Keebio had a great video demonstrating the drag-soldering technique using flux. I have not soldered with external flux yet, so the more video demonstrations I see of it, the better. Thought some others here might find it helpful.
https://twitter.com/keebio/status/1258230249973862400
I heard there are some downsides to using flux, particularly damage to the iron tip. Anyone here have any knowledge on that?
Using flux a lot will eat up a tip a little quicker then just soldering with rosin core solder, but you got remember that rosin core is flux in a harder form. You can not solder without flux, if you tried to use just straight up solder without flux it would not stick to much of anything very well. The flux is meant to flow over the surfaces to be bonded to & clean away anything that would affect the solder sticking to the copper. A nice side effect of flux is that it also keeps solder from sticking to itself when copper is around. So for quick fixes on pins that are very close together drag soldering reigns supreme, but hot air soldering is much more efficient at installing/removing components that have many pins very close to each other. It should also be noted that if you are doing a good bit of drag soldering there are specialized tips like knife shaped & conical angled tips that work much better than your average conical or chisel shaped tip for it.
I will say this though, flux is definitely something that should be in every KB builders soldering arsenal. The uses for it are numerous & it can make life much easier in many situations. Having a hard time desoldering a certain joint, reflow it with fresh solder & drown it in flux before the next desoldering attempt. I promise it will clean after that. Getting bridges in a tight area? Drown it in flux then reheat the joints & the solder will jump onto only the surfaces it should be on. Have a SMD chip that needs reflowed, I’m sure you get the picture. IMHO the little bit more external flux eat up a tip is a sacrifice well worth making for the benefits using external flux can give you.
Edit: Another thing that should be in your solder arsenal now that I’m thinking of it is tip tinner. If you use it on your tips after each soldering session it’ll keep them useful much longer than just covering them in a solder ball IME.
I think that @Rob27shred enumerated all the use cases and benefits of using flux!
As he said everybody should have a flux syringe ready at least for solder joint rework. Also it works wonders with desoldering wick (witch I do recommend to have also) because flux not only cleans the metal surface but also conduct heat very well: your iron tip heats the surface to desolder much quickier using flux as a result.
I have used tip tinner in the past to revive some badly oxydated tips and it works well.
But I found that by taking regular care of your tip it is not needed.
Two factors come to play for a tip to oxydize: how aggressive rosin/flux is and how high your soldering temperature is.
In my case I use only unleaded solder so I have both of these problems: rosing/flux in unleaded solder is quite agressive and you often have to crank up your iron past 300°C.
I found that regularly cleaning tip with a brass wool before/during/after soldering session does a good enough job.
After each soldering session I check that solder that I melt on the tip goes evenly on the tip surface, if not brass wool again.
Then melt a big blob of solder on the tip and let the iron cool as is.
This video is quite instructive on the oxydation effects and how to clean your tip: