The mechanism involved in the formation of the solder bond and the source of its strength is described. The soldering of two metals requires the establishment of an interface where the solder is bonded to each of the two metals, thus giving the desired metallic continuity in the solder joint. The formation of the interface in a solder joint requires three elements: the solder alloy itself, the base metal, and the flux or atmosphere in which the process takes place. The thermodynamic equilibrium between the surface energies, and the use of the dihedral angle in the evaluation of wetting conditions are described. A simplified atomic picture is used to illustrate the structure of the solder joint and to explain the source of its strength. Intermetallics, alloying, and mutual diffusion are also discussed. Finally, the paper shows the sequence of events leading to the solder joint formation and indicates how the wetting information can be translated into a practical quality control system.