Question

Define alloy?what are the constituent solder alloy

Answer 1

An alloy is a mixture of metals or a mixture of a metal and another element. Alloys are defined by a metallic bonding character.[1] An alloy may be a solid solution of metal elements (a single phase) or a mixture of metallic phases (two or more solutions). Intermetallic compounds are alloys with a defined stoichiometry and crystal structure. Zintl phases are also sometimes considered alloys depending on bond types 
Solder is an alloy of lead and tin. Low melting point of solder makes it suitable for welding electrical wires.

Answer 2

An alloy is a combination of metals or a combination of one or more metals with non-metallic elements.

This is achieved by melting the alloy and then cooling it down. The choice of specific alloy depends on its melting point, chemical reactivity, mechanical properties, toxicity, and other properties. Hence a wide range of solder alloys exist, and only major ones are listed below. Since early 2000s the use of lead in solder alloys is discouraged by several governmental guidelines in the European Union, Japan and other countries