J. O. Suh

Mechanism of electromigration-induced failure in the 97Pb–3Sn and 37Pb–63Sn composite solder joints

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 ...

Electromigration in flip chip solder joints having a thick Cu column bump and a shallow solder interconnect

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 ra...

Effect of current crowding and Joule heating on electromigration-induced failure in flip chip composite solder joints tested at room temperature

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...

Preferred orientation relationship between Cu6Sn5 scallop-type grains and Cu substrate in reactions between molten Sn-based solders and Cu

A strong crystallographic orientation relationship between the Cu6Sn5 scallop-type grains and their Cu substrate has been found by synchrotron micro-x-ray diffraction study. Even though the crystal structures of Cu6Sn5 (monoclinic) and Cu (face-centered-cubic) are very different, angular distributions of crystallographic directions between Cu6Sn5 and Cu revealed a strong orientation relationship. Both SnPb solder and pure Sn showed the same result, indicating that this is general behavior between Sn-based solders and Cu. The strong orientation relation suggests that Cu6Sn5 forms prior to Cu3Sn in the wetting reactions. A total of six different orientation relationships were found. In all the cases, the [1¯01] direction of Cu6Sn5 preferred to be parallel to the [110] direction of Cu with a misfit of 0.24%. Due to pseudohexagonal structure of the Cu6Sn5, the six relationships can be categorized into two groups. From the orientation distribution, one group was found to be less rigid then the other group.

Electromigration Study in Flip Chip Solder Joints

We have studied the effect of thickness of Cu under bump metallization (UBM) from 5 mum, 10 mum to 50 mum on electromigration induced failure mechanism in flip chip solder joints. In the case of 5 mum Cu UBM, due to the direct current crowding effect at the UBM/solder interface, the failure mode induced by electromigration was the loss of UBM and the interfacial void formation at the cathode contact interface between the interconnect line and the solder bump. The current crowding effect in flip chip solder joints were reduced when the Cu UBM thickness was increased to 10 mum, and the flip chip joint with 10 mum Cu UBM showed much longer mean-time-to-failure than that with 5 mum Cu UBM because the 10 mum Cu UBM was enough to contain the current crowding inside the UBM. However, even when the current distribution was uniform in the solder area in this case, the final failure mode was the same as the case of 5 mum UBM. The failure occurred by a two-stage consumption of the 10 mum thick Cu UBM in the joint where electrons flowed from the chip to the substrate. In the first stage, the 10 mum Cu UBM dissolved layer by layer at the entire Cu UBM/solder interface. After half of Cu UBM was dissolved, the asymmetrical dissolution of Cu UBM was concentrated at the corner where electrons entered from Al interconnect to Cu UBM. A small number of Kirkendall voids were found in the Cu3Sn layer during the electromigration testing, but they were not a serious concern in this case. When the Cu UBM was 50 mum thick, more uniform distribution of current density was obtained in the solder bump, and the flip chip joint showed a very strong resistance against electromigration-induced failure. In the case of a 50 mum thick Cu UBM combined with a 20 mum height shallow solder interconnect, the flip chip joint did not fail after 720 hours of current stressing at 100degC with a current density of 6.75 x 104 A/cm2 calculated at the pad opening on the Si chip side. The reduction of current crowding in the solder region by using thick Cu UBM and the small ratio of Sn to Cu in the bump structure improved the reliability against electromigration-induced failure. The effect of Cu thickness on reducing current crowding during electromigration has been confirmed by simulation. However, in the case of 50 mum thick Cu UBM, Kirkendall void formation was found to be much more serious and the formation was enhanced by electromigration at the Cu/Cu3Sn interface due to the large Cu/Sn ratio. 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 50 mum thick Cu UBM based interconnects. To provide a comparison for the microstructure evolution caused by electromigration and by thermal annealing alone, a flip chip joint with 50 mum thick Cu UBM and with 20 mum height shallow solder bump was investigated without any current stressing but with aging at 150degC for 720 hours. In the case of thermal aging, thick Intermetallic Compounds (IMCs) were observed not only under the Cu column bumps but also on the sidewall finish of the substrate because there was no electromigration and no temperature gradient for thermomigration between the chip and the substrate. The Kirkendall void formation was not very serious in the case of thermal aging as compared to the case of electromigration because Sn moved to both sides to form IMCs during thermal aging.

