Use of multicomponent phase diagrams for predicting phase evolution in solder/conductor systems

Although the complete phase equilibrium is never reached in interconnection applications, the assumption of local equilibrium at the interfaces is generally valid in most systems composed of dissimilar materials. Therefore, the tie lines in ternary (or multicomponent) phase diagrams-together with the relevant stability diagrams and the mass balance requirements-can be used for predicting the phase sequences (i.e. diffusion paths) formed, for example, in solder/conductor joints. Generally, binary phase diagrams alone cannot, provide sufficient information on the phase formation in solder/conductor systems because they do not bear any information on the relative stabilities between different binary phases in multicomponent systems. As examples, the formation of intermetallic compounds in several solder/conductor systems with Au- or Cu- metallization was studied with the help of ternary phase diagrams as well as experimentally. The phase diagrams were calculated using thermodynamic methods. The experimental results confirmed that the dependence of formation of intermetallic compounds on temperature and solder composition is clearly represented by the phase diagrams supplemented with the stability diagrams.