Unless you limit your modelling activities to sailing RTR (Ready To Run) models and buy the same when in need of replacement, it is almost inevitable that the skill of soldering will be required (or wished for?) at some time in this hobby. Many modellers have learnt and successfully practice the rituals needed to solder but perhaps without a true understanding of what the process involves. These are sometimes the ones who try to make soldering more complicated than it really is, maybe in an attempt to preserve their illusion of superiority? This might be fine until something goes wrong, there is only "magic" in the process of joining two or more pieces of metal together with solder if you don't understand what is happening. Perhaps the worst crime is to suggest that soldering is like using glue in that the solder "sticks" to the surface!
Welding is simple to comprehend, supply enough heat to locally melt the metals you are trying to join, probably introducing more metal into the molten mixture, then allow to cool. The result is a solid joint between the metals and, if done correctly, it is very strong and tough. But, soldering is clearly different since we operate at temperatures much lower than the melting points of the metals we are trying to join. I have yet to melt any copper wires let alone steel, with my soldering iron.
If you take two metals with different melting points and heat them up together then you might expect the one with the lowest melting point to melt first. Then, as the temperature rises, the second metal eventually liquefies on reaching its melting point. However, it is common for metals to form alloys, which is in the liquid state they mix together and do not remain physically separate. This results in the metal with the lowest melting point liquefying first and in effect "dissolving" the second metal at well below its melting point.
Usually these alloys do not have a single meting point but a range over which they solidify. Solidification starting with an alloy rich in the highest meting point metal. Melting such an alloy being the reverse process in that an alloy rich in the lowest melting point metal will start to melt first. Some people might remember the technique of repairing old domestic lead water pipes by using a solder that had such a temperature range over which it solidified thus allowing the semisolid solder to be "wiped" into a neat smooth surface with a rag.
By adding different metals together to make an alloy, it is possible to achieve surprisingly low melting points. You may have seen the joke of giving someone a spoon made from "Woods Metal" to stir a hot cup of tea in which the spoon promptly melts! It is worth adding that this alloy being made from Bismuth, Lead, Tin and Cadmium, none of which are wise to ingest, means that after laughing at the joke the tea must definitely not be drunk.
Thus, the process of soldering is simply applying a suitable metal which melts a conveniently low temperature onto the base metal surface which it can start to dissolve into and form an alloy with. Once this occurs, the heat is removed and the molten metal at the surface is allowed to solidify. The result is a joint which has no definite boundaries as the chemical composition changes from the base metal to solder at the outside, Fig 1.
So, with this understanding of how soldering works, it is not hard to appreciate what is needed for success. If it still sounds a little too "magical", just ask yourself how we get rid of ice frozen onto footpaths in winter. Throwing another equally cold solid onto the ice doesn't sound very promising yet, that just what we do with salt! The ice quickly melts and shows that things can behave in ways that might seem odd until you understand what is happening (hopefully readers who enjoy warm winters can still appreciate this example?).
Glynn Guest