductivity not had issues thus far with the stuff I use, but I'd bet money they're all conductive when in a liquid state. Specifically I tend to use Kester 186 (RMA that borders on RA performance), Kester 1544 (RA), and MG Chemicals 835 (RA) for liquid flux.

For paste/gel flux, I use Kester RF741 (reasonably priced in the US, and easy to obtain). For solder paste (63/37), I use Kester Easy Paste 256 (usually referred to as EP256), which is simply EP256 + 63/37 solder balls.

BTW, all of these can be left on the boards for most cases (I like to clean mine though).

Regarding your experiment, I wonder if the solvent was fully boiled off while in the liquid/melted state. The reason is, once the resin is cooled (all solvent gone + remaining deposit hardens), it fully contains any activators that are part of the flux content making them inert under normal conditions (max PCB temp never reaches the activation temperature).

P.S. It's a bit long, but hopefully the detail will be of help to someone.  :)
 
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