Flux left on circuit boards

abenn

Senior Member
I've been making circuits, mainly PICAXE, for many years now and have always left the flux from my resin-cored solder wire on the PCB, as I had read on the internet that it's safe to do so. But recently I've found that it's causing malfunctions on a couple of my boards, causing adjacent transistors to trigger at the same time when each is supposed to be triggered independently by a different output from the PICAXE. I know it was the flux because scraping it off cured the problem, but the boards had been working correctly for over a year without a problem. I surmise that perhaps the flux had absorbed moisture from the atmosphere, even though the board was shrink-wrapped and located in a dry but unheated garage, making it slightly conductive.

I'm aware that professional PCBs are laquered, so what's the best (simplest) procedure for cleaning up and protecting a board after manufacture please?
 

abenn

Senior Member
Thanks johnlong. I have pure acetone, the main ingredient of nail varnish remover, so will use that. Is there any chance that this, or any other solvent, might have a bad effect on components?
 

johnlong

Senior Member
Hi
I tend to use chip holders and buy dip components so any components that get washed are normally passives
ressistors capacitors and such not experianced any problems with them. Use to use electro solve spray librally in the control pannel
at work to clean away the high dust (manufacturing absorbances) deposits the product left and chase away moisture, but would check the spec sheet of any component you may have worries about
 

mortifyu

New Member
With over 30 years experience (prior to 2008 when I gave away repairing) repairing televisions, a/v equipment, microwaves etc. etc. etc. I can say that I do not believe I ever found flux to be the cause of issue. Indeed I found plenty of issue with certain glues used on PCB's and deflection yoke assemblies which did have corrosion that had formed due to moisture between the glue and PCB. But never once can I recall the flux causing conductive (leakage) issues. This is not to say moisture in some way has not been your issue. Without knowing your circuit, perhaps your resistor values are too high making the circuits overly sensitive switching on possibly even to humidity. To clean flux from PCB's, acetone has been the chemical I have used for many years without problems. Dedicated PCB lacquers are available. However in more recent times I have seen PCB's coated with a type of epoxy that seals everything. I have also seen jelly like substances keeping components from the environment and potential of moisture. Another surprisingly common fault I have come across is a phenomena known as 'Silver Migration' where the metals of the pins would physically migrate between B-C-E of transistors (particularly TO-92 packages) forming a conductive path that would cause random intermittent faults as transistors would turn on unexpectedly without intentional drive.


Regards,
Mort.
 

abenn

Senior Member
Mortifyu, poor choice of resistor values may be a factor in my case because I'm not very experienced at circuit design (though I did take advice on this forum in this case). The circuit appeared not to be damp, and the flux powdered as I scraped it off and, most importantly, the fault was cured after I'd scraped it. So slight moisture absorption in the flux is all I can think of.

As it happens, I've got another different circuit (flashing strobe lights on a R/C model aircraft) that's just started behaving erratically, so I'll give that a good clean with acetone after today's flights and see if it cures it.
 

mikeyBoo

Senior Member
It depends on the flux type, but some flux will definitely corrode circuit board connections.
Ever wonder what to do with that old electric toothbrush that only stays charged for a couple of days?
Well, an electric toothbrush with alcohol makes a wonderful board cleaner.
 

MartinM57

Moderator
Ever wonder what to do with that old electric toothbrush that only stays charged for a couple of days?
Great idea - ta!

This works for me - I found that acetone/acetone-based/IPA/IPA-based cleaner does clean off the flux in my solder/solder paste off but leaves an unsightly white hard deposit (e.g. https://www.avrfreaks.net/forum/white-residue-after-cleaning-pcb):
- Wurth Brake Cleaner from a 500ml aerosol can (pretty cheap), sprayed on, agitated with a brush (/old electric tooth brush) - does a great job dissolving the flux, but if you let it evaporate, you'll just end up with a thin layer of flux all over the board, so, before it dries...
- ... PCB flux cleaner (https://www.allendale-ultrasonics.co.uk/flux-remover-ultrasonic-solution-5-litre) - I tried using just it in my ultrasonic cleaner but it was hopeless - sprayed on and agitated...this goes slightly soapy and holds the flux in suspension before the brake cleaner evaporates, so then...
- ...distilled water (from my dehumidifier in the shed) sprayed on until it's dripping off clean
- air dry/hairdryer dry

Elf'n'Safety would say googles/air mask etc are required.

