Electromigration in thin-film solder joints

Intermetallic compound (IMC) formation and growth are critical reliability issues with interconnected microelectronics, especially for small-scale solder joints in three-dimensional integrated circuits (3D IC) packaging. In this study, the growth behavior of IMCs in ultrathin 3D-IC solder joints under electromigration was investigated using a field-emission scanning electron microscope (FE-SEM) equipped with an electron backscatter diffraction (EBSD) analysis system. The joints consisted of a Cu/Sn/Cu configuration with a Sn thickness of ~ 30 μm. We determined a strong dependence of the IMC growth and Cu pad depletion on the β-Sn orientation upon electron current stressing. When the c-axis of the Sn grain was oriented in the direction of the electron flow, aggressive CuSn IMC growth and Cu pad depletion occurred at the anode and cathode, respectively. A mathematical analysis based on crystallography and electromigration theory was used to rationalize the strong dependences that were observed.