Soldering terminology

Also known as Gibbs' phase rule after its developer, J. W. Gibbs (1839-1904). It states that if there is a thermodynamic equilibrium in a heterogeneous ( multi-phase) system, the chemical potentials of substances in the system must be equal. Let p be the number of phases in the heterogeneous system and c the number of substances. Then:

f=c-p+2

Here, f is the number of degrees of freedom (independently changeable state variables).

The system consists of a single substance (c = 1), present in liquid, solid or gaseous form (p = 3), then f = 0, i.e. the temperature and pressure are constant. If the system is fixed and the pressure is constant, the degree of freedom is less : f = c - p + 1.

During the actual soldering, the diffusion zone is not a system in equilibrium and so the phase rule is not applicable. However, it still applies for the solder itself.