This results in very shiny and spotlessly clean PCBs that look like they've just come out of a good quality commercial facility. YMMV.
 

techElder

Well-known member
@abenn Don't get all flustered with flux! Forget the acetone (it is dangerous.) Get a can of "flux cleaner" anywhere soldering stuff is sold. When you finish soldering a board, spray the cleaner on and brush with an old (or new) toothbrush. You will clean off the flux and make a shiny PCB. Then go through with a magnifier and look for problems with soldering.

I've spent years in manufacturing and repair, in my previous life, and never soldered anything without cleaning the flux off and inspecting my work; just something I learned early on.

If you think flux will absorb water, pour (put) some in a bottle of water and see if it will dissolve. That will give you a clue.
 

abenn

Senior Member
techElder, I'm not sure that whether or not it dissolves in water means it can't absorb water and become slightly conductive. Anyway, there's no doubt in my mind that flux was forming a path across two tracks, since removing it -- and not doing anything else -- solved the problem.

From now on I shall be cleaning. I tried one of my boards this afternoon with acetone (because I happen to have it) and a paintbrush, but it needed something more robust than a soft paintbrush, so an old toothbrush is the ideal solution. I've checked online, and some products sold as flux remover look, from their specs., suspiciously like acetone or alcohol, and they're much more expensive. Some others convert the flux into a soap-like material which needs washing off with water. I'm happy with this afternoon's result, so I'll stick with acetone for the moment.

Thanks to all who contributed to this thread.
 

rq3

Senior Member
I've been making circuits, mainly PICAXE, for many years now and have always left the flux from my resin-cored solder wire on the PCB, as I had read on the internet that it's safe to do so. But recently I've found that it's causing malfunctions on a couple of my boards, causing adjacent transistors to trigger at the same time when each is supposed to be triggered independently by a different output from the PICAXE. I know it was the flux because scraping it off cured the problem, but the boards had been working correctly for over a year without a problem. I surmise that perhaps the flux had absorbed moisture from the atmosphere, even though the board was shrink-wrapped and located in a dry but unheated garage, making it slightly conductive.

I'm aware that professional PCBs are laquered, so what's the best (simplest) procedure for cleaning up and protecting a board after manufacture please?
True rosin for rosin cored electronic solder is basically purified pine sap, and is not designed to be left on the circuit board. It does absorb moisture from the air, and can lead to the issues you are seeing. The solid rosin is dissolved in isopropyl alcohol, which is also an effective solvent to remove rosin residue, followed by a soapy water wash and throrough clean water rinse (sometimes even distilled water, depending on how much ionic contamination is allowed after the rinse).

Acetone is commonly used also, because it's cheap and easy to find, but acetone is not a particularly good rosin solvent, but it IS a pretty good epoxy solvent, and risks damage to FR4 epoxy circuit boards. It is also very good at wicking up the legs and leads of epoxy encapsulated integrated circuit packages (like a Picaxe).

There are also synthetic water soluable fluxes which are designed to be easily washed off without resorting to solvents, and other synthetic fluxes that are designed to be left on the board. Rosin is neither of those.

Even rosin flux comes in different levels of "aggresiveness", generally acheived by the addition of various acids. No rosin is designed to be left on the board, all though it often is at the hobbyist level (or even the cheap consumer level), and usually does no harm. In the short term.

Industrially, ultrasonic cleaning is rarely used for electronic cleaning unless a careful analysis has been made of its effect. Many components have unsupported wire bonds within their package (quartz crystals are one good example). The wire bonds tend to acoustically resonate in the ultrasonic bath, destroying the bonds.

For the hobbyist using rosin flux, a good scrubbing with a clean toothbrush and 90% Isopropyl alcohol, followed by a good scrubbing with Dawn dish detergent and a thorough rinse with clean water works well. For a circuit board used outside long term, a spray coating of gloss acrylic can help, if the circuit has no very high impedance areas (greater than 1 mega-ohm). Even acrylic will absorb some moisture, and the matt acrylics are worse in that regard.
 

Goeytex

Senior Member
I suspect that scraping the flux off removed a tiny solder bridge on the board. It is unlikely that the flux itself could carry enough current to fully turn on a transistor at relatively low switching speeds.

That being said, it is still a bad practice ( unprofessional) to leave flux on a circuit board. IT is ugly and can cause issues in certain circumstances. Clean it with alcohol then examine under a magnifier for bad solder joints or solder bridges.

To protect the board after cleaning and testing, you can apply a thin conformal coating. Or if the budget is low maybe even spray it with clear acrylic or urethane. Exposed copper WILL corode over time.
 

westaust55

Moderator
Tin whiskers are a potential problem. Not directly related to the leaving of flux but could explain bridging between tracks/pads on a PCB.

