Detailed Phase Evolution of a Phosphorous-Rich Layer and Formation of the Ni-Sn-P Compound in Sn-Ag-Cu/Electroplated Ni-P Solder Joints

The interfacial microstructure of Sn-3Ag-0.5Cu/Ni-P with various phosphorous contents was investigated by analytical transmission electron microscopy (TEM) and field-emission electron probe microanalysis (FE-EPMA). As the Ni-Sn-P compound was formed between the solder matrix and Ni-P under bump metallization (UBM), the so-called phosphorous-rich layer was transformed to a series of layer compounds, including Ni3P, Ni12P5, and Ni2P. The relationship between Ni-Sn-P formation and the evolution of P-rich layers was probed by electron microscopic characterization with the aid of the Ni-P phase diagram. In addition, the thickness of Ni-P UBM affects the Ni-Sn-P formation. Based on TEM, selected-area diffraction, as well as FE-EPMA, the detailed phase evolution of P-rich layers in the Sn-Ag-Cu/Ni-P joint was revealed. Moreover, in consideration of the mechanical properties of the joint, Ni-Sn-P phase formation, and fabrication feasibility of Ni-P UBM, the phosphorous content and suitable thickness of Ni-P UBM are discussed.