[Amps] current threads about silver solder and resoldering

[Manfred Mornhinweg]

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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