 

MartinM57

Moderator
As a hobbyist I do clean PCB only scrubbing it with a tooth brush and isopropyl alcohol as proposed by rq3 in step 1. II wonder if the cleaning with detergent and the following rinse are really necessary?
My thoughts (and in practice, by doing the wash and rinse) is that the IPA dissolves the flux, so you end up with a flux/IPA "soup". The IPA evaporates, but there's no way it makes the flux evaporate, so the flux now must be left on the board, distributed evenly over it, instead of just at the solder joints...
...so you end up with "sticky-to-the-fingers" PCBs that are worse than before. IMHO.

Of course, if you use lots of IPA/multiple scrubbings with fresh IPA that drips off the board, then that is the way the flux is getting physically removed from the board.
 

Hemi345

Senior Member
^-- this. Toothbrush and IPA 93% or higher. Rinse with fresh IPA and dab up with Kim wipes. Quick dry with the hot air station when I'm excited to test it out.
 

premelec

Senior Member
The only time I've seen trouble with flux conducting was with extremely high input impedamce electrometer circuits... I don't usually clean the flux residue from good grade multi core flux solder... that said the guy who taught me soldering 70 years ago used to use Nokorode soldering paste - I DO NOT recommend it... :rolleyes:
 

neiltechspec

Senior Member
Never particularly come across this issue myself, but I do clean important boards.

As my main interest is Classic Automotive, the product I use is called 'Panel Wipe'.

Not sure which ingredients it's based on, but the higher VOC ones are either naptha or hexane based,
lower VOC are IPA based.

The one I use isn't IPA based - from the smell of it, but it is very effective & not expensive if bought in 5L tins
for other purposes.

I have tried IPA in the past but didn't find it very effective, was very little better than water !.

Neil.
 

techElder

Well-known member
I have tried IPA in the past but didn't find it very effective, was very little better than water !
I've (and the techs that worked with/for me) always used a commercial cleaning product for that same reason, and used it at a cleaning/inspection station with ventilation.

Again, the main reason is to remove the impediments to a visual (with magnification) inspection of the PCB after soldering.
 

abenn

Senior Member
On reading what people have said about acetone (I use it to clean my equipment when using epoxy resin for fibreglassing) I'll be sourcing some IPA to do my PCB cleaning from now on. I'm resonably confident it was hygroscopic flux that caused my problem, for if it had been a solder bridge -- however fine -- it would surely have been apparent straight away, rather than waiting about a year to reveal itself.
 

mortifyu

New Member
In light of personally performing in excess of 50,000 repairs (as per my invoice numbering) to electronic equipment thus far in my life time, I have never had residual flux ever cause issues of any kind, even in 15kv+ high voltage applications. For this reason I will never bother to ‘wash’ the flux from any solder jobs I do and certainly do not regard this approach as ‘unprofessional’.

High impedance circuits most certainly is the place where moisture could cause issue in rare circumstances especially where Field Effect Transistors are used due to the very low gate currents required.


Regards,
Mort.
 

mikeyBoo

Senior Member
Rather than start an argument I suggest folks Google “will flux corrode circuits” & draw your own conclusions.

Now, I worked in heavy industry for 30+ years (inside/outside) not counting the 10 years I worked on aircraft (avionics etc.) when I was young & had 2 brains.
What I have seen numerous times is that if you put a circuit-board in a metal box, mount it on a silo where there are huge changes in temperature & humidity, dew point becomes a factor (air condensing into water). Now, leaving flux on a board mounted in such a situation adds to the corrosion problem (that nasty green “fur” growing around solder connections).

So, in hot, humid climates with cold nights & hot days, not removing flux is not an option. In fact, sometimes it’s necessary to spray sealer on a circuit-board (making it a pain to work on later) if it’s used outdoors.

However, if you live in a hot dry climate with minimal temperature excursions, might be ok to not remove flux.

What I do know is that in the deep south (USA) or on boats it’s always a good idea to remove flux.
 

westaust55

Moderator
I'm resonably confident it was hygroscopic flux that caused my problem, for if it had been a solder bridge -- however fine -- it would surely have been apparent straight away, rather than waiting about a year to reveal itself.
Tin whiskers as I mentioned at post 14 also take some to to grow and bridge tracks/pads.
 

abenn

Senior Member
OK, I'd never heard of 'tin whiskers' growing. Google tells me that 'A single accepted explanation of the mechanism has not been established', and one photo shows one growing across two terminals in thin air without any support. Dendrite growth sounds more believable, though that is more likely to be visible, at least under magnification. Anyway, a cleaning regime will be implemented in future. Happy Christmas.
 

