Bump resistance measured by Kelvin bump probe

The Kelvin probes are able to detect the early stage of void formation. Figure 29 shows the SEM image for another bump stressed by 0.8 A at 150°C for 37.8, 110.2, 177.8 and 384.0 hours. The bump resistance increased from 0.60 mΩ to 0.62 mΩ after the current stressing, which corresponds to a increase of 1.03 times. Small voids started to form under the IMC layer at the left corner of the passivation opening, where current crowding occurred most seriously in Figure 29(a). Another bump was stressed at the same condition for 110.2 hours, and the current was terminated when the bump resistance reached 1.2 times of its initial value.

The cross-section image was shown in Figure 29(b). Larger voids were found in the interface of the solder and the IMC. The voids almost depleted the passivation opening. However, the bump resistance increased to only twice of its initial value, which was about 0.1 mΩ. Under

the stressing conditions, voids started to form at approximately 5% of the failure time, and they grew for the rest of the stressing time. The incubation time for void formation is relatively short compared with the failure time. This may be attributed to the fact that the cross section of the UBM opening is quite large, and thus it takes time for the voids to propagate and deplete the UBM opening.

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3.5 Investigation of void nucleation and propagation on Joule heating effect

To verify the void nucleation and propagation on Joule heating effect on electromigration,

A sample was stressed for different lengths of time and then examined by Kelvin bump probes, x-ray microscopy and infrared microscopy. Figures 31(a) to 31(d) show the x-ray images of the sample stressed at 0.8 A at 150 °C for 0, 50.0, 941.4, and 1087.9 h, when the bump resistance reached 1, 1.2, 6 and 10 times of its initial value. Larger voids were formed with the increase in stressing time, and the voids propagated from the left hand side to the right-hand side. In addition, the voids became irregular as they propagated, as shown in Figure 31(b) to 31(d).The corresponding IR images were shown in Figure 32(a) to 32(d).

Figure 33 shows that the temperature increases in the Al pad for the bump B3 in Figure 11(d) when stressed by 0.8 A through bumps B2 and B3. In this case, the average temperature in the Al pad was obtained by averaging the temperatures in a square of 40 μm × 40 μm in the

center of the pad, as illustrated by the dotted lines in Figure 32(a). The average temperature increase due to the current stressing was as high as 14.5 ℃ before current stressing. Figure 33 illustrates the average temperature increase for the four stressing time and it increased with the increase in stressing time. It increased to 15.1 ℃ when stressed by 50 hrs, and it increased to 17.8 ℃when stressed by 941.4 hrs. In addition, the depletion percentages of UBM opening were 41.3%, 75.2%, and 94.1% for the four current stressing times, as labeled in Figure 33.

An interesting finding is that: Figure 34(a) shows the IR image when current stressing time

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is 1267.9 h, when the bump resistance reached 20 times of its initial value. A hot spot on the Al trace was found as higher as 22.2 ℃ under current stressing. To facilitate the observation of the location of the hot spot, the passivation and UBM openings were marked on the image by two dotted white circles. After current stressing at 0.8 A at 150 °C for 1267.9 h, the same sample was examined again by x-ray microscope in Fig. 34(b). Then it was examined by SEM to reveal the location of voids, as shown in Fig. 34(c).

The increase in bump resistance may enhance the local Joule heating effect. At later stages, current crowding may become very serious due to the dramatic of the contact opening.

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Figure 31 Plan-view x-ray images of the joint after stressing for (a) 0 h. (b) 50.0 h.

(c) 941.4h (d) 1087.9 h.

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Figure 32. IR images of the joint after stressing for (a) 0 h. (b) 50.0 h. (c) 941.4 h (d) 1087.9 h.

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Figure 33. The average temperature increase for the four current stressing time

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Figure 34. (a) IR image for the hot spot was found by 1267.9 h

(b) X-ray image of the same joint after current stressing. The hot spot was labeled.

(c) Cross-sectional SEM image of the joint.

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3.6 Failure morphology of the flip chip solder joints due to electromigration

The failure modes for the above stressing conditions are quite similar. Figure 35 shows the cross-section SEM image for the typical failure morphology. The failure always occurred in the bump with electron flow from chip side to the substrate side. The UBM layer was consumed and void formed near the chip side.

