A diffusion-kinetic model for predicting solder/conductor interactions in high density interconnections

A combined thermodynamic and diffusion-kinetic approach is very viable for developing microjoining and interconnection materials and processes, in particular, whenever thinner metallizations, coated overlayers, or smaller solder-joint volumes are encountered in very high density electronics. A diffusion-kinetic model based on the utilization of integrated diffusion coefficients and mobilities is introduced and discussed for calculating the layer growth of intermetallic compounds between metal conductors and tin-based solders and is exemplified with a relatively simple ternary Cu/SnBi system. The model has also been used for calculating the local nominal composition of the effective joint or contact region. Moreover, the mobilities of Sn and Cu in Cu6Sn5 and Cu3Sn intermetallic compounds are determined, and the role of both stable and metastable phase diagrams is discussed in predicting the appearance of possible reaction products as well as the driving forces for the dissolution, diffusion, and precipitation processes.