Soldering

On Friday, during my Physics internship, I learned how to solder! Sure, just a little bit and it took me quite a while and it has little to do with Physics itself, but still.
I had to bind a contact to a solar cell. First one has to apply rosin to both the solder and the soldered material. I don't know, how it precisely works, but it's there to clean the surface and remove oxides. Also, you use different rosin to solder different materials, so I guess it somehow reacts with the impurities.
Temperature is not that high - some 300C, it's because you use a quickly melting alloy to melt and then to get solid again (in my case it was some tin-lead alloy). Reportedly, lead alloys are going to be prohibited for this purpose; I am just not sure how it will help nature, when other alloys have to be heated to temperatures about 100 degrees higher, thus what you get by not introducing lead to nature, you lose by energy consumption...
My work was quite crude and, considering the cell was rather small, I wonder how it is going to work now.