Mechanism of interfacial reaction for the Sn-Pb solder bump with Ni/Cu under-bump metallization in flip-chip technology

Nickel-based under-bump metallization (UBM) has been widely used in flip-chip technology (FCT) because of its slow reaction rate with Sn. In this study, solder joints after reflows were employed to investigate the mechanism of interfacial reaction between the Ni/Cu UBM and eutectic Sn-Pb solder. After deliberate quantitative analysis with an electron probe microanalyzer (EPMA), the effect of Cu content in solders near the interface of the solder/intermetallic compound (IMC) on the interfacial reaction could be probed. After one reflow, only one layered (Ni1-xCux)(3)Sn-4 with homogeneous composition was found between the solder bump and UBM. However, after multiple reflows, another type of IMC, (Cu1-y,Ni-y)(6)Sn-5, formed between the solder and (Ni1-x,Cu-x)(3)Sn-4. It was observed that if the concentration of Cu in the solders near the solder/IMC interface was higher than 0.6 wt.%, the (Ni1-x,Cu-x)(3)Sn-4 IMC would transform into the (Cu1-y,Ni-y)(6)Sn-5 IMC. The Cu contents in (Ni1-x,Cu-x)(3)Sn-4 were altered and not uniformly distributed anymore. With the aid of microstructure evolution, quantitative analysis, elemental distribution by x-ray color mapping, and related phase equilibrium of Sn-Ni-Cu, the reaction mechanism of interfacial phase transformation between the Sn-Pb solder and Ni/Cu UBM was proposed.