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Effects of Crystallographic Orientation of Sn on Electromigration Behavior
Keywords: Electromigration, SnAgCu, Flip-chip
In advanced electronic packaging structure, electromigration is one of the serious problems in fine pitch flip-chip solder joints because the current density of each solder joint increases rapidly as continuously decreasing the dimension of solder joint for high performance and miniaturization. The failure induced by electromigration in flip-chip solder joints are mainly categorized into two modes, the depletion of under bump metallurgy (UBM), such as Cu and Ni, and the formation of interfacial voids between the solder and the intermetallic compounds. Many researchers reported that one of the main reasons about the Cu depletion is the current crowding at an entrance point from a metal trace to a solder bump, and the interfacial voids are formed by Sn migration. However, most of bumps show a different type of failure even if all bumps are tested under the same experimental condition, and the time to failure also different. So, it is important to know when the different types of failure occur. Most of the lead-free solder alloys are mainly composed of Sn, which has a body centered tetragonal crystal structure and highly anisotropic diffusion characteristic. Diffusivity of Cu atoms along the c axis of Sn grain is 500 times faster than that of a axis. Also, the nature of solidification of a Sn-Ag-Cu solder open fit into a few large Sn grains. Therefore, the crystal orientation of Sn grains is believed to play an important role in Sn based solder joints. In the present work, the relationship between the Sn grain orientation and the electromigration behavior were investigated. The test vehicle was a Cu/Sn-3.0Ag-0.5Cu/Cu dummy flip-chip structure, and the applied current density was 15 kA/cm2 at 160 C. The depletion of Cu atoms at the cathode side was a major cause of the circuit failure. Electromigration behavior and the growth of intermetallic compounds were strongly depend on the orientation of c axis of Sn grain with respect to the electron flow. Rapid failure, which caused by Cu UBM depletion, occurred when the c axis of Sn grain almost parallel to the direction of electron flow, also a little microstructural change and improved electromigration properties were observed when the c axis is at a large angle to the direction of electron flow.
Kiju Lee, Ph.D Student
Osaka University
Ibaraki, Osaka 567-0047,
Japan


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