Technoman

Senior Member
Found in a French PhD thesis (2011) about soldering in high reliability equipment under RoHS : tin whisker growing have been reported related to the use of SAC (SnAgCu) for boards being exposed a long time (hundreds hours) at high temperature (100°C). The cause is unclear.
Seems the use of SnPb is safe.
 

rq3

Senior Member
Never particularly come across this issue myself, but I do clean important boards.

As my main interest is Classic Automotive, the product I use is called 'Panel Wipe'.

Not sure which ingredients it's based on, but the higher VOC ones are either naptha or hexane based,
lower VOC are IPA based.

The one I use isn't IPA based - from the smell of it, but it is very effective & not expensive if bought in 5L tins
for other purposes.

I have tried IPA in the past but didn't find it very effective, was very little better than water !.

Neil.
This is one of those items conducive to the home experiment. Pure rosin is easily and cheaply available at any music store, as it is universally applied to violin bows. A good sized block costs a few dollars. Break off 3 chunks. Place one in a small bottle of water, another in a small bottle of isopropyl alcohol (preferably at least 90%), and the third in a small bottle of acetone.

Check the results the next day. The remaining rosin you can dissolve in more isopropanol to yield a lifetime supply of liquid soldering flux for electronics touch-up work, especially where just a drop will make surface mount soldering almost enjoyable. Just don't forget to scrub off the residue when the soldering job is complete ;-)

Happy Holidays!
 

abenn

Senior Member
Follow-up -- don't use acetone!

As I mentioned earlier in this thread I planned to use acetone because that's what I had to hand, and it certainly made a good job of cleaning. Two identical boards used for controlling points on my model railway, and one board that strobes lights on a model aircraft, were cleaned and coated with laquer. Soon after I noticed that the LED indicator lights, driven from my point controllers, were bright yellow instead of green, which indicates too much current flowing through them. Those particular boards are all through-hole components except for one surface-mounted NSI50010 driver that limits the current to the indicator LEDs to 10mA. On testing and inspection today, it turned out that in each case the drivers had failed in closed mode, thus passing whatever current the LED could (or couldn't) take. Replacing the NS150010 drivers, and not cleaning off the flux, cured the problem.

The strobe-light controller in my model aircraft has recently begun intermittently flickering the lights rapidly instead of regular strobe flashes, so I suspect one or more of its components has been affected by the acetone cleaning too. It's all surface-mounted components, so it could be anything, including the PICAXE 08M2. I'll probably completely rebuild that board, rather than trying to trouble-shoot.

rq3 mentioned earlier in this thread the risk of acetone affecting resin encapsulated components, and I think I've proved him 100% right. I'll be using alcohol for cleaning from now on.
 

rq3

Senior Member
Follow-up -- don't use acetone!

As I mentioned earlier in this thread I planned to use acetone because that's what I had to hand, and it certainly made a good job of cleaning. Two identical boards used for controlling points on my model railway, and one board that strobes lights on a model aircraft, were cleaned and coated with laquer. Soon after I noticed that the LED indicator lights, driven from my point controllers, were bright yellow instead of green, which indicates too much current flowing through them. Those particular boards are all through-hole components except for one surface-mounted NSI50010 driver that limits the current to the indicator LEDs to 10mA. On testing and inspection today, it turned out that in each case the drivers had failed in closed mode, thus passing whatever current the LED could (or couldn't) take. Replacing the NS150010 drivers, and not cleaning off the flux, cured the problem.

The strobe-light controller in my model aircraft has recently begun intermittently flickering the lights rapidly instead of regular strobe flashes, so I suspect one or more of its components has been affected by the acetone cleaning too. It's all surface-mounted components, so it could be anything, including the PICAXE 08M2. I'll probably completely rebuild that board, rather than trying to trouble-shoot.

rq3 mentioned earlier in this thread the risk of acetone affecting resin encapsulated components, and I think I've proved him 100% right. I'll be using alcohol for cleaning from now on.
Not just alcohol. ISOPROPYL alcohol, often called IPA. Here in the states, it's available in various "grades", usually 70%, or 90%. The 70% is what the nurse swabs your arm with before a shot, and the 90% is considered a bit too "strong" for application to skin.
ALL alcohols are hydroscopic; i.e., they absorb moisture. It's almost impossible to get to 100% alcohol, because it will be constantly absorbing moisture from the air. Under rigid lab conditions, 98% is about ther best you can do, but is not relevant here.

