This investigation elucidates stress evolution in situ in tin strips under electromigration using synchrotron radiation x-ray. Minute variations in stress are determined precisely using intense x-rays. Back stresses gradient with the values of 5.5 and 16.5 MPa/cm, which are induced by the current densities of 1 × 103 and 5 × 103 A/cm2, respectively, are measured directly. The effective diffusivities that include both grain and lattice diffusion at various current densities are determined. The Joule heating is observed, ranging from 5 to 15 °C, according to various current densities passed through the stripes. Results of this study suggest that the protective oxide layer on the surfaces significantly influences the kinetics of stress evolution.

Eutectic SnPb solder has been widely used in packaging for several decades. The stability of the interface between solder and under-bump metallization (UBM) is an important issue that has led to many studies. Even though Ni atoms dissolve much slower into SnPb solder than Cu, the intermetallic compound, Ni3Sn4, which forms when eutectic SnPb solder reacts with Ni(V)/Ti UBM, is not stable on Ti layer, creates V-rich zone, and causes spalling. To prevent the phenomenon, and the resulting reduction of mechanical reliability in solder joints, we propose the addition of a layer of Cu thin film to serve as a sacrificial layer. Both eutectic SnPb solder and composite solder (high-Pb solder with eutectic SnPb solder) were studied in severe reflow conditions to simulate the worst case of die attach and later reflow process. Cu film first was consumed completely to form a compound. Due to lower interfacial energy between Cu6Sn5 and Ni(V), the interface was stable and no spalling occurred. However, the same thickness of Cu was insufficient to prevent Ni from diffusing into solder or compound. Not only diffusion of Ni atoms was observed; Sn atoms also diffused into the Ni(V) layer. The Sn–Ni reaction caused the interface between the compound and Ni(V) to retreat into the Ni(V) layer. The compound was not stable at the interface, and spalling could be seen. Due to the interdiffusion of Ni and Sn, many Kirkendall voids were also observed at both side of the interface.

White Sn (β-Sn) thin film stripe shows a voltage drop about 10% when subjected to electromigration testing. Since β-Sn has anisotropic crystal structure, it possesses different resistivity along a-, b- and c axis. The direction of the axes determines the resistance in each grain. Under electromigration, low resistance grain tends to grow in the expense of the neighboring high resistance grains. The changes of grain orientation in the Sn stripe before and after electromigration was studied by synchrotron x-ray microdiffraction (∼1νm diameter) to achieve grain-by-grain analysis. Grain growth involves grain boundary migration and rotation of neighboring high resistance grains. A model different from normal grain growth is proposed to describe the condition and mechanism of microstructural evolution under electromigration.