[Amps] current threads about silver solder and resoldering

[Bill Fuqua]

Right now I am speaking of lead and tin only.
  With out a uniform ratio the plastic region is extended over a broader 
range of temperature. This is where some material is melted and some is not 
on a microscopic scale.
As one materials science book put it, the atoms exchange electrons and 
"loosely" form molecules but in a way that you can call them compounds. 
When the ratio is exactly 3 and the alloy is homogenous the
plastic region is completely eliminated. It is more than a mixture of 
materials, it is an alloy. When you plate lead and tin on a PC board it is 
not an alloy it has to be fused, melted,
so that the materials can diffuse though each other to become a homogenous 
alloy.
   I did not say that all integer ratios are eutectic, only 3.


At 04:15 PM 9/2/2012 +0000, you wrote:
>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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