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Effects of Ni-coated Carbon Nanotubes addition on the electromigration of Sn–Ag–Cu solder joints
Journal of Alloys and Compounds 581 (2013) 202–205
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effects of Ni-coated Carbon Nanotubes addition on the electromigration of Sn–Ag–Cu solder joints Zhongbao Yang, Wei Zhou, Ping Wu ⇑ Department of Applied Physics, Institute of Advanced Materials Physics, Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Faculty of Science, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2012 Accepted 9 July 2013 Available online 17 July 2013 Keywords: Intermetallics Atomic diffusion Composite materials
a b s t r a c t The electromigration behaviors of line-type Cu/Sn–Ag–Cu/Cu interconnects with and without Ni-Coated multi-walled Carbon Nanotubes addition were investigated in this work. After soldering, the (Cu,Ni)6Sn5 intermetallic compounds formed at the solder/Cu interface. The electromigration analysis shows that the presence of Carbon Nanotubes can suppress the atomic diffusion in the solder induced by electromigration effectively. And finite element simulation indicates that the Carbon Nanotube networks can reduce the current density in the solder matrix, which results in the improvement of electromigration resistance of composite solders. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In virtue of the environmental safety and human health concerns, the research in developing lead-free solders has been accelerated in recent years. At present, there are many promising alternative lead-free solders, for example, Sn–Ag, Sn–Zn and Sn– Bi systems. Among all the candidates, Sn–Ag–Cu alloys have been regarded as one of the most promising candidates to replace Sn– Pb solder due to their good mechanical properties, adequate wetting characteristics as well as the comparable melting temperature [1]. With the trend of continuous miniaturization in electronic products, the flip chip solder joints are downsizing and the current density applied to the interconnects is up to 104 A/cm2. Recently, electromigration (EM) becomes a serious concern for packaging reliability, which may induce voids, hillocks, solute segregation, growth and dissolution of the intermetallic compounds (IMCs) at the electrodes [2,3]. The accumulation and growth of the voids will lead to eventual electric failures of solder joints [4]. As a result, conventional solders can no longer guarantee the required performance for device reliability during service. Therefore, new interconnect materials with high qualities are exigent for device functionalities to provide adequate EM resistance. As we all know, one approach to improve the performance of solders is to introduce second phases to conventional solder alloys to form composite solders. Many studies have reported the positive effects of foreign
⇑ Corresponding author. Tel.: +86 22 27408599; fax: +86 22 27406852. E-mail address: [email protected] (P. Wu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.07.058
particulate reinforcements on the properties of solder materials [5,6]. Carbon Nanotube (CNT), which possesses almost 100 times the tensile strength (150 GPa) of high-strength steel alloys and 105 times the conductivity of pure copper, has aroused increasing scientific and research interest since discovered in 1991 by Iijima [7–12]. Its extraordinary qualities made it suitable for making many excellent composites. Nai et al. primarily incorporated the multi-walled Carbon Nanotubes (MWCNTs) in Sn–Ag–Cu solder via powder metallurgy method and studied the influence of MWCNTs addition on the mechanical properties of the composite solders. Their studies indicated a marginal increment in improving the mechanical properties of the composite solders [13]. Afterwards, Kumar et al. adopted the same technique to manufacture single-wall Carbon Nanotubes (SWCNTs) enhanced Sn–Pb and Sn–Ag–Cu solders. They reported that the addition of appropriate amount of SWCNTs could improve microstructural and tensile properties of solder alloys [14]. However, the solder joints must survive a combination of the mechanical stress and electric current. So it is essential to understand the effect of CNTs addition on the electrical properties of solder joints. In the present study, the EM behaviors of Sn–3.5Ag–0.7Cu (SAC) composite solder joints under the current were investigated. Two-dimensional finite element simulation was performed to obtain the current distribution in both solder joints. Electrical resistivity tests were also conducted on such composite solders with different weight percentage of CNTs. During our experiment, Ni surface coating was applied on CNTs to overcome the weak bonding between Sn and CNTs. On one hand, Ni has good wetting characteristics with SAC solder and it can form stable phases in the Ni–Sn binary system [15].
