A Lecture on Soldering By The Soldering Craftsman 1-2

Soldering tin is an alloy of stannum (Sn) and plumbum (Pb) in proportion of 6 to 4, the fusing point of which is as low as 183. Nowadays, the tin solder containing lead is mostly called eutectic crystal solder or lead-containing solder, and either of the descriptions is used for being discriminated from the Pb-free soldering tin.

2.1 Eutectic

When two or three metals are combined, the mixed metal with the lowest fusing point is called eutectic alloy. The above is a simplified sketch of a two-element alloy. There is no need to memorize it specially. Now what I am emphasizing is that when the lead containing rate is 100%, the fusing point is 327.5℃; when the tin containing rate is 100%, the fusing point is 231.9℃.
It’s a common sense that when the two metals are mixed, the fusing point will be somewhere in between at around 270℃. So it is incredible but true that in the case of eutectic alloy with tin and lead compounded in the proportion of 6 : 4, the fusing point will be as low as 183℃.
It is like water turning into solid (ice) at 0℃, but if some salt is added into the water, it will not turn into ice at 0℃. The scenario is the same with metals.