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Electro-Migration Behavior in Micro-Joints of Sn-57Bi Solder and Cu Post Bumps
Keywords: Electro-migration, Sn-Bi solder, intermetallic compound
The bonding technique for High density Flip Chip(F.C.) packages requires a low temperature and a low stress process to have high reliability of the micro joining. Sn-Bi solder has been noted as a low temperature bonding material. Electro-migration behavior of Sn-57Bi flip chip interconnection with Cu post bumps was investigated. The flip chip bumps used for this experiments consisted of Cu post formed with plating and Sn-57Bi solder. Two types of under bump metal(UBM) of organic substrate were studied, that is, Cu pad and electroless Ni(6um)/Au(0.5 um) on Cu pad. Electron flow to induce the electro-migration was from organic substrate side (Cu pad) to chip side (Cu post) with current density of 40000A/cm2 at 125 degree C. Both the case, Bi migrated to accumulate on the anode side (Cu post) and Sn migrated to the cathode side(substrate pad). And interconnect resistance has increased until about 150 hours. However, after that it was stabilized for more than 3000 hours. With Ni/Au UBM pad, Cu3Sn/Cu6Sn5 intermetallic compounds(IMCs) were formed at the Cu bump side. And under the Bi layer Cu6Sn5/Ni-Sn compound were formed. But we didn't observe the failure like cracks or voids at the Ni layer. With Cu pad, only Cu3Sn IMC at the Cu bump side and under the Bi layer Cu6Sn5/Cu3Sn compound were formed after 4000 hours, respectively. Although the voids were observed at Cu3Sn/Cu interface, good electrical connection was obtained.
Kei Murayama,
Shinko Electric Industries Co., LTD.
Nagano-shi, Nagano 381-0014,
JAPAN


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