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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).
The solder flux should have an angle of less than 25 degrees (as in the drawing above) in relation to the starting point. A well soldered joint is also mechanically solid.
From July 2006: lead-free soldering
As per European Law solder has to be lead-free from July 2006 onwards. Generally, lead-free solder has a higher soldering temperature than leaded solder.
Since metal diffuses during soldering, the silver of silver plated surfaces (silver plated ceramic in top quality equipment, silver plated copper cable) is lifted and dissolved in the solder, especially when soldered repeatedly.
Silver solder prevents this from happening.