Dissolution in service of the copper substrate of solder joints

It is well known that during service the layer of Cu₆Sn₅ intermetallic at the interface between the solder and a Cu substrate grows but the usual concern has been that if this layer gets too thick it will be the brittleness of this intermetallic that will compromise the reliability of the joint, particularly in impact loading. There is another level of concern when the Cu-rich Cu₃Sn phase starts to develop at the Cu₆Sn₅/Cu interface and an imbalance in the diffusion of atomic species, Sn and Cu, across that interface results in the formation at the Cu₃Sn/Cu interface of Kirkendall voids, which can also compromise reliability in impact loading. However, when, as is the case in some microelectronics, the copper substrate is thin in relation to the volume of solder in the joint an overriding concern is that all of the Cu will be consumed by reaction with Sn to form these intermetallics. This paper reports an investigation into the kinetics of the growth of the interfacial intermetallic, and the consequent reduction in the thickness of the Cu substrate in solder joints made with three alloys, Sn-3.0Ag-0.5Cu, Sn-0.7Cu-0.05Ni and Sn-1.5Bi-0.7Cu-0.05Ni. A simple model developed for the reduction of the Cu thickness as a result of diffusion controlled reaction with Sn to form Cu₆Sn₅ was found to fit the experimental data well. The results reported in this paper provide an example of the way in which microstructural features that can affect joint reliability are affected by small alloying additions.