Myth busting solder-flux cleaning with isopropanol alcohol

[helius]

Like I said, saponifiers are used in aqueous cleaning systems. A large, multi-ton apparatus using counter-current flow in several baths.
I don't expect that this is a common item in hobbyist workshops.

I use MG Safety Wash II (4050A) for most things, although I have also used Heavy-Duty Flux Remover (413B) for some projects with very old and hard residue. As I wrote in reply #91 above, it has plastic incompatibility issues, but the powerful smell is the main reason I rarely use it. It isn't a horrible smell, just very strong.

The Chemtronics ES896B is a rather strong flux remover that should be handled with gloves because of its n-hexane content. It should work well, but I try to avoid working with toxic chemicals unless necessary.

[Zhao]

If you wish to spend a little more hassle, and you don't care about cosmetics, try to use a solid block of rosin It is very safe to leave it as-is on the PCB, even if you apply way too much of it. I use the leg of a 2W resistor as applicator, heat it up, dip it into the rosin and stir to coat a small blob of rosin onto the tip, and then melt the blob wherever needed. This method works fine for both thru-hole and SMD. I even use rosin block to store some small SMD parts which are otherwise easily lost.

I haven't found any sign of corrosion on the boards that are fluxed this way, even on 10+ year old PCBs with exposed copper traces.

Not all the rosins are alike, though.
 - Natural rosin has heat stability issue and can char and oxidize at relatively low temperature. They are ok for leaded soldering (people has been doing that for decades), but tend to burn too quickly for lead-free. I find myself having to reapply it very often when soldering at higher than 300 degree C.
 - Hydrogenated rosin (also called water white rosin) are much better at high temperature. It also has lighter color and doesn't tend to turn black after prolonged exposure to heat.
 - Pure rosin are not very active. They tend to take longer to tin oxidized, uncoated copper. Some solid rosin specifically designed to be used as flux might contain other activators that make it more active. They seem to be as safe as natural or hydrogenated rosin at room temperature.

Attempting to clean solid rosin residual without a big tank of solvent will leave you with a sticky mess.