Local Melting Induced by Electromigration in Flip-Chip Solder Joints
C. M. Tsai1, Y. L. Lin1, J. Y. Tsai1, and C. R. Kao1, 2,* 1 Department of Chemical & Materials Engineering 2 Institute of Materials Science & Engineering National Central University Jhongli City, Taiwan (* E-mail:
[email protected] Phone/Fax: +886-3-4227382)
Abstract A new electromigration failure mechanism in flip chip solder joints is reported. The solder joints failed by the local melting of PbSn eutectic solder. The local melting occurred due to a sequence of events induced by the microstructure changes of the flip chip solder joint. The formation of a depression in current crowding region of solder joint induced a local electrical resistance increased. The rising local resistance resulted in a larger Joule heating, which, in turn, raised the local temperature. When the local temperature rose above the eutectic temperature of the PbSn solder, the solder joint melted and consequently failed. The results of this study suggests that a dynamic, coupled simulation that takes into account the microstructure evolution, current density distribution, and the temperature distribution may be needed to fully solve this problem.
Keywords: Electromigration, flip chip, PbSn solder, and local melting.
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1. Introduction The electromigration failure is one of the major reliability concerns in flip chip technology [1-3]. In today’s circuit design, each solder joint will carry more than 0.2 A, and it will reach 0.4 A in the near future. At present, the diameter of a solder joint is about 100 μm, and it will be reduced to 50 μm soon. It means that the average current density in such a 50 μm joint is about 104 A/cm2 when a 0.2 A current is applied. Furthermore, due to the geometry of a flip chip solder joint, current crowding occurs at the solder/interconnect interface, which is also the entrance of the electrons into the joint. The current density in current crowding region is about one order of magnitude higher than the average current density in solder joint [4]. In addition, the current density needed to cause electromigration in eutectic PbSn is two orders of magnitude smaller than that for Al and Cu interconnects [5-7]. These are the reasons why electromigration in flip chip solder joints can be the major reliability problem in microelectronic devices. Many studies had been carried out recently on this subject [8-20]. In the literature, the void formation-and-propagation mechanism has been identified to be a major failure mechanism due to electromigration in flip chip solder joints [8]. In this mechanism, a void first initiates from the current density crowding region. Then it extends along the UBM (Under Bump Metallurgy)/solder interface across the entire contact and
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forms a gap between the UBM and solder. The formation of the gap at the UBM/solder interface induces an open circuit failure. Furthermore, during the propagation of the void, the presence of the void and the microstructure changes induced by the void also affect the electrical characteristics of the solder joint. With the presence of the void, the effective current density increases as the total available cross-sectional area for electrical conduction decreases. In addition, the resistance of the contact area rises due to this smaller conduction area. These two factors make the Joule heating at the contact area increase. This, in turn, raises the temperature of the solder joint. Thus, the temperature of the solder joint increases as the void grows. If the temperature rises above the melting point of the solder before a complete gap forms between the UBM and the solder, the solder joint will melt, which may lead to the failure of the joint. The objective of this study is to see if the melting of the solder may indeed occur. The in-situ approach of observation was used in this study. The samples were cross-sectioned to the center of the solder joints before applying the current. The solder used in this study was the PbSn eutectic solder. The UBM on the chip side was a disk-shaped, thick Cu layer, and the bond pad on the substrate side was coated with the Au/Ni surface finish. The effect of electromigration on the temperature change and the resistance change were monitored. The failure mode of the solder joint was investigated.
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2. Experimental A schematic diagram for the configuration of the samples used is shown in Fig. 1. The solder was the eutectic PbSn, and the solder joint had a normal diameter of 125 μm. The UBM on the chip side was a layer of 8.5 μm thick Cu with a dish-shaped rim, and the metallization on the substrate side was a 0.25 μm Au layer and a 3 μm Ni layer over the Cu conducting trace. The on-chip Cu lines had a width of 50 μm and a thickness of 5.5 μm. The opening diameter of contact window to the solder was 90 μm. In order to have a direct observation of the joint under current stressing, the sample was cross-sectioned to the center of the solder joints before applying current. During electromigration, the applied current was 1 A and the average current density at the 90 μm diameter contact window was 3.1×104 A/cm2. This was a nominal average value, and did not reflect the actual current density distribution in the current crowding region. In Fig. 1, the electrons entered the solder joint from the upper-right corner, went down the solder joint, and exited the joint through the substrate. The current stressing experiment was conducted with the samples placed at the room temperature.
