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I am trying to research potential modifications to lead-free solders (intended to be used in electronics). Base alloy is eutectic $\ce{Bi_{58}Sn_{42}}$, and I am going to add various combinations of a bit of copper, silver, gold, platinum, gallium, lithium, and indium.

Is there something there which is known to not work? I am wondering this because I see that gold, platinum, gallium, and lithium are not typically used as dopants for electronic soldering alloys. Should I expect a lot of problems attributed to intermetallic compounds? (I know there are issues due to a Sn-Cu reaction, but nothing about the rest).

Is it theoretically possible to find a combination with significantly high copper content (> 10-20%) while still having low melting temperature (< 250 $^{\circ}\mathrm{C}$)?

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Lithium might be tricky to alloy in an oxidizing atmosphere (e.g. air), and Pt might take some effort to form an alloy, because of its high m.p., which, however, is still less than than the boiling points of Bi or Sn. Other than that, there should be little trouble forming alloys.

You can search on line for "phase diagram alloy components" to find the melting point for at least some of the alloys. E.G. an online search for

phase diagram "copper-indium" alloy

turned up this link at Springer-Verlag GmbH, with the diagram below.

phase diagram Cu-In alloy

That said, I would expect the "10-20 % copper" requirement precludes melting point less than 300 C, even with low-melting In, Hg, Ga etc.

BTW, I would not use "dopant" for a component of an alloy that will not appreciably change its electronic properties; it is just another component, be it minor, of a ternary, quaternary etc. alloy.

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