Interfacial Reactions Between Lead Free Solder Alloys and Metallic Substrates: Thermodynamics and Kinetics

Ternary alloys based on Sn-rich, Sn-Cu and Sn-Ag binary eutectics have attracted considerable attention as potential Pb-free solders.However, intermetallic layers formed at the interface between the solder and pads of a printed circuit board may adversely affect the reliability of solder joints due to excessive growth during storage and service. The inherent complexity of the substrate-solder systems and a large variety of possible reactions in these materials (their sequential and concurrent occurrence) makes it extremely difficult to develop a predictive model that describes quantitatively morphological evolution within the solder joints. This problem becomes more complex with the continuing trend towards increasing miniaturization of microchips.
In this work some fundamental thermodynamic and kinetic aspects of interfacial reactions between Sn-Ag-Cu alloys and Cu, Ni and Cu-Ni substrates, confronted with experimental results, will be discussed:
- How the size and the composition of the solder ball can affect the occurrence of meta-stable equilibria.
- The growth kinetics of intermetallics under meta-stable conditions.
- The role of a change in the driving force of diffusion, due to the presence of meta-stable equilibria, on the growth kinetics of interfacial layers.