Figure 35. Cross-section SEM image for the typical failure morphology Chip

Substrate

e

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Chapter IV Conclusions

The Joule heating effect in the solder joints has been investigated using an IR microscope and a 3D coupled thermal-electrical simulation. The temperature distribution in joints can be determined thoroughly. A hot spot was found in the vicinity of the entrance point of the Al trace, which is detrimental to the electromigration lifetime of the solder joints.

Temperature distributions in the Al traces and the Al pads directly above the solder bumps were measured from 0.1 A to 1.0 A. It is found that the dimension of the Al trace plays a crucial role in the Joule heating effect. Under the same stressing current, the solder joints with longer Al trace had higher temperature increase, and the solder joints with 40-μm-wide Al trace had higher Joule heating effect than those with 100-μm-wide Al trace. These results indicate that the electromigration life time depends strongly on the dimension of the Al trace, since the Al trace is the major heat source.

X-ray microscopy can detect void nucleation and propagation in flip-chip solder joints.

The voids nucleated in the vicinity of the entrance point of Al trace, and their shape was quite irregular. The growth velocity was measured to be around 0.3-1.8 μm/h at various stages under 6.5×10³ A/cm² at 150°C for SnPb solder joints with thick-film Cu/Ni UBM. The Joule heating effect does not increase when small voids are formed. Yet, when larger voids are formed at later stages of electromigration, temperature in solder joints increases significantly.

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References

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List of Tables

Table I. Thermal conductivities, electrical resistivities, and temperature coefficients of resistivity for the materials used in the simulation model.

Table II. Failure time of the solder joints with various lengths of Al traces under current stressing.

Table III. Thermal resistance and material properties used in the lumped thermal resistance model.

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Table I. Thermal conductivities, electrical resistivities, and temperature coefficients of resistivity for the materials used in the simulation model.

Material Thermal conductivity (W/m-°C)

UBM(Ti+Cr/Cu+Cu) 147.61 5.83 4.9

SnAg3.5 33.00 12.3 4.6

Note : The materials not given in electric resistivity are assumed to be electrical insulators.

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Table II. Failure time of the solder joints with various lengths of Al traces under current stressing.

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Table III. Material properties used in the lumped thermal resistance model.

Material Chip Solder Underfill Substrate

Thermal conductivity (W/m °C) 147 50 0.5 0.25

Thermal resistance (°C/W) 0.073 38.9 4.92 85.7

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Publication list

1. S. H. Chiu and Chih Chen, Investigation of void nucleation and propagation during electromigration of flip-chip solder joints using x-ray microscopy, APPLIED PHYSICS LETTERS 89, 262106 (2006) (SCI paper)

2. S. H. Chiu, T.L. Shao , C. Chen, Infrared microscopy of hot spots induced by Joule heating in flip-chip SnAg solder joints under accelerated electromigration. APPLIED PHYSICS LETTERS 88 (2): Art. No. 022110 JAN 9 2006 (SCI paper)

3. S. H. Chiu, D.J. Yao, and Chih Chen, Effect of Al-trace dimension on electromigration failure time of flip-chip solder joints, Journal of Electronic Materials, 35(9): 1740-1744, 2006. (SCI paper)

4. S. W. Liang, S. H. Chiu, and Chih Chen, Effect of Al-trace degradation on Joule heating during electromigration in flip-chip solder joints, APPLIED PHYSICS LETTERS 90, 082103 (2007) (SCI paper)

5. T. L. Shao, S. H. Chiu, Chih Chen, D.J. Yao, and C.Y. Hsu, Thermal Gradient in Solder Joints under Electrical Current Stressing, Journal of Electronic Materials, 33(11):

1350-1354, 2004. (SCI paper)

6. T. L. Shao, Y. H. Chen, S. H. Chiu and Chih Chen, Electromigration Failure Mechanisms for SnAg3.5 Solder Bumps on Ti/Cr-Cu/Cu and Ni(P)/Au Metallization Pads, Journal of Applied Physics, Vol. 96, 8, 4518 (2004) (SCI paper)