Soldering terminology

If two metals (A and B) are mixed in a predetermined ratio to obtain an alloy, the melting point of the alloy is less in some cases than the melting points of the individual metals. The alloy with the lowest melting point is called eutectic alloy. A euctetic composed of two metals (A and B) is a binary eutectic while a eutectic consisting of A, B and C metals is a ternary eutectic: E euctetic Cooling HeatingA+B.

The illustration shows the state diagram (also known as phase diagram) of the alloy of lead and tin. It shows that a eutectic is formed at a tin content of 63 % (mass fraction ) and a corresponding amount of lead. While the melting point of lead is 327 °C and the melting point of tin is 232 °C, that of the eutectic alloy is 183 °C. This is the most common alloy used for solders. In this eutectic alloy, lead and tin are merely mixed with each other very finely , but do not react chemically. Therefore, eutectic alloys are regarded as mechanical mixtures. The eutectic  is the alloy with the lowest potential energy. Note that while Sn and Pb simultaneously crystallize from the melt at the eutectic temperature, this does not happen instantaneously, but takes some additional time. In the zones towards the ends of the phase diagram, Sn forms a solid solution in Pb, and vice versa.