Comment by atoav

The variables aren't that hard to learn. The one thing many beginners struggle with is that they don't really get the principles involved. They treat a soldering iron as some sort of heated paint brush and wonder why they can't get the paint off the brush.

In reality there are multiple factors at play, but a simple and useful abstraction is to think about solder as something that flows more easily to places that are hot. That means the solder always likes to go onto the iron, since that is likely the hottest thing around.

This means your first goal is simply to heat up the places you want to solder far enough so they can melt the solder if you touch them with the solder wire¹.

This usually means heating two things at once: be it two wires you solder together or a pad and the leg of a resistor. That means knowing the geometry of your tip well is useful: How to wedge it in to heat the two things at once. Typically only the shiny, plated bits of the tip are really good at conducting heat². A little drop of solder on the iron can help conduct the heat to a bigger surface, a bit like the fat in a frying pan.

But generally you heat the parts with the iron and then you feed the solder wire into the parts. If they are hot e ough, the solder melts. If not, you wait.

The solder wire typically has a resin-like fluid inside, that helps with soldering as it reduces the surface tension of the metal (this is most of the stuff in the fumes). Capillary effect is the main thing that makes solder flow where you want it to. When the flux is evaporated soldering gets harder, so you may need to add fresh solder after a while or you use flux from a dispenser that you add manually.³

Then there is a lot about logistics. Heated air travels up, on earth solder tends to be affected by gravity and has a tendency to go down (although capillary effects are really the dominant force here), gravity affects parts, wires bend where their material wanta them to etc. The logistics part means: you have only two hands, an iron, solder wire and typically two or more parts that need to be joined.

Being good at soldering is (1) having a good mental model and intuition about how and why the solder flows the way it does and (2) have skill and experience to get the logistics right. The first you can learn to understand, the latter is a matter of practise.⁴

I have thought soldering sucessfully to well over 400 people during the past decade.

¹ This means your iron and tip need to be able to deliver enough heat to heat the parts. If you try to heat up a 200W copper CPU cooler with a 14W soldering iron you can wait forever for the solder to melt. This also has to do with thermal mass and regulation of the iron. Modern irons with modern tips have the temperature probe inside the tip. This helps a ton with avoiding temperature dips when you start to touch something and makes soldering much easier.

² This means keeping tips clean with a proper cleaner literally before every solder joint is crucial. If your tip is toast and has no shiny bit anymore, clean it. If that doesn't help use Ersa Solder Tip Reactivator or buy a new tip. Of course your parts should also be clean (Isopropanol)

³ There are ways to remove excess solder. The simplest being using the right motion with the tip on a coldish joint, making the excess solder stick to the iron and not to the (colder) joint. Other things are spring loaded suction devices (very useful to get solder out of through holes!) and solder wick (very useful to clean old pads)

⁴ There a tools like third hands and so on, and watching some videos by experienced solderers is a good way (nowadays) to learn it, but a steady hand only comes with practise