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Mass extrusion occurs in electromigration at the anode in cross-sectioned Sn-0.7Cu flip-chip solder joints. In a pair of joints, the hillock squeezed out at the anode on the board side is more serious than the whisker grown at the anode on the chip side. The difference of mass extrusion has been found to be affected by the amount of intermetallic compound ͑IMC͒ formation in the solder bump. It is found that when a large amount of Cu-Sn IMCs form in the grain boundaries of Sn grains, small hillocks are extruded on the anode end. It is proposed that the excessive IMC formation may be able to block the diffusion path of Sn atoms, so the growth of both the Sn whiskers and hillocks are retarded.

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