Intermetallic compound formation and diffusion path evolution in eutectic tin‐lead flip chip solder bumps after aging

In the flip‐chip technology (FCT) used in current microelectronic packages, a Ni‐based under‐bump metallurgy (UBM) is widely used due to its slow reaction rate with Sn. In this study, solders joints of eutectic Pb‐Sn with a Ni UBM were employed to investigate the intermetallic compound (IMC) formation after aging at 150°C 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 evaluated with an electron probe microanalyzer. The interfacial morphologies were revealed with the aid of a field‐emission scanning electron microscope through a special etching technique.

At the centre of the chip side, two IMCs were found between the solder and Ni metallization. The scalloped‐like IMC was determined to be (Cu, Ni)₆Sn₅, while the nodule‐like IMC was (Ni,Cu)₃Sn₄. However, at the edge of the chip side, three IMCs were revealed. The scalloped‐like IMC was (Cu_1−y,Ni_y)₆Sn₅, the nodule‐like IMC was (Ni_1−x,Cu_x)₃Sn₄, and the layer‐type IMC was (Cu_1−z,Ni_z)₃Sn.

On the basis of the elemental distributions from the quantitative analysis of the IMC and the related phase transitions during the IMC formation, two distinct diffusion paths are proposed to illustrate the interfacial reaction and phase transformation between IMCs and solder in Sn‐Pb joints aged at 150°C. These diffusion paths demonstrated two kinds of phase equilibrium, including (Cu_1−z,Ni_z)₃Sn/(Cu_1−y,Ni_y)₆Sn₅/solder and (Ni_1−x,Cu_x)₃Sn₄/(Cu_1−yNi_y)₆Sn₅/solder.