Thanks, I'm new to soldering, so no clean does that mean I need to buy some liquid to clean it? I see several times clean / no clean
There's all kinds of fluxes... a lot of them leave a bit of residue on the circuit board around the pad or lead or whatever you solder when you're done soldering .
You need to think of fluxes as liquids that act like acids attacking the surfaces of the metals you want to solder when they get heated up by your soldering iron tip. They attack the surfaces removing oxides and crap making it possible for the solder to chemically react with the metals in the pads and leads of components.
The solder has a core of solid flux in the middle of the wire which turns liquid or paste like at lower temperature than the actual solder around it ... as you bring the solder wire near the iron tip, the flux goes liquid and pours on the surfaces you want to solder and "attacks" them cleaning the oxides on the surfaces.
The flux residue that remains around pads or leads, on the solder... very strong fluxes can continue to act like acids slowly eating into the pads and leads and over time you could end up with broken traces or other issues. Other flux residues can be a tiny bit conductive, so it could happen that the flux residue on a circuit board could behave like a very high value resistance between two pads on the circuit board, and therefore affecting the circuit.
No-clean fluxes means the residue is not conductive and is not considered strong enough to continue attacking the surfaces once you're done. So, you don't have to clean the area once you solder. The same applies for most rosin based fluxes - you have basic Rosin flux, and there's Rosin Mildly Activated or RMA flux which is rosin plus some additives which make it a bit better, and you have Rosin Activated fluxes which is rosin with yet more additives and even stronger. Most of these 3 also don't need to be cleaned off the board, but always double check the datasheet.
If you do want to clean the residue from the boards, it can be done with very easy to find isopropyl alcohol (probably anything above 97% purity would be fine, the 70% purity varieties are for hygiene, for killing bacteria off your hands, the lower concentration is better at killing stuff) or acetone (nail polish remover)... but keep in mind acetone can smudge paints, like printed text on plastic sleeves on components for example
There's some fluxes you should stay away from, like for example anything that says "Organic" or "water soluble". Organic fluxes are often much more powerful than needed and the fumes are much worse for your lungs and as for water soluble fluxes ... unlike no clean fluxes and rosin based fluxes, these must be removed from the boards and it's a complicated to clean them... the name is tricky, it doesn't mean you can just wash the board with regular water.
Flux is very important in the process of soldering... it really helps. So a 3$ investment in a bottle you'll probably use for years is really worth it.
Even with new components and new circuit boards, I like to apply a small drop of liquid flux on the pads of something I want to solder. Yes, the solder has some flux inside it, but more flux rarely hurts.
It's also very useful at tinning wires for example... you can strip the insulation off a wire, put the wire inside liquid flux to get it wet with flux. Then, you can put some solder on your iron tip, basically make a big blob of solder ... by this time all the flux in the solder is burnt out, your iron tip has only solder... so the flux on the wire will do the work.... so now you can simply bring the blob of solder close to the wire and move it from side to side and solder will get onto the wire and tin it.
See the videos below ...
The first two explain really well how to solder... yeah, the videos SEEM outdated, but the techniques and explanations are still valid, so try to watch the two videos. First explains about solder and flux, the 2nd explains the technique of soldering... just don't stress too much about all that cleaning of pads and leads as it's not really that needed with modern stuff
The third video is about tinning wires ... the 4th shows a bit over the top tinning of a wire, but pay attention to the steps he makes and the technique
solder and flux :
technique :
tinning wires and soldering to terminals (terminals are not used so much anymore these days)
tinning wires paranoid level