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On the other hand, Ni atoms can form a continuous coating on CNT with a significant binding energy [16–17]. 2. Experiment procedures For the experiments, Sn–3.5Ag–0.7Cu (wt.%) solder and Ni-Coated multi-walled Carbon Nanotubes (Ni–MWCNTs) were used as the matrix alloy and the reinforcement respectively. The composite solders with varying weight percentages of Ni– CNTs (0 wt.%, 0.05 wt.%, 0.2 wt.%, 0.5 wt.% and 1 wt.%) were synthesized. The desired Ni-CNTs were dispersed in ethanol under ultrasonic agitation at first and then dried. A ferrous spoon was used to mill the dried Ni–CNTs agglomerates to improve the homogeneity of the mixture. The composite solder was then prepared by blending for 10 h in a ball mill. Subsequently, a watersoluble flux was added into the mixed powder and the mixture was blended manually for at least 30 min. The paste mixture was then printed onto a glass using a stainless steel stencil and reflowed at 240 °C to prepare the solder balls with about 600 lm diameter. The reflow time above the melting temperature was shorter than normal reflow profile in order to avoid the CNTs floating on the surface of meting solder balls. The bulk solders were fabricated in a specific glass groove and the electrical resistivities of them were measured with the four-point probe method. The composite solder with 1 wt% Ni–CNTs (SAC/1Ni–CNTs) was chosen to carry out the electromigration test. The fabrication process of the Cu line/solder ball/Cu line samples is reported elsewhere [18]. Finally the fabricated samples were currently stressed with a direct current of 10 A (the current density is about 1 104 A/cm2) for 260 h, 640 h and 960 h respectively. The microstructural evolution of the solder joints was examined by a scanning electron microscope (SEM), and the chemical compositions of the IMC were identified by an energy dispersive X-ray spectroscopy (EDX).
3. Results and discussion The representative SEM images showing the IMC layers formed between the solders (SAC and SAC/1Ni–CNTs) and Cu substrates after soldering are exhibited in Fig. 1. Scallop-type Cu6Sn5 layer with an average thickness of 2.10 lm can be found at the SAC/Cu interface, as shown in Fig. 1(a). Generally, Cu6Sn5 is the main reaction product between most Sn-based solder alloys and Cu at low temperature [19]. A markedly different interfacial reaction was observed in composite solder joints, as illustrated in Fig. 1(b). The morphology of IMC layer was transformed from scallop-type to thin layer-type after introducing Ni–CNTs to the solder. Thecompound was confirmed to be (Cu,Ni)6Sn5 by EDS analysis, whose compositions were 47.55 at.% Cu, 3.99 at.% Ni and 48.46 at.% Sn. During the reflow, most Ni atoms coated on CNTs arrived at the solder/Cu interface, inducing the formation of the Ni-containing Cu6Sn5 ((Cu,Ni)6Sn5) IMC. In the solder matrix Ni atoms accounted for 0.12% by the EDS mapping analysis. As also can be seen from Fig. 1(b), the thickness of (Cu,Ni)6Sn5, which is 2.93 lm, is bigger than that of Cu6Sn5. This phenomenon can be explain by the fact that the activation energy of (Cu,Ni)6Sn5 was smaller than that of Cu6Sn5 [20], which resulted in the increasing in high growth rate constants of Cu6Sn5 after introducing Ni into the SAC solder despite of low Ni content. Fig. 2 shows the microstructure of SAC solder joints with Cu pads after current stressing of 0 h, 260 h and 640 h, with Fig. 2a, c, and e showing the cathode side and Fig. 2b and d, and f showing the anode side. It is obvious that the polarity effect, which the current enhanced the growth rate of IMC at the anode whilst limiting
Fig. 1. Representative SEM images showing the IMC layers formed at the interfaces after reflow: (a) SAC/Cu and (b) SAC-1CNT/Cu.
Fig. 2. SEM images of the cross-sectioned SAC/Cu after current stressing for 0 h; 260 h and 640 h: (a, c and e) the cathode side; (b, d and f) the anode side.
the rate at the cathode, was clearly produced in the specimen. Cu is expected to be the dominant electromigration-diffused specie in SAC solder joints [21]. When the interconnection was exposed to current, the Cu atoms decomposed from the Cu6Sn5 intermetallics as well as Cu pad (the cathode side) and moved withelectron flow. The Sn atoms were driven back to the cathode interface by backstress. As the EM time increasing, Cu6Sn5 at the cathode was almost consumed and replaced with Sn, as shown in Fig. 2(e). At the anode side, IMC cliffs developed on the surface of the IMC layer along the anode side after EM for 260 h, as shown in Fig. 2(d). With
Fig. 3. SEM images of the cross-sectioned SAC-1CNT /Cu after current stressing for 0 h; 260 h and 640 h: (a, c and e) the cathode side; (b, d and f) the anode side.