The three alcohols readily available are:

Ethyl (grain alcohol), distilled from, you guessed it, grain, or another starch source. The stuff that gets you drunk. Commonly available in the states as "Everclear", a 198 proof (94%) food grade alcohol. This is about as concentrated as any alcohol can get without water "contamination", as alcohol absorbs water.

Methyl (wood alcohol), destructively distilled from wood. When consumed or absorbed through the skin, makes you blind. Used for Sterno fuel and the like, and sometimes used to "cut" ethyl alcohol because it is cheap and easy to make. Also used to "denature" other alcohols and solvents, on the assumption that everyone knows what "denaturing" means.

Isopropyl alcohol (IPA), from various (usually petroleum) sources. The stuff swabbed on your arm at the doctor, before the needle goes in.

Each of these has a very different polarity, and only IPA dissolves rosin flux with any degree of success.

Acetone is a different animal altogether. It's a terrific epoxy solvent, and has no place anywhere near already cured epoxy, like FR grade epoxy circuit boards, or epoxy encapsulated integrated circuits (almost all integrated circuits the average user will see). It's toxic, flammable, and has its uses when fabricating epoxy components from liquid epoxy resin. Cleaning solder flux is not one of those uses.
 
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abenn

Senior Member
I intend to use IPA, I was simply being too brief when I said 'alcohol'. I'm surprised I can't get it from my local chemist (pharmacy) given its common useage, but I can get it on the internet, but just haven't got around to it yet.

The reason I have acetone to hand is for cleaning equipment after mixing and applying epoxy resin when fibreglassing. I didn't know it would attack cured resin though.
 

abenn

Senior Member
. . . Isopropyl alcohol (IPA), from various (usually petroleum) sources. The stuff swabbed on your arm at the doctor, before the needle goes in. . . .
It seems the reason I can't get it from my pharmacist is that here they use "surgical spirit" for cleaning wounds and preparing for needles. It's based on methyl salicylate and methyl alcohol with castor oil thrown in, perhaps to discourage the usage that hippy has alluded to :)
 

hippy

Senior Member
I recall isopropyl alcohol used to be used for cleaning vinyl records. Ignoring the debate about whether that works and the safety issues, that there's still a vinyl community may mean there is still a High Street source for it.
 

abenn

Senior Member
Thanks Hemi. I've got lots of hits for the smaller quantity I want; just haven't got around to placing an order yet.
 

tmfkam

Senior Member
Perhaps I've been lucky but I've never had any problems that have been attributable to flux left on circuit boards.

I've been involved in servicing, maintaining and repairing consumer electronics for more than 40 years professionally now and it hasn't been an issue. I have always used standard 60:40 solder, with included flux. The solder is claimed to be 'no clean' and as a rule, most of my work I don't clean.

The last five - ten years I have also been designing PCBs where I've created circuits, software and boards which I, and my colleagues, then hand solder and again not found flux residue to give problems. I do often (though not always) clean the boards, once made, in an ultrasonic tank but that is more to make them look *pretty* than to clean off the flux. My work colleagues and I have made thousands of boards that have been designed, and made this way for commercial products and bar the odd component failure they have been in use daily for up to five solid years.

We had one problem with a batch of boards which we had made by a UK manufacturer that had no 'glaze' applied to the outside of the board. These boards weren't used for some period of time and when we got around to building them up, they had intermittent problems. When I investigated, I could measure resistive leakage between almost all adjacent pins. If they were heated up they worked correctly, but when cold the leakage returned. Still, not a flux issue, and soon remedied by fitting some additional pull up resistors.
 

Lenp

New Member
I'll leave one thought about acetone.
DO NOT use it on certain plastics like polyacrylates often referred to as Lexan.
We made some custom chamber tubes from polyacrylate tube stock and cleaned them before shipping with acetone since it seemed to have no immediate effect on the plastic and removed any glue reside quickly.
A week later we were replacing about $2000 worth of tubes because they all stress cracked from the acetone.
Some chemicals are well worth using but you must check compatibility or you might need to check your bank account!
IPA 70-90% appears to be safe for most plastics, and in normal times, it is readily available.
 

thunderace7

New Member
I clean my circuit boards with a small wire brush. Its a rotary brush for a Dremel but i use it by hand. It might be a bit 'caveman' but it works for me.
I doubt it would be beneficial to SMDs, though. I only do through hole components.
 
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