The surface microstructure changes were monitored with a scanning electron microscope (SEM). In order to perform the observation, we had to stop the current-stressing, and place the specimen under the microscope. After the observation, the current-stressing was
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resumed using the same specimen. In other words, the current-stressing was interrupted several times. The reported current-stressing time was cumulative. That is to say that the reported current-stressing time was the summation of all the time that current had passed through the specimen before a specific observation. A detailed description of the experimental details had been reported elsewhere [20].
The temperature on the topside of the chip was monitored using a thermocouple (Fig. 1). The resistance change of the joints was monitored by measuring the voltage drop across the joint. Knowing the applied current, which was constant, the resistance could then be calculated from the voltage drop across the joint. The temperature and resistance measurements were performed on samples that were different from those used for in-situ microstructure observation. The reason for doing so was because during in-situ microstructure observation the current stressing had to be interrupted several time. In order to obtain un-interrupted temperature profiles and resistance profiles, new samples had to be used.
3. Results and Discussion Figures 2 (a), (b), and (c) shows the evolution of microstructure of the same solder joint during in-situ current stressing for 0 hr, 10 hrs, and 30 hrs, respectively. The zoom-in
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micrographs of the upper-right corner of Fig. 2 (a)-(c) are shown in Fig. 3 (a)-(c), respectively. The microstructure of the original solder joint before applying the current is shown in Fig. 2 (a) and Fig. 3 (a). The intermetallic compound on the chip side was Cu6Sn5, the intermetallic on the substrate side was too thin to be resolved. In the solder, the brighter phase was the Pb-rich phase, while the darker phase was the Sn-rich phase. After 10 hrs of current stressing as shown in Fig. 2 (b) and Fig. 3 (b), depression of the solder surface was visible near the upper-right corner of the contact opening, where electrons entered the joint and was the main current crowding region. The maximum depth of this depression was about 4-6 μm at this stage.
This depression became even deeper (8-10 μm max.) when the time reached 30 hrs,
as shown in Fig. 2 (c) and Fig. 3 (c). Concurrent with the developing of the depression, a lump rose above the solder surface in the lower part of the joint, as shown in Fig. 2 (b) and (c). The formation of the depression and the lump was the reflection of materials being driven by electron flow from the cathode side to the anode side. In addition to the depression and the lump, there were many small particles, between Cu and Cu6Sn5, extruding above the surface, as shown in Fig. 3 (b). Analysis using electron microprobe revealed that they were newly formed Cu6Sn5.
When the time reached 80 hrs, an intriguing phenomenon occurred, as shown in Fig. 4. The fine grains in Fig. 4 (a) suggested that this region had become molten and re-solidified. The question is whether the entire joint had melted or this melting was local? As shown in 6
Fig. 4 (b), in the lower part of this joint there were several large Pb-rich grains that did not melt. This suggests that the melting was local. To the best of our knowledge, the local melting of the current crowding region had never been reported before. This result indicates that, in addition to the void formation-and-propagation mechanism, solder joints under current stressing could also fail through the local melting mechanism.
The top surface temperature of the chip during the current stressing is shown in Fig. 5. It should be noted that the temperature profile shown in Fig. 5 was from a different sample shown in Figs. 2-4. The sample shown in Fig. 5 followed the same failure mechanism as that of the sample shown in Fig. 2-4, and the temperature and resistance profiles should also be quite similar. The temperature increased rapidly from the room temperature to 108 ℃ within 5 mins of
the current stressing. Subsequently, the temperature increased very slowly until
the current stressing time reached 85 hrs, after which the temperature rose rapidly and went beyond the PbSn eutectic temperature at 183 oC. More specifically, temperature rose from 122 ℃ to 150 ℃ in 40 mins, and from 150 ℃ to 183 ℃ in 6 mins. The global meting occurred when the temperature went beyond 183 oC. The resulting surface microstructure is shown in Fig.6 (a).
