Why is the melting point of solder lower than Lead or Tin. 3

[Gav1]

I would like to know why solder has a lower melting temperature than the lowest melting temperature of either alloy Lead and Tin, also are there any other alloy combinations that exhibit these characteristics.

 

[phex]

can't really answer that question as i'm no materials specialist, but i'd like to point out the formation of minimum boiling azeotropes in liquid-liquid mixtures. there's no real cause why metal alloys should not behave like that.

hth,
chris

 

[Retracnic]

Unfortunately, I know the answer to this one. I say 'unfortunately' because I learned it spending my youth working at an electronics plant doing, guess what- That's right, soldering.

Basically the alloy of tin and lead is not crystalline in structure, unlike most pure metals (and many alloys). This has the effect of lowering the melting point because the atoms can execute a phase change much more easily when not bound into a lattice structure. Also once the molten solder comes into contact with certain other metals (notably copper), chemical reactions begin that speed the flow of solder.

Good luck, and watch those cold solder joints.
Bryan Carter.

 

[25362]

Another explanation I found in a general chemistry book: An alloy is a homogeneous mixture of two or more metals. Their structures are more complicated than those of pure metals because the two or more types of metal atoms have different sizes. The packing problem is akin to that of storekeeper trying to stack oranges and grapefruit in the same pile. Some alloys are softer than the component metals. The presence of bigger lead atoms (radius 180 pm) compared with tin's (radius 145 pm) help to soften and lower the solder's MP, much as melons destabilize a stack of oranges.

A low MP alloy of lead, tin, and bismuth is used to control water sprinklers in certain fire-extinguishing systems.

 

[baglamas]

All alloys have a melting point lower than that of the pure elements they contain. Just look at the Pb-Sn phase diagram:

http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/pics/Pb_Sn.GIF

 

[25362]

To dangelop, I beg to disagree. Although most alloys behave as you say, there are alloys with melting points higher than at least one of the components. For example, Pt-Ir, "constantan" (Cu-Ni 55-45), bronzes, brasses, etc.