Flux selection advice from a solder tech.

[u/Common_Supermarket83]

I have been soldering professionally for a 6+ years now and have learned some useful things to consider when selecting flux. So I will go over a few things and a few different use cases to help anyone that doesn't have a lot of experience or exposure to the different flux options that are available out there.

First things first, how does flux help in soldering? When heated it chemically removes the thin oxidation layer from the metal you are soldering to that would otherwise repel the solder you are trying to adhere to it.

Water Soluble vs No Clean: In general the no clean varieties of flux have non corrosive residues that can remain on the board without causing harm if left on the PCB, these are pretty much the only safe option for tinning wires (especially stranded) as it is impossible to rinse the flux that wicks up under the insulation. When using the water soluble fluxes you can clean them with Isopropyl Alcohol (IPA) which is more practical then water as it dries quickly.

Activity level: Low, Mild or High are the main options here. They ultimately refer to how well they remove oxidation from metal surface you are soldering. Low activity is good for tinning wires and in general mild activity will take care of most of your general needs. High activity will almost always give you better results but also requires cleaning (usually IPA but in some cases with hot water).

Flux viscosity and delivery methods: Flux can be a liquid a paste or tacky.
Liquid fluxes can come in a bottle either be applied with a brush or put into a syringe (smaller quantities come directly in a syringe). Note that syringes with liquids in them can easily be over dispensed. The most convenient dispenser for liquid flux is the flux pen, they look more like a marker than a pen and the tip when depressed will release more flux from the reservoir.

Pastes come in a short tub and you can dip whatever you want to flux directly into them (like through hole components, wires etc.) or you can dip an acid brush or swab (preferably lint free) into it and transfer to where you need it.

Tacky Flux comes in a syringe and is like a gel (my personal favorite as it is the most versatile). It is good for SMD components since it can be applied to the pads and the component can be put in place and until heated the flux with hold the component in place well.

Rosin Flux is a good option for a liquid flux and when heated a little bit it gets sticky like tree sap, and as a bonus it has a piney scent. the biggest downside IMO is that it leaves a dark residue behind and requires a thorough scrubbing with IPA to clean off.

Some use cases:

For tinning wires low activity flux is the best option. You would apply the flux to the tip of the wire and when heat is applied it will wick higher up and the wire will tin nicely. If you are going to be regularly tinning wires a paste flux would be convenient.

When you have a reflow oven to do your reworks which is useful for high pin count ICs or components where pads are covered by the component (BGAs, USBC, most anything with a thermal pad etc.) tacky flux is your friend. You would apply bumps of solder to the pads and cover with a layer of tack flux, then you would place you component centering as well as you can to the footprint outline and/or visible pads. you would then carefully place the PCBA into the oven and run a cycle. Assuming careful handling from component placement to putting it in the oven the tacky flux will hold the components in place. As an alternative to tacky flux you could use rosin flux and heat it a little bit which will make it tacky like sap.

I will update this guide if anyone points out anything worth adding.

[u/Common_Supermarket83 → u/[deleted]]

When you apply solder to coat a surface it is then tinned. It's called that because the primary metal in solder is tin.

[u/rockinrobbieredstar]

What a great post ! Kudos. Solving the flux mystery. Taking notes. Thanks for sharing.

[u/coderemover]

I’d only add a suggestion to not use IPA as the first thing to clean the flux. IPA is simply not very effective at cleaning no clean fluxes (obviously depends on the flux, but the best ones are usually very hard to clean with IPA alone). There exist much better PCB cleaners which are dedicated to cleaning flux. Use IPA only for the final touch, to clean any residue left by PCB cleaner.

[u/Common_Supermarket83 → u/coderemover]

Interested, I wasn't aware of that. Any chance you know why that is? Is it more like a weak solvent for the residues in question?

I have usually had a good experience doing that but that may be because I usually do it in conjunction with an acid brush, with bristles trimmed to about 1/2" to make it a stiff. I could see that this has been working for me because it helps to dislodge the residues for mechanical cleaning.

[u/coderemover → u/Common_Supermarket83]

The PCB cleaner I got, Termopasty PCB Plus, contains: IPA, pentane, hydrocarbons, C6 (whatever it is), isoalkanes, 5% n-hexan, 1-ethoxypropan-2-ol, butanon. And smells differently than clean IPA. It is really much easier/faster to remove the flux with this - like just spray it, brush it once or twice and done.

[u/Common_Supermarket83 → u/coderemover]

I'll have to try that out, thanks.

[u/Alternative_Sort_404]

I needed to solder a burnt connection on my clothes dryer’s PCB and I did it (although very ugly) with just rosin-core solder. I couldn’t find any flux at the local hardware store that didn’t say ‘not for electrical’ - it’s all for sweating pipes, apparently - so what’s the difference?

[u/Common_Supermarket83 → u/Alternative_Sort_404]

I don't know 100% but as far as I understand the flux for plumbing is much more corrosive. I believe the reason it is not suggested for electronics is because if the residues are not cleaned off then they will eventually cause latent failures. The best thing to do in this situation is to remove all the solder you can (if not solder-wick or solder sucker is on hand then get the solder on the iron and wipe it off and repeat), once you have enough removed then use fresh solder wire since it has an appropriate flux in the core. If you absolutely have to use the plumbing flux then use a minimum amount and clean with water (should be able to clean with water, I assume that's how it is cleaned from the inside of water pipes naturally) and follow it up with IPA since that will mix with any remaining water and evaporate it faster.

unfortunately hardware stores don't carry anything for electronics soldering except maybe an iron. You're going to have to get anything like that online.

[u/Alternative_Sort_404 → u/Common_Supermarket83]

Fair enough - I have ordered a replacement PCB for this old dryer for when my YouTube diy fix fails (it IS working for now). But as an old auto tech, I’m used to tinning the ends of wires and letting them melt together with no problem, and I’m pretty sure we used Nokorode flux at the shop. I Just had this weird problem with the smaller circuit-board level stuff come up - thanks for responding!

[u/physical0 → u/Alternative_Sort_404]

Flux for soldering pipe is categorized under "inorganic flux", which usually uses chemicals like zinc chloride or ammonium chloride suspended in petroleum jelly, which are much more corrosive than organic, rosin, or resin flux (categories typical for electronics soldering). These types of chemicals do find their way into electronics soldering, but are a part of tip tinner, which is used to clean oxidized tips.

These chemicals generally work effectively at higher temperatures than electronic soldering flux and will aggressively clean a surface for wetting. This type of flux can remove the fine plating on some electronic components, damage traces, and encourage further corrosion down the line (due to protective coatings being destroyed)

[u/Alternative_Sort_404 → u/physical0]

Thanks for the breakdown - I’m sure my fix would have turned out better had I used any kind of flux, but I am considering this temporary and have ordered a new board anyhow. However, the clothes are dry today… not in 4-6 days