Mechanical strength and failure characterization of Sn-Ag-Cu intermetallic compound joints at the microscale

Continuous miniaturization of microelectronic devices has led the industry to develop interconnects on the order of a few microns for advanced superhighdensity and three-dimensional integrated circuits (3D ICs). At this scale, interconnects that conventionally consist of solder material will completely transform to intermetallic compounds (IMCs) such as Cu ₆Sn ₅. IMCs are brittle, unlike conventional solder materials that are ductile in nature; therefore, IMCs do not experience large amounts of plasticity or creep before failure. IMCs have not been fully characterized, and their mechanical and thermomechanical reliability is questioned. This study presents experimental efforts to characterize such material. Sn-based microbonds are fabricated in a controlled environment to assure complete transformation of the bonds to Cu ₆Sn ₅ IMC. Microstructural analysis including scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD) is utilized to determine the IMC material composition and degree of copper diffusion into the bond area. Specimens are fabricated with different bond thicknesses and in different configurations for various tests. Normal strength of the bonds is measured utilizing double cantilever beam and peeling tests. Shear tests are conducted to quantify the shear strength of the material. Fourpoint bending tests are conducted to measure the fracture toughness and critical energy release rate. Bonds are fabricated in different sizes, and the size effect is investigated. The shear strength, normal strength, critical energy release rate, and effect of bond size on bond strength are reported.