Intermetallic Compound Formation and Diffusion Path Evolution in the Flip Chip Sn-37Pb Solder Bump after Aging

In the flip-chip technology of current microelectronic package, Ni-based under-bump metallurgy (UBM) is widely used due to its slower reaction rate with Sn. In this study, solders joints of eutectic Pb-Sn/Ni-UBM were employed to investigate the intermetallic compound (IMC) formation after aging at 150 degC for various periods of time. The compositions and elemental re-distribution in the IMC formed due to the interfacial reaction between the Ni/Cu UBM and eutectic Sn-Pb solders were deliberately evaluated with an electron probe microanalyzer. The interfacial morphologies were revealed with the aid of a field-emission scanning electron microscope through special etching technique. In the center of the chip side, two IMCs were found between solders and Ni metallization. The scalloped-type IMC was (Cu,Ni)₆ Sn₅, while nodular-type IMC was (Ni,Cu)₃Sn ₄. However, in the edge of the chip side, three IMCs were revealed. The scalloped-type IMC was (Cu_1-y,Ni_y) ₆Sn₅, nodular-type IMC was (Ni_1-x,Cu _x)₃Sn₄, and layer-type IMC was (Cu_1-z,Ni_z)₃Sn. On the basis of elemental distribution in the quantitative analysis for the IMC and the related phase transition among the intermetallic compound formation, two distinct diffusion paths were proposed to illustrate the interfacial reaction and phase transformation between IMCs and solder in the Sn-Pb joints aged at 150 degC. These diffusion paths demonstrated two kinds of phase equilibrium, including (Cu_1-z,Ni_z)₃ Sn/(Cu_{1 - y},Ni_y)₆Sn₅/solder and (Ni_1-x ,Cu_x)₃Sn₄/(Cu_1-yNi _y)₆Sn₅/solder