There is nothing worse than having remnants of unactivated flux on the board. it will eat copper over time. People forget that unactivated flux is very corrosive stuff. Activated flux still poses problems. it can be electrically conductive (especially rosin based) , or absorb moisture and saponify (water soluble)
Manufacturers describe certain fluxes as "no-clean flux" because it is not essential to clean it off the board after soldering. Appropriately designated fluxes do no harm if left behind.
Provided it is fully activated ! unactivated no-clean is just as bad as , if not worse than , regular flux ...
If the iron has too much thermal mass or too fast a response time you will spatter the flux leading to a bad joint.
If you splatter the flux you are using too high a temperature. The flux should melt and softly bubble, not splatter or burn.
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That is why the temperature on the tip should 'dip'. Look at a reflow curve. there is a time for warmup , an activation time, a ramp to liquid phase , a liquid phase and a cooldown.
The same is true for solder wire. the formulation for the powder flux in solder wire is different from the liquid fluxes out of a pen.
The flux inside hollow core wire can not come out of the wire before the solder has gone liquid !
Liquid flux is added before the solder goes liquid. The flux core comes out once the solder has gone liquid.
I see many times peple mixing fluxes. one type in the core , another type in the pen ... big nono !
You need to stick to the same manufacturer of solder and flux. And you need to use compatible liquid flux that is appropriate for the wire being used.