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Fig. 4. Thickness of IMC at SAC/Cu and SAC-1CNT/Cu interfaces of anode side after current stressing for 0 h; 260 h; 640 h and 960 h.
EM progressing, more Cu atoms were driven to the anode side and a large amount of Cu6Sn5 formed along the electron flowing direction. When the Cu atoms were driven to the interface (the anode) they would react with Sn to form Cu6Sn5. Owing to the advent of Cu atoms there had not enough space to accept the new production, which resulted in Cu6Sn5 developed upwards along the anode side. Moreover, it is noted that some IMC hillocks generated on the surface of matrix during current stress, which was attributed to stress relaxation behavior owing to the ambulation of atoms [22]. Fig. 3 shows the cross-sectioned Cu/SAC-1CNTs/Cu SEM images of the anode and the cathode after current stressing for 0 h, 260 h and 640 h. There were less IMC grains scattering near the anode region and none IMC hillocks produced on the surface of matrix. The average thickness of the IMCs at the anode side just increased by 0.35 lm after exposure to current for 260 h while that in unreinforced solder joints increased by 3.21 lm, which was 9.2 times thicker than that of the composite sample, as shown in Fig. 4. Accordingly, the addition of Ni-CNTs as reinforcement in the solder suppressed EM effects effectively. The presence of CNTs in the composite solder matrix after fine polishing and etched is shown in Fig. 5(a) and (b) respectively. It can be seen that CNTs distributed randomly in the solder matrix. Several CNTs joined with each other and had a good interfacial bonding with the matrix. After various measures during experiment, Ni–CNTs could bond with solder singly. With the addition of increasing amounts of CNTs in the solder, there would be areas where CNTs come into contact with each other to form networks. Thus CNTs in the composite solder was believed to be present in the forms of singly dispersed CNTs and networks randomly
Fig. 6. Electrical resistivity results of monolithic and composite solders.
buildup by CNTs. It is well known that CNT was provided with excellent conductivity. Electrical resistivity results of the composite solders also revealed that the incorporation of CNTs lowered the electrical resistivity of such solders, as can be seen from Fig. 6. In the solders alloyed with CNTs, some high conductivity paths (the networks) may be built. When the solder joint was exposed to current, electrons would go through the high conductivity paths preferentially rather than collided with atoms in the solder. As a result, a relatively small number of atoms moved from cathode to anode and the effect of EM was suppressed. Hence, it was proposed that the presence of these CNTs played a role of fast channel for electron to pass through in this study. In order to get a better understanding of the different EM behaviors, two-dimensional finite element simulation was performed to obtain the current distribution in the both joints, as shown in Fig. 7. A current density of about 1 104 A/cm2 is powered through the sample in the simulation. CNTs are intentional distributed in two region of composite solder to get a better observation of CNTs effect. Obviously, the electrical current accumulates at the CNT networks and its vicinity in composite solder joint. Moreover, when the CNTs join with each other as Fig. 5(b), the electrical current will accumulate entirely at the intermediate CNTs among the network, where the current density reaches to 106 A/cm2. The current density at region B in composite solder matrix is about 6 103 A/cm2, which is obviously lower than that at region A in its monolithic counterpart. Since the resistivity of CNTs is lower than that of solder, the electrical current will primarily choose the CNTs as the flowing path rather than the solder matrix. So when entering the solder, the electrical current will head to and accumulate at the CNT networks, resulting in the reduction of current density in solder matrix. As is well-known, CNT has excellent
Fig. 5. SEM images showing the presence of Ni–MWCNTs in solder matrix after: (a) Fine polishing and (b) etched.
