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Investigation of the Current Crowding Effects during Electromigration of Lead-Free Flip Chip Solder Joint
Keywords: Electromigration, UBM, flip chip
Electromigration is a critical reliability issue for flip chip solder joints as current density increases with decreasing bump pitch/size. One specific phenomenon in electromigration of flip chip is the current crowding effect that is caused by the non-uniform current density in the solder bump due to the change in electron flow direction at the contact window. This results in selective damage of the UBM, leading to an earlier failure of the solder bump. The most commonly studied current crowding effect is where the highest current density is at the entrance edge of the contact interface. In this presentation, an electromigration study of lead-free Sn-0.7Cu (in wt%) solder bumps on a Cu UBM was performed in the temperature range of 180-200oC. A different current crowding effect was observed at the other edge of the contact interface. This new effect resulted in selective damage of the UBM at the cathode side. Two dimensional computer simulations clearly showed higher current density at this location. The proposed mechanism is that after major current crowding, electron flow all the way to the metallization edge, resulting in an abrupt change in the direction toward the anode. The current density at this location is not as great as at that of primary current crowding so this phenomenon requires thermal diffusion assistance or a higher current density. This is a new phenomenon that can possibly result in failure of flip chip bumps during electromigration.
Jin Wook Jang, Packaging Engineer
Tempe, AZ

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