Effect of electromigration on mechanical behavior of solder joints

The combination of electromigration effect and stress effect was investigated in lead free solder joints with a diameter of 300/spl mu/m. One dimensional structures, metal (wire)-solder (ball)-metal (wire) was developed. Mechanical force and current could be applied serially or simultaneously. The current density of electromigration was 1/spl sim/5/spl times/10/sup 3/ A/cm/sup 2/. The working temperature was 100-150/spl deg/C. Tensile test and shear test were taken before and after electromigration to make the comparison. The tensile strain rate was 3/spl mu/m/min. The experiment results show that, the samples broke at the middle of solder without applying current. However, after applying current of electromigration for 1 day, 2 days or longer, the failure always occurred at cathode interface. And the tensile strength was lower with longer electromigration time or higher current density. Shear test also illustrates the electromigration effect on mechanical property in composite solder joints.

Electromigration in Pb-free solder bumps with Cu column as flip chip joints

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 mum thick and 50 mum diameter for eutectic SnPb solders and 80 mum thick and 85 mum diameter for eutectic SnAg solders. We observed that these flip chip joints did not fail after 1 month at 100 degC with a current density of 1times10 4 A/cm2. 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/Cu 3Sn interface in the eutectic SnPb case due to t he smaller amount of Sn 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

Effects of current density on electromigration-induced failure in flip chip composite solder joints at room temperature

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 MTTF (mean time to failure) and failure modes were found to be strongly dependant on the change of current density. The composite solder joints did not failed after one month stressed at 4.07 /spl times/ 10/sup 4/A/cm/sup 2/, but they failed after 10 hours of current stressing at 4.58 /spl times/ 10/sup 4/ A/cm/sup 2/. At a slightly higher current stressing of 5.00 /spl times/ 10/sup 4/ A/cm/sup 2/, they failed after only 0.6 hours by the melting of the composite solder bumps. Due to the precipitation and growth of Cu/sub 6/Sn/sub 5/ intermetallic compound at the cathode side, the Cu under bump metallurgy (UBM) was quickly consumed and followed by void formation at the contact area. The void reduced the contact area and displaced the electrical path, thus it affects greatly the current crowding and Joule heating inside the solder bump. A large Joule heating inside solder bumps can cause melting of the solder bump and the failure occurred quickly. The effect of void propagation on current crowding and Joule heating has been confirmed by simulation.

Morphology, growth and size distribution of Cu/sub 6/Sn/sub 5/ intermetallic compound by flux-driven ripening at SnPb solder and Cu interface

The size distribution and growth of scallop-type Cu/sub 6/Sn/sub 5/ intermetallic compound at the interface between molten SnPb solder and Cu was investigated, along with morphological change of Cu/sub 6/Sn/sub 5/ according to SnPb solder composition. Cu/sub 6/Sn/sub 5/ showed round scallop-type morphology when SnPb solder composition was from eutectic (63Sn37Pb) to about 40Sn60Pb. In other compositions, the intermetallic compounds showed faceted morphology. This change of morphology is due to variation of interfacial energy with solder composition. The scallop growth rate is proportional to cube root of time, and size distribution is in good agreement with the flux-driven-ripening (FDR) theory. Comparison with other works showed that intermetallic compound scallops with shape close to hemisphere give better agreement with the FDR theory.

Synchrotron radiation based X-ray micro-diffraction study on reliability issues of solder joints in electronic packaging technology

Orientation distribution of Cu⁶Sn⁵ and its relationship with orientation of substrate Cu was studied with synchrotron based micro X-ray diffraction. We have obtained Laue spots both from Cu⁶Sn⁵ and Cu at the same time. From the Laue pattern, orientation distribution maps of Cu⁶Sn⁵ and Cu were obtained. Orientations Cu⁶Sn⁵ scallops had strong dependence to that of Cu. [001] direction of Cu⁶Sn⁵ was always parallel to [110] of Cu, to minimize misfit. It was also found that there are loses in its texture as reflow time increases