Bill,
I recalled that the ratios of lead and tin atoms had to be an integer
to for the lowest melting point.
But that's not right either! I hate to be so overly teacherish in this
thread, but facts remain facts!
In all possible compositions of tin-lead alloys, only one single ratio
is eutectic. Exact ratios of atoms such as 1:1, 2:1, 1:2, are NOT
eutectic, and fully melt at a higher temperature than the eutectic mix.
Have a look at these two web pages for more detailed information, or
google for phase diagrams of tin lead solders or other alloys.
http://www.ami.ac.uk/courses/topics/0244_tsm/index.html
http://spaceflight.esa.int/impress/text/education/Solidification/Phase_Diagrams.html
Note that the eutectic point of tin-lead is given as 61.9/38.1, rather
than the conventional 63/37. That's why there is so much 60/40 solder
around. The 63/37 solder is only marginally better. The reason to use
63/37 rather than 61.9/38.1 is in the inability to make an absolutely
homogeneous alloy, and the fact that the pasty phase is shorter if the
mix deviates from the eutectic point toward the tin side, then if it
does toward the lead side. So, if the tolerance of an alloy happens to
be 2%, it's likely that a nominal 63/37 mix will statistically end up
having a shorter pasty range than a nominal 61.9/38.1 mix.
Manfred
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