Fracture surface analysis of aged and drop tested Sn-Ag-Cu solder joints

A propagating crack can follow several different paths, depending mainly on the energy associated with the advancing crack tip. The crack can follow single phase boundaries (grain boundaries), interphase boundaries, or propagate across a continuous lattice. The Sn-Ag-Cu system, at the composition and temperature range common for solder alloys, would have as equilibrium phases nearly pure Sn, Cu6Sn5 and Ag3Sn. However, near the Cu terminals, the local equilibrium involves a much higher Cu content, which leads to the formation of Cu3Sn between the Cu pad and the Cu6Sn5 layer. Also, during aging, the formation of Kirkendall voids between the Cu pad and the Cu3Sn layer can occur. Each propagation path appears with a very specific look, and readily identifiable chemical composition. This paper focuses on providing some guideline on how to identify the relation between crack propagation paths and microstructure on Pb-free solder joints, and what are the visual and chemical characteristics that provide the basis for path identification