Microstructural evolution in Pb-free solder microjoints

Both interfacial and bulk microstructures are important to the behavior of solder joints. The interfacial microstructures of Pb-free solder joints require study since these involve novel intermetallic phases and interfacial reactions and that are not well characterized. Moreover, miniaturization of the solder joints has the consequence that the joints themselves may have only a few grains in the cross-section, making their mechanical properties difficult to predict or reproduce. The present paper discusses the microstructure and the evolution of microstructure in several new Pb-free joints, including Sn-Au and Sn-Ag solders on Cu and Ni substrates. The nature and morphology of the intermetallic phases developed at the interface is discussed along with their development during exposure at moderate to high temperature. The microstructure of the bulk is described and related to the solidification conditions. Its evolution on annealing at high temperature is discussed and contrasted with its evolution during periodic thermal cycling.