The electromigration-induced failure in the composite solder joints consisting of 97Pb–3Sn on the chip side and 37Pb–63Sn on the substrate side was studied. The under-bump metallization (UBM) on the chip side was 5 μm thick electroplated Cu coated on sputtered TiW/Cu and on the substrate side was electroless Ni/Au. It was observed that failure occurred in joints in a downward electron flow (from chip to substrate), while those joints having the opposite current polarity showed only minor changes. During electromigration, in addition to the compositional change by the moving of Pb atoms in the same direction as the electrons, current crowding was observed inside the UBM and it enhanced the phase transformation of Cu to Cu3Sn and to Cu6Sn5 at the UBM/solder interface. Due to the growth of Cu6Sn5, the Cu UBM was consumed rapidly, resulting in void formation-induced failure at the cathode side. The Cu6Sn5 intermetallic compound and void were first initiated from the upper left-hand side corner of the contact window which matches the current crowding region. The sequence of Cu UBM consumption and void formation is presented. The current crowding has been confirmed by simulation. The mechanism of electromigration-induced failure in the composite solder joint structure is discussed.

In advanced electronic products, current crowding induced electromigration failure is one of the serious problems in fine pitch flip chip solder joints. To explore a strong resistance against current crowding induced electromigration failure, a very thick Cu column bump combined with a shallow solder interconnect at 100μm pitch for flip chip applications has been studied in this paper. Results revealed that these interconnects do not fail after 720h of current stressing at 100°C with a current density of 1×104A∕cm2 based on the area of interface between Cu column bump and solder. The reduction of current crowding in the solder region by using thick Cu column bumps increased the reliability against electromigration induced failure. The current distribution in a flip chip joint of a Cu column bump combined with a shallow solder has been confirmed by simulation. However, Kirkendall void formation was found to be much serious and enhanced by electromigration at the Cu∕Cu3Sn interface due to the large Cu∕Sn ratio. Since this is a system of a limited amount of Sn and an infinite supply of Cu, the Cu6Sn5 transforms to the Cu3Sn after all the Sn content in the solder bump is consumed and the Cu3Sn can grow very thick; the vacancy flux that opposes the Cu flux will condense to form Kirkendall voids. The mechanism of electromigration induced Kirkendall void formation in the Cu column with the shallow solder joint is discussed. Furthermore, a very large temperature gradient exists across the shallow solder interconnects, leading to thermomigration. Electromigration accompanied by thermomigration could replace current crowding as a serious reliability issue in using Cu column based interconnects.

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Wetting reaction between molten Sn-based solders and Cu produces scallop-type Cu6Sn5. In the present wetting study, a (001) single crystal Cu is used as substrate and a dramatic change in the morphology of Cu6Sn5 is observed: instead of scallop type, the authors observed a rooftop-type Cu6Sn5 grains, elongated along two preferred orientation directions. This was confirmed by electron beam backscattered diffraction and white beam synchrotron x-ray microdiffraction. The results indicate that the nucleation, growth, and ripening behavior of Cu6Sn5 on single crystal substrate can be quite different from the conventional case of wetting on randomly oriented polycrystalline Cu substrates.

The electromigration of flip chip solder joints consisting of 97Pb–3Sn and 37Pb–63Sn composite solders was studied under high current densities at room temperature. The mean time to failure and failure modes were found to be strongly dependent on the change in current density. The composite solder joints did not fail after 1month stressed at 4.07×104A∕cm2, but failed after just 10h of current stressing at 4.58×104A∕cm2. At a slightly higher current stressing of 5.00×104A∕cm2, the composite solder joints failed after only 0.6h due to melting. Precipitation and growth of Cu6Sn5 at the cathode caused the Cu under bump metallurgy to be quickly consumed and resulted in void formation at the contact area. The void reduced the contact area and displaced the electrical path, affecting the current crowding and Joule heating inside the solder bump. Significant Joule heating inside solder bumps can cause melting of the solder and quick failure. The effect of void propagation on current crowding and Joule heating was confirmed by simulation.