As can be seen in this figure, a clear gap had appeared between the
UBM and the bulk of the solder. The surface in Fig. 6 (a) was slightly polished to reveal inner microstructure, which is shown in Fig. 6 (b). An extensive dissolution of the Cu UBM and part of the Cu trace can be observed. The dissolved regions were back-filled with the solder. 7
The dissolved Cu atoms migrated into the solder and formed a large amount of Cu6Sn5 intermetallic compounds near the anode side. The site of open circuit was at the upper-right corner, where the Cu traced had been replaced with the solder. The microstructure shown here is similar to that reported by Hu et. al. [9]. The investigation to uncover the connection between the results of this study and that of Hu et. al. is underway.
The resistance change during current stressing was shown in Fig. 7. The temperature profile in Fig. 5 and the resistance profile in Fig. 7 were from the same sample.
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behavior of the resistance was similar to that of the temperature, other than the fast temperature rise region. The resistance rose slowly from 0.34 Ω to 0.39 Ω. Afterward, the resistance increased sharply. The resistance rose from 0.39 Ω to 0.45 Ω in 40 mins, and from 0.45 Ω to open circuit in 6 mins. In view of the microstructure evolution, the resistance profile, and the temperature profile, we propose that the microstructure change induced the resistance change, which, in turn, caused the temperature change. The power dissipation P=I2R, where I is the applied current and R is the resistance.
As the applied current was
constant in this study, the power dissipation was directly proportional to R. Consequently, an increase in R will raise P, which will cause the temperature to rise. As the depression between the UBM and solder enlarged, the available conducting area decreased, which cased the resistance to increase. In addition, the depression was located near the main current crowding region. As a result, the local resistance rise near the main current crowding region was higher 8
than the rest of the contact region. This factor combined with the highest local current density near the main current crowding region made the local melting happen at the main current crowding region. The rise of the local temperature also accelerated the local electromigration rate as electromigration was a diffusion process, and electromigration rate depended exponentially on the temperature. This vicious cycle produced a faster microstructure change, local resistance rise, and local temperature rise. Eventually, the solder joint failed catastrophically.
4. Conclusions It was shown through in-situ observation that flip chip solder joints under current stressing could fail through a local melting phenomenon. Before the local melting, a depression formed due to electromigration near the main current crowding region. It was proposed that this depression induced a local resistance rise. The rising local resistance combined with the higher local current density resulted in the rise of the local temperature at the main current crowding region. When the local temperature went beyond the melting point of solder, local melting occurred. After the local melting, the joint would fail rather quickly.
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Acknowledgment.
This work was supported by the National Science Council through
grants NSC-93-2214-E-008-008 and NSC-93-2216-E-008-010.
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Reference 1.
S. Brandenberg and S. Yeh, Proceedings of the 1998 Surface Mount International Conference and Exhibition (SMTA, Edina, MN, 1998) p.337.
2.
K. Zeng and K. N. Tu, Mater. Sci. & Eng. R 38, 55 (2002).
3.
J. D. Wu, P. J. Zheng, K. Lee, C. T. Chiu, and J. J. Lee, Proceedings of the 2002 Electronic Components and Technology Conference (IEEE, NY, 2002) p.452.
4.
E. C. C. Yeh, W. J. Choi, and K. N. Tu, P. Elenius, and H. Balkan, Appl. Phys. Lett. 80, 580 (2002).
5.
C. Y. Liu, C. Chen, C. N. Liao, and K. N. Tu, Appl. Phys. Lett. 75, 58 (1999).
6.
C. Y. Liu, C. Chen, and K. N. Tu, J. Appl. Phys. 88, 5703 (2000).
7.
Q. T. Huynh, C. Y. Liu, C. Chen, and K. N. Tu, J. Appl. Phys. 89, 4332 (2001).
8.