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composite solder joints showed that the presence of reinforcement can suppress the effect of EM effectively. According to the finite element simulation, the CNT networks were chosen primarily as the path for current flux, which resulted in the reduction of the current density in solder matrix. Electrical resistivity results of the composite solders also revealed that the incorporation of CNTs degraded the electrical resistivity of such solders. Acknowledgments This work was supported by the National Natural Science Foundation of China (51074112) (11247224), the Key Program of Tianjin Natural Science Foundation (11JCZDJC22100). References
Fig. 7. Current density distribution in the: (a) SAC solder joint and (b) SAC/1Ni–CNT solder joint (the resistivity of Cu, Sn, Cu6Sn5 and CNT is 1.7 lX cm, 11 lX cm, 18 lX cm and 0.01 lX cm, respectively).
current carrying capacity [10]. Thus current crowding in CNTs does not make damage to solder joint but improves of electromigration resistance of composite solders. 4. Conclusions The addition of Ni–MWCNTs in solder altered the morphology of the IMC layer after reflow. At melting state the Ni coated on CNTs reacted with Sn and Cu in the solder matrix and at the interface forming (Cu,Ni)6Sn5. EM analysis of the monolithic and
[1] K. Zeng, K.N. Tu, Mater. Sci. Eng. R 38 (2) (2002) 55–105. [2] W.H. Lin, A.T. Wu, H.J. Lin, T.H. Chuang, K.N. Tu, J. Electron. Mater. 36 (2007) 753–759. [3] K.N. Tu, J. Appl. Phys. 94 (2003) 5451–5473. [4] H. Ye, C. Basaran, D.C. Hopkins, Int. J. Solids. Struct. 40 (2003) 4021–4032. [5] H.T. Lee, Y.H. Lee, Mat. Sci. Eng. A 419 (2006) 172–180. [6] , J. Alloys Comp. 453 (2008) 180–184. [7] S. Iijima, Nature 354 (1991) 56–58. [8] M.F. Yu, B.S. Files, S. Arepalli, R.S. Ruoff, Phys. Rev. Lett. 84 (2000) 5552–5555. [9] H. Dai, Surf. Sci. 500 (2002) 218–241. [10] J.S. Delmotee, A. Rubio, Carbon 40 (2002) 1729–1734. [11] K.T. Lau, D. Hui, Compos. Part B: Eng. 33 (2002) 263–277. [12] R.B. Pipes, P. Hubert, Compos. Sci. Technol. 62 (2002) 419–428. [13] S.M.L. Nai, J. Wei, M. Gupta, Mat. Sci. Eng. A 423 (2006) 166–169. [14] K.M. Kumar, V. Kripesh, Andrew A.O. Tay, J. Alloys Comp. 450 (2008) 229–237. [15] P. Yao, P. Liu, J. Liu, J. Alloys Comp. 462 (2008) 73–79. [16] E. Durgun, S. Dag, V.M.K. Bagci, O. Gulseren, T. Yildirim, S. Ciraci, Phys. Rev. B 67 (2003) 201401 (R). [17] S.H. Yang, W.H. Shin, J.W. Lee, S.Y. Kim, S.I. Woo, J.K. Kang, J. Phys. Chem. B 110 (2006) 13941–13946. [18] W. Zhou, P. Wu, J. Electron. Mater. 38 (2009) 866–872. [19] T.B. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak, Binary Alloy Phase Diagram, 2nd ed., ASM International, Materials Park, OH, 1990. pp. 1481–1483. [20] Jeong.-Won. Yoon, J. Alloys Comp. 486 (2009) 142–147. [21] L.D. Chen, M.L. Huang, S.M. Zhou, J. Alloys Comp. 504 (2010) 535–541. [22] H.Y. Liu, Mat. Sci. Eng. A 528 (2011) 1467–1471.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effects of Ni-coated Carbon Nanotubes addition on the electromigration of Sn–Ag–Cu solder joints Zhongbao Yang, Wei Zhou, Ping Wu ⇑ Department of Applied Physics, Institute of Advanced Materials Physics, Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Faculty of Science, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2012 Accepted 9 July 2013 Available online 17 July 2013 Keywords: Intermetallics Atomic diffusion Composite materials
a b s t r a c t The electromigration behaviors of line-type Cu/Sn–Ag–Cu/Cu interconnects with and without Ni-Coated multi-walled Carbon Nanotubes addition were investigated in this work. After soldering, the (Cu,Ni)6Sn5 intermetallic compounds formed at the solder/Cu interface. The electromigration analysis shows that the presence of Carbon Nanotubes can suppress the atomic diffusion in the solder induced by electromigration effectively. And finite element simulation indicates that the Carbon Nanotube networks can reduce the current density in the solder matrix, which results in the improvement of electromigration resistance of composite solders. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In virtue of the environmental safety and human health concerns, the research in developing lead-free solders has been accelerated in recent years. At present, there are many promising alternative lead-free solders, for example, Sn–Ag, Sn–Zn and Sn– Bi systems. Among all the candidates, Sn–Ag–Cu alloys have been regarded as one of the most promising candidates to replace Sn– Pb solder due to their good mechanical properties, adequate wetting characteristics as well as the comparable melting temperature [1]. With the trend of continuous miniaturization in electronic products, the flip chip solder joints are downsizing and the current density applied to the interconnects is up to 104 A/cm2. Recently, electromigration (EM) becomes a serious concern for packaging reliability, which may induce voids, hillocks, solute segregation, growth and dissolution of the intermetallic compounds (IMCs) at the electrodes [2,3]. The accumulation and growth of the voids will lead to eventual electric failures of solder joints [4]. As a result, conventional solders can no longer guarantee the required performance for device reliability during service. Therefore, new interconnect materials with high qualities are exigent for device functionalities to provide adequate EM resistance. As we all know, one approach to improve the performance of solders is to introduce second phases to conventional solder alloys to form composite solders. Many studies have reported the positive effects of foreign
⇑ Corresponding author. Tel.: +86 22 27408599; fax: +86 22 27406852. E-mail address: [email protected] (P. Wu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.07.058
particulate reinforcements on the properties of solder materials [5,6]. Carbon Nanotube (CNT), which possesses almost 100 times the tensile strength (150 GPa) of high-strength steel alloys and 105 times the conductivity of pure copper, has aroused increasing scientific and research interest since discovered in 1991 by Iijima [7–12]. Its extraordinary qualities made it suitable for making many excellent composites. Nai et al. primarily incorporated the multi-walled Carbon Nanotubes (MWCNTs) in Sn–Ag–Cu solder via powder metallurgy method and studied the influence of MWCNTs addition on the mechanical properties of the composite solders. Their studies indicated a marginal increment in improving the mechanical properties of the composite solders [13]. Afterwards, Kumar et al. adopted the same technique to manufacture single-wall Carbon Nanotubes (SWCNTs) enhanced Sn–Pb and Sn–Ag–Cu solders. They reported that the addition of appropriate amount of SWCNTs could improve microstructural and tensile properties of solder alloys [14]. However, the solder joints must survive a combination of the mechanical stress and electric current. So it is essential to understand the effect of CNTs addition on the electrical properties of solder joints. In the present study, the EM behaviors of Sn–3.5Ag–0.7Cu (SAC) composite solder joints under the current were investigated. Two-dimensional finite element simulation was performed to obtain the current distribution in both solder joints. Electrical resistivity tests were also conducted on such composite solders with different weight percentage of CNTs. During our experiment, Ni surface coating was applied on CNTs to overcome the weak bonding between Sn and CNTs. On one hand, Ni has good wetting characteristics with SAC solder and it can form stable phases in the Ni–Sn binary system [15].
Z. Yang et al. / Journal of Alloys and Compounds 581 (2013) 202–205
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On the other hand, Ni atoms can form a continuous coating on CNT with a significant binding energy [16–17]. 2. Experiment procedures For the experiments, Sn–3.5Ag–0.7Cu (wt.%) solder and Ni-Coated multi-walled Carbon Nanotubes (Ni–MWCNTs) were used as the matrix alloy and the reinforcement respectively. The composite solders with varying weight percentages of Ni– CNTs (0 wt.%, 0.05 wt.%, 0.2 wt.%, 0.5 wt.% and 1 wt.%) were synthesized. The desired Ni-CNTs were dispersed in ethanol under ultrasonic agitation at first and then dried. A ferrous spoon was used to mill the dried Ni–CNTs agglomerates to improve the homogeneity of the mixture. The composite solder was then prepared by blending for 10 h in a ball mill. Subsequently, a watersoluble flux was added into the mixed powder and the mixture was blended manually for at least 30 min. The paste mixture was then printed onto a glass using a stainless steel stencil and reflowed at 240 °C to prepare the solder balls with about 600 lm diameter. The reflow time above the melting temperature was shorter than normal reflow profile in order to avoid the CNTs floating on the surface of meting solder balls. The bulk solders were fabricated in a specific glass groove and the electrical resistivities of them were measured with the four-point probe method. The composite solder with 1 wt% Ni–CNTs (SAC/1