Metallurgical questions about alloying of Sn with Cu/Ag/Au

[Archie Medes]

Hi

My question relates to electronics soldering, where the main metal in the solder is Sn, with small amounts of Ag and/or Cu

If a rod of Cu, Ag, or Au is placed in molten Sn, which of the rods would melt away fastest?

Is there a threshold temperature up to which the solid metals won't melt/alloy? Or do they start alloying as soon as they are placed in the molten Sn?

For solid Cu or Ag, if some of the different metal is already alloyed with the molten tin, how will it change the alloying process?

And most importantly (for my curiosity), why?

Thank you,

Archie

 

[Henryk]

Gold dissolves in molten tin very, very quickly. Once, someone suggested to me to try a regular solder to attach a piece of a gold wire, didn't work, as soon as I touched the wire with the solder, it was all dissolved by the molten tin.
I don't know about silver.
Copper does alloy with tin. This is the basis for tin-based soldering of copper. It creates a region of Sn/Cu alloy and that helps creating a strong bond.

 

[Archie Medes]

Thank you

I forgot to stress it is the silver I am most interested in, because I can't find information on it either, whereas gold and copper information (anecdotally at least) seems much more available.

Reason I ask is for fashioning custom soldering heads. I want to know how silver behaves compared to copper, and why. I am slowly collecting my own anecdotal data, but I don't have much soldering to do, and I'm curious about the chemistry/physics. Cu,Ag,Au fall under the same column in the periodic table, with Sn being on the middle row with silver, and something about silver requiring more energy to free electrons, but that is about all I've got.

 

[Henryk]

Here is an article, phase diagrams of tin with various metals including Cu, Ag, Au. It appears that tin-silver diagram is very similar to tin-copper. Gold is much different.
http://iweb.tms.org/PbF/JOM-0212-45.pdf

 

[Archie Medes]

Thank you :)

 

[dipstik]

If a rod of Cu, Ag, or Au is placed in molten Sn, which of the rods would melt away fastest?
-whichever has the lowest melting temp for a given composition

Is there a threshold temperature up to which the solid metals won't melt/alloy? Or do they start alloying as soon as they are placed in the molten Sn?
-they will melt at the melting temp and dissolve according to phase diagrams. diffusion can be calculated using ficks laws.

For solid Cu or Ag, if some of the different metal is already alloyed with the molten tin, how will it change the alloying process?
-the phases will depend on the phase diagram for the given concentrations.

And most importantly (for my curiosity), why?
-phase diagrams can be derived by evaluating gibb's free energy for given concentrations and temperatures, and finding the phase energetically favorable for the state of the system. the dynamics governed by ficks are related to activation energies (thermal drivers), concentration gradients (chemical potential) and other thermodynamic phenomena.