Browsing by Author "Nah, Jae-Woong"

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Electromigration in Pb-free solder bumps with Cu column as flip chip joints
(2006)
Nah, Jae-Woong
;
Suh, J. O.
;
Tu, K. N.
;
Yoon, Seung Wook
;
Chong, Chai Tai
The electromigration in flip chip joints of Cu column bump with two different kinds of solder bumps; eutectic SnPb or eutectic SnAg was studied. The Cu columns were 50 mu m thick and 50 mu m diameter for eutectic SnPb solders and 80 mu m thick and 85 mu m diameter for eutectic SnAg solders. We observed that these flip chip joints did not fail after 1 month at 100 degrees C with a current density of 1 x 10(4) A/cm(2). In these flip chip joints of Cu column bumps with solder bumps, the current distribution in the solder region was uniform and they had strong resistance against electromigration induced failure. The effect of Cu column bump on current distribution in the solder joint has been confirmed by simulation. When we compared the difference in microstructural change between the Cu column with eutectic SnPb and the Cu column with eutectic SnAg, we found that electromigration enhanced the formation of Kirkendall voids at the Cu/Cu3Sn interface in the eutectic SnPb case due to the smaller amount of Sri in the solder bump. Therefore, the flip chip joints of Cu column bumps with eutectic SnPb solder bumps would be weakened at the interface after high current stressing.
Mechanism of electromigration-induced failure in flip-chip solder joints with a 10-mu m-thick Cu under-bump metallization
(2007-03-01)
Nah, Jae-Woong
;
Chen, Kai
;
Tu, K. N.
;
Su, Bor-Rung
;
Chen, Chih
The electromigration-induced failure in flip-chip eutectic SnPb solder joints with a 10-mu m-thick Cu under-bump metallization (UBM) was studied without the effect of current crowding in the solder region. The current crowding occurred inside the UBM instead of in the solder joint at the current density of 3.0 x 10(4) A/cm(2) because of the spreading of current in the very thick Cu UBM. In these joints, the failure occurred through a two-stage consumption of the thick Cu UBM in the joint where electrons flowed from the chip to the substrate. In the first stage, the Cu UBM dissolved layer by layer rather uniformly across the entire Cu UBM-solder interface. In the second stage, after half of the Cu UBM was dissolved, an asymmetrical dissolution of Cu UBM took place at the corner where electrons entered from the Al interconnect to the Cu UBM. Experimental observation of dissolution steps of the 10-mu m-thick Cu UBM is presented. The transition from the first stage to the second stage has been found to depend on the location of current crowding in the flip-chip joints as the UBM thickness changes during the electromigration test. The current distribution in the flip-chip solder joints as a function of UBM thickness was simulated by three-dimensional finite element analysis. The dissolution rate of Cu UBM in the second stage was faster than that in the first stage. The mechanism of electromigration-induced failure in the flip-chip solder joints with a 10-mu m-thick Cu UBM is discussed.