Microstructural influences on thermomechanical fatigue behaviour of third generation high Ag content Pb-Free solder alloys

Abstract

New third generation lead free alloys are being targeted for use in harsh environments requiring resistance to both thermal and mechanical loading. In applications such as the engine compartment of motor vehicles where extreme thermal cycles and long dwell are common, it has been necessary to look for strengthening mechanisms that better survive extremes of temperature and strain. Alloys are being designed to improve toughness of interfacial intermetallic layers and provide solid solution strengthening to compensate for the loss of strength once the Ag3Sn particles coarsen. Results show that while solid solution-strengthening by additions of Sb is effective, it is not sufficient, and other strengthening options through the introduction of additional precipitates and solid solution strengthening using Bi is most desirable. The role of Ni is difficult to discern but it is most concentrated in the Cu6Sn5 intermetallic phase located at the interface between the solder and the copper substrate. This presumably has a toughening effect on the intermetallic thereby inhibiting crack formation and growth.