Ron

If you are mixing 2 metals you get a change in the melting temperature.
Generally said, the pure metals have the highest melting points, mixing
them leads to a decrease in the melting temperature. Hence, there must be a
minimum, the eutectic alloy. You see this effect nicely in the phase
diagram.
Now as you add a third metal you can look at the eutectic alloy as a pure
metal being mixed with another pure metal. This means that the melting
point again is going to be lower and that there will be another minimum,
the ternary eutecticum. You see this effect with Sn63Pb37 ( Melting point
183°C) and Sn62Pb36Ag2 ( Melting point 179 °C ).
If one solders on various pad surfaces ( reflow or repair ) this alloying
will occur and basically the melting point will be lower again. However, it
is not sure whether the surface material of the pad distributes evenly in
the whole solder joint or whether you get local enrichment which may lead
to local formation of intermetallics. The whole metallurgy of soldering is
far from easy. Just to say, Sn62Pb36Ag2 does form alloys with Ni and Au but
these 4-metal alloys (quarternary alloys) are not well known. The problem
you mentioned also becomes important with leadfree solder. As an example I
would like to mention the use of Bi containing solder. These solder alloys
would be a great replacement for tin-lead. However, never let them come
into contact with residues from lead containing solder ( repair ). Bi will
form an alloy with lead with a melting point of 93°C.

Hmmm, maybe nothing is clear now anymore. Well you see metallurgy is a passion.

Best regards

Guenter

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