Significantly enhanced shear strength of Sn-Ag-Cu/Co-P ball grid array solder joints by CoSn₃ intermetallic compound

Co-P films with thicknesses of 3.5 and 12.5 μm were used as metallization in Sn-Ag-Cu/Co-P ball grid array solder joints. The solder joints were annealed at 150°C up to 1000 h. The growth and mechanical properties of interfacial intermetallic compounds (IMCs) in the Sn-Ag-Cu/Co-P joints were studied, and the influence of Co-based IMCs on the shear strength of the Sn-Ag-Cu/Co-P joints was analyzed. The shear strength of the Sn-Ag-Cu/Co-P (3.5 μm) and Sn-Ag-Cu/Co-P (12.5 μm) joints after solid-state aging was increased by 19.5% and 88.0% in comparison with the Sn-Ag-Cu/Cu joints, respectively. The shear strength of the annealed Sn-Ag-Cu/Co-P (12.5 μam) joints reached 103.0-116.6 MPa, which was due to the formation and growth of CoSn₃ interfacial IMC. CoSn₃ with a thickness of tens of micrometers formed at the joint interface after annealing, and it possessed much larger hardness and Young's modulus in comparison with Sn-Ag-Cu solder; therefore, the shear strength of the Sn-Ag-Cu/Co-P joints was significantly improved. The experimental data show that CoSn₃ can be used to enhance the shear strength of solder joints and Co-P is a promising candidate for metallization used in Pb-free solder joints.