K. N. Tu, J. Appl. Phys. 94, 5451 (2003)
9.
Y. C. Hu, Y. H. Lin, and C. R. Kao, J. Mater. Res. 18, 2544 (2003).
10.
T. Y. Lee, K. N. Tu, and D. R. Frear, J. Appl. Phys. 90, 4502 (2001).
11.
H. Ye, C. Basaran, and D. Hopkins, Appl. Phys. Lett. 82, 1045 (2003).
12.
H. Ye, C. Basaran, and D. Hopkins, Int. J. Solids Struct. 40, 4021 (2003).
13.
J. W. Nah, K. W. Paik, J. O. Suh, and K. N. Tu, J. Appl. Phys. 94, 7560 (2003).
14.
Y. C. Hsu, T. L. Shao, C. J. Yang, and C. Chen, J. Electron. Mater. 32, 1222 (2003).
15.
T. L. Shao, K. C. Lin, and C. Chen, J. Electron. Mater. 32, 1278 (2003).
11
16.
A. T. Wu, K. N. Tu, J. R. Lloyd, N. Tamura, B. C. Valek, and C. R. Kao, Appl. Phys. Lett., 85, 2490 (2004).
17.
K. Jung and H. Conrad, J. Mater. Sci. 39, 1803 (2004).
18.
Y. H. Lin, C. M. Tsai, Y. C. Hu, Y. L. Lin, and C. R. Kao, J. Electron. Mater., 34, 27 (2005).
19.
Y. H. Lin, C. M. Tsai, Y. C. Hu, Y. L. Lin, J. Y. Tsai, and C. R. Kao, Mater. Sci. Forum, 475-479, 2655 (2005).
20.
Y. H. Lin, Y. C. Hu, C. M. Tsai, C. R. Kao, and K. N. Tu, Acta Mate., 53, 2029 (2005).
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Figure Captions Fig. 1
Schematic drawing showing the configuration of the flip chip solder joint and the direction of electron flow.
Fig. 2
SEM micrographs of the same solder joint during in-situ current stressing. (a) Before current stressing, (b) after 10 hrs current stressing, (c) after 30 hrs current stressing.
Fig. 3
Zoom-in micrographs for the upper-right corner of Fig. 2 (a)-(c), respectively.
Fig. 4
SEM micrographs of the solder joint after 80 hrs current stressing. (a) The zoom-in view of the current crowding region. The fine grains suggested that this region had become molten and re-solidified. (b) A micrograph showing the same joint in (a) but has a larger field of view. In the lower part of this joint, there were several large Pb-rich grains, which did not melt. This suggests that the melting was local.
Fig. 5
Top surface temperature of the chip during the current stressing. The eutectic temperature of the PbSn solder is 183 ℃.
Fig. 6
(a) SEM micrograph of the failed solder joint. (b) The same joint in (a). The surface was slightly polished to reveal the internal structure.
Fig. 7
Joint resistance of the solder joint during the current stressing.
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Fig. 1
Schematic drawing showing the configuration of the flip chip solder joint and the direction of electron flow.
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Fig. 2
SEM micrographs of the same solder joint during in-situ current stressing. (a) Before current stressing, (b) after 10 hrs current stressing, (c) after 30 hrs current stressing.
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Fig. 3
Zoom-in micrographs for the upper-right corner of Fig. 2 (a)-(c), respectively.
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Fig. 4
SEM micrographs of the solder joint after 80 hrs current stressing. (a) The zoom-in view of the current crowding region. The fine grains suggested that this region had become molten and re-solidified. (b) A micrograph showing the same joint in (a) but has a larger field of view. In the lower part of this joint, there were several large Pb-rich grains, which did not melt. This suggests that the melting was local.
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Fig. 5
Top surface temperature of the chip during the current stressing. The eutectic temperature of the PbSn solder is 183 ℃.
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Fig. 6
(a) SEM micrograph of the failed solder joint. (b) The same joint in (a). The surface was slightly polished to reveal the internal structure.
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Fig. 7
Joint resistance of the solder joint during the current stressing.
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