Wondering if using flux on our usual pcbs (Aion's, Lectric FX, PPCB, etc.) is really necessary ?

eh là bas ma

eh là bas ma

Well-known member
An esteemed comrade forumite from diystompboxes often have some very dirty boards to troubleshoot.

I just realized he was using "fluids" to help the soldering process. Once his work is done, the pcb looks covered with flux residues. Even after cleaning with isopropyl alcohol, there are still some remains. Looks like he's also struggling to clean the fluid on the board with Q-tips...

Some friends told me it was even more unhealthy to solder things with flux. Like the fumes are even more dangerous ?

I always thought most pcb had already received some chemical treatment, some sort of dried flux on the pcb, so that the solder goes more easily on the pads ?

I can understand that flux can be useful in some applications, but is it really useful when building guitar circuits, with PedalPCB boards, for example ?

Every comments and observations are welcome !
 
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eh là bas ma said:
A comrade forumite from diystompboxes often have some very dirty boards to troubleshoot.

I just realized he was using "fluids" to help the soldering process. Once his work is done, the pcb looks covered with flux residues. Even after cleaning with isopropyl alcohol, there are still some remains. Looks like he's also struggling to clean the fluid on the board with Q-tips...

Some friends told me it was even more unhealthy to solder things with flux. Like the fumes are even more dangerous ?

I always thought most pcb were already receiving some chemical treatment, some sort of dried flux on the pcb, so that the solder goes more easily on the pads ?

I can understand that flux can be useful in some applications, but is it really useful when building guitar circuits, with PedalPCB boards, for example ?

Every comments and observations are welcome !
Click to expand...

most modern solder contains a flux core, so additional flux isn't usually needed.

however, IME sometimes there are scenarios where additional flux is useful (flux GEL in a syringe).

- toggle switches (well, 'mini toggles' as used on pedals) - i will always use flux gel.
i have learned the hard way that mini toggles are pathetic, fragile, and really don't like heat. (imo 3PDT switches aren't much better, but can get away without using flux gel)

- soldering onto large / odd / heat sinking surfaces - e.g. back of full-size/24mm pot, floyd rose / trem claw grounding

- reflowing shitty solder joints, or when reflowing anything where i need to minimize heat exposure as much as possible.

- sometimes can be handy or necessary when desoldering
 
eh là bas ma said:
A comrade forumite from diystompboxes often have some very dirty boards to troubleshoot.

I just realized he was using "fluids" to help the soldering process. Once his work is done, the pcb looks covered with flux residues. Even after cleaning with isopropyl alcohol, there are still some remains. Looks like he's also struggling to clean the fluid on the board with Q-tips...

Some friends told me it was even more unhealthy to solder things with flux. Like the fumes are even more dangerous ?

I always thought most pcb had already received some chemical treatment, some sort of dried flux on the pcb, so that the solder goes more easily on the pads ?

I can understand that flux can be useful in some applications, but is it really useful when building guitar circuits, with PedalPCB boards, for example ?

Every comments and observations are welcome !
Click to expand...
I use rosin core solder, but otherwise only use flux to clean the iron tip or when de-soldering.
 
Thanks for your replies !

I understand it can be useful in some occasions, but it's not really meant to be applied on the whole pcb to solder each and every parts, right ?

Also, i doubt that flux is even slightly necessary for a non-professional pedal builder, making 2 or 3 stompboxes per month. I never felt the need for it in 4 or 5 years, with lots of desoldering and reflowing.... I am sure it could have been often useful, but not really necessary, there was always another way.

jhaneyzz said:
I use rosin core solder, but otherwise only use flux to clean the iron tip or when de-soldering.
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What about the health side of the matter ? Rosin is the english word for colophane. This thing is very corrosive and very volatile, present in the fumes coming from the iron when soldering.

As I understand it, the main risks while soldering are the pyrolysis gas from the flux burning, and from the pcb's plastic ? I read somewhere that the lead isn't volatile enough to be inhaled with the solder fumes, but I couldn't check this info yet...

It's impossible to avoid making flux fumes from the solder itself, and I guess putting even more flux everywhere isn't a problem as long as we ventilate the room correctly.
 
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I have a little pot of flux that I dip wire ends into, makes tinning them easier and quicker. That’s the only time I use flux. What is already in the solder is enough for pcb soldering, pot and switch lugs, etc.

Whenever I have to desolder something I just add fresh solder first, then suck it all up with either braid or a sucker.

I dont do any SMD work though, it seems nice flux in a syringe is a requirement for rework with SMD.
 
mnemonic said:
it seems nice flux in a syringe is a requirement for rework with SMD
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If flux can help protecting something from the heat, as implied in owlexifry's answer, it makes sense.

I also wonder if flux residues can create shorts after some time ? Can it oxydize after 10 years and damage the board, or something ?

​

 
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eh là bas ma said:
If flux can help protecting something from the heat, as implied in owlexifry's answer, it makes sense.

I also wonder if flux residues can create shorts after some time ? Can it oxydize after 10 years and damage the board, or something ?

​

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That might depend on the composition of the flux and what sort of environment it’s in. If the flux stays soft and gummy, or if it gets soft and gummy when warm (and your pcb is in a tube amp or something that gets warm), then it could absorb contaminants in the environment that eventually become conductive.

Also avoid corrosive flux like plumbers use, don’t want to melt a hole though your board.

I have an old pot of ‘uniflux’ my dad bought at i think RadioShack probably over 25 years ago, it says ‘corrosion resistant’ on it and I’ve never had a problem with it in the almost-decade I’ve been using it.

I also have a new pot of chipquik CQ4300 that specifically mentions ‘no clean’. Though that one is more sticky so I don’t use it as much, the pot is full to the top and sticks to the lid, and i get it on my desk when taking the lid off
 
Rosin flux is not an electrical issue to leave on the board. Clean it or don't, it only matters from an aesthetic point of view. But as noted if you have a board caked in a layer of dried flux it can make debugging more difficult as it can hide things like bad solder joints.

The main solder I use for "production" builds is Kester 331 organic core solder, the flux of which is actually conductive and will cause electrical problems if it isn't washed off. So why do I use it? Because it's water soluble, all I have to do is give the PCBs a spin in the washing station for 5-7 minutes then throw them in the drying oven for 15 minutes and I have beautifully clean boards with no electrical issues.
 
I use a flux pen when doing switches, ICs and transistors. The goal being to minimize soldering time as much as possible. It's a minimal amount and adds little time to the front or back end.
For switches I add it to the lugs themselves. Other components, to the pads and clean the legs with isopropyl.
I did populate a board fluxing all pads a couple of weeks ago, in the name of science. I found it easier to quickly solder and faster flow through the hole -but- it was an absolute mess to clean. Won't be doing that again in the future.
 
Lots of great info from many people here before me, however...

@eh là bas ma, I'm trying to sum your questions up here:




Heat "Protection" — What Owlexifry and jwin615 said, flux used to MINIMISE solder time. It's NOT that flux protects anything from heat, it's that it helps solder flow better-faster — so you can get the iron in/out faster and therefore expose the component/board to less heat ie less time that the component is heated.​
​
Fumes — Flux-fumes are bad: it's my understanding from reading about the topic over time in various forums/places that yes, FLUX-fumes are what's bad for you (though the other fumes aren't good either) definitely the worst.​
​
Shorts — as Mnemonic and Nathan noted, some flux IS conductive, but not Rosin. Getting the right flux for the job is key.​
​
​
PCBs — most do NOT have flux on them, in fact, I've never come across a board that had "factory-flux". As Owlexifry said, most solder we use is rosin-core based, so additional flux is most-often not needed. I'm going to try the wise-Owl/Jwin's idea to use it on mini-switches. I'm definitely going to use flux to solder FV-1 chips, I've got a few projects coming up.​


Great thread.
 
I don't use flux on the pcbs. I have a flux pen, and the only times I've used it was for stubborn connections like jacks.
 
I have a box fan I keep blowing fumes away from me while soldering and I wash my hands after soldering so whatever lead residue on my hands is removed. I think this solder I've been using has a rosin core, but I'd have to double check. It does leave behind a little residue that I don't bother cleaning up.
 
I’ve been using flux on ALL solder joints. Maybe it’s my el cheapo soldering station, but I don’t get good solder joints without it, and I have the iron pretty hot. Lately, I’ve been using MG Chemicals 836-P Nov-clean flux pen. First time trying a pen. I like it, but it’s hard to control how much flux comes out. I’m gonna try a different flux from them tho, because this stuff WILL corrode the board if left on too long. I haven’t noticed it on PCB’s but on vero the copper strips will turn green rather quick.
 
I went through 30 or so years of soldering electronics projects without using any additional flux (beyond that which is in the tube of solder “wire.” As soon as I started SMD, I needed solder flux, and got a liquid that I can load into a needle applicator squeeze bottle. Since I have that at hand, I do use it on occasion; when the solder just doesn’t seem to flow properly.

For something like a tremelo claw, I’ve always sanded off a small spot through the plating; this makes soldering much easier.
 
I wonder if there is a relation between the amount of flux residues from the solder, and the iron temperature.

Isn't it possible to minimize the amount of residues by finding the right temperature for the iron ? The right setting will probably vary according to the size and the brand of the solder.

I'd think there are more flux residues around the pads when the iron is set too high, for example.

I will try and report soon, when my next order will be delivered.

There will be a Cream Pie Fuzz, I guess it won't matter if I test on this one, it's just a fuzz after all...
 
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eh là bas ma said:
I wonder if there is a relation between the amount of flux residues from the solder, and the iron temperature.

Isn't it possible to minimize the amount of residues by finding the right temperature for the iron ? The right setting will probably vary according to the size and the brand of the solder.

I'd think there are more flux residues around the pads when the iron is set too high, for example.

I will try and report soon, when my next order will be delivered.
Click to expand...
I could see it spreading a bit further with a hotter iron. Especially if you're overheating it and warming the board too much. Honestly, for me, using less solder on ppcb boards, due to the smaller pads, made.a huge difference. I still use more than needed at times resulting in a ball on the component side.
I'm going to guestimate that it takes 35-40% of the solder on a ppcb board than a madbean board.
I actually prefer madbean sized pads, especially for the odd power diode that barely fits in a ppcb hole...
But feed rate is something I have to consciously consider when soldering these boards.
 
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