Solder flux selection, what should I look for and why?

[FriedMule]

I am not so much asking for what is best because I imagine it would be like asking what solder is best.
It is fairly easy and logical to select type of solder, you simply select the mixture dependent on its reaction while soldering, maybe add some silver if you are working with that and thickness is fairly logical. The only "problem" is to choose a brand.

But what about flux, You do of cause have Mildly activated, no cleaning and so on, but what is else the difference, why should I choose lirumlarium mixed with ruskumsnuskum instead of gluarium and stickioum? (invented chemical words).

[xavier60]

I have settled on general purpose type RMA fluxes that do not cause leakage problems whether it has been cleaned from the PCB or not.
There are fluxes that leave a residual leakage regardless of how well the PCB has been cleaned.
It needs to be easily available with no risk of it being a faked substitute as Amtech products often are.
And this is it, https://caig.com/product/caig-rosin-flux-rsf-r80-2/. Sometimes branded "Deoxit". I can buy it locally at Jaycar.
I dispense it from plastic syringes, I get the type that don't have the rubber piston.
https://www.aliexpress.com/item/32906201931.html?spm=a2g0o.order_list.0.0.47131802hY9bSb

I use the flux for both hand soldering and reflow, leaded and lead-free solders.

[tooki]

Some tips on usage:

When talking about external flux (liquid or gel) used for hand soldering (as opposed to reflow), you must clean all flux residues, even no-clean. Most no-clean fluxes work by having two important high temperatures: one at which it activates, and another where it’s neutralized again. Any such flux that is heated enough to activate, but not hot enough to neutralize will remain in the active state! In hand soldering, this is easy to have happen, because some of the flux will run off and be close enough to a joint to get hot enough to activate, but not to neutralize.

In some testing I did a few years ago, almost every flux I tested was corrosive if left on unheated or only partially heated. (Test was applying flux to both stripped ends of untinned stranded copper wire with clear insulation. One end was then tinned with solder. Then left in a drawer.) Some gel fluxes caused corrosion overnight. Some took a year. Some only on the heated side.

Some other tests on bare copper PCB showed another phenomenon with liquid fluxes: a thin layer that could dry quickly led to no corrosion at all. But droplets that took a day to fully dry (to the point of not being tacky) led to corrosion.

My favorite external flux is Chip Quik SMD291NL: it’s a transparent gel (surprisingly handy property compared to ones that are not, especially for microscope work) which doesn’t give off an acrid snell when heated. It smells like toasted hazelnuts instead. Easy to get from Digi-Key and mouser.

[JenniferG]

Thanks a lot for all the help y'all.   I believe I recall one guy on youtube saying he mixed some of his flux paste (from a jar) with some isopropyl alcohol and made a thick liquid with it, and that's how he applied it.  Is that all the commerical products are?  $20 tube which just paste mixed with isopropyl alcohol?