Soldering means that materials have to be joined, i.e. the atoms of material
A have to penetrate material B and vice versa. The process is called "alloying".
When soft soldering there are usually two transitions:
1) Metal A <-> solder
2) 2) Solder <-> metal B
Metal a could be a wire of a component, metal B as well or the copper clad
of a PCB. Soldering generates a multiple layered system: metal-mixed crystal-solder-mixed
crystal-metal (see picture below):
The soldering temperature should be almost 100 degrees higher than the solidification
temperature (i.e. around 325 degrees).
A too low temperature prevents the two metals' alloying process, the joined
areas only stick together.
A too high temperature generates a mixed crystal layer that is too thick,
resulting in reduced conductivity and stability.
The components to be soldered and the soldering tip should be clean (just wipe
the hot soldering tip with a wet sponge).
The part with the biggest mass is the most difficult to get to the right
temperature; it should be, therefore, heated up with the soldering tip.
Afterwards both part should be held together and the solder applied. After
about 4 seconds this process should be completed (it takes a bit longer when
using a smaller soldering tip or when soldering large areas).
According to the table above lead-free solder has a melting temperature that
is about 35 degrees higher than leaded solder.
Silver solder prevents this from happening.