Break Down Failure Process of the Flip Chip Bumps Caused by Current Stressing

Break down failure process by electromigration in flip chip bump connections under current stressing at certain temperatures was studied. The flip chip bumps used for these experiments consisted of Cu post formed with electroplating and SnAg3.0Cu0.5 solder. Under bump metallurgy was electroless plated Ni(6um)/Au(0.5 um) on Cu pad. Electron flow to induce the electromigration was from organic substrate side (Cu pad) to chip side (Cu post) with current density of 4.0×104A/cm2 at 165 ℃. As a result of that, the failure process of the bumps under current stressing was found to be progressed in several stages. As the first stage, diffusion of Au was significantly occurred within a few hours. In the second stage, Cu6Sn5 intermetallic compound (IMC) formation was enhanced on the surface of the Cu post and the IMC was reached to the surface of the under bump metallurgy. In the third stage, this stage was frequently occurred at the same time of the second stage, Ni diffusion into Cu6Sn5 IMC and the formation of (Cu,Ni)6Sn5 IMC were simultaneously occurred. In the last stage, narrow gaps and voids were generated at the intra- and interfaces of (Cu,Ni)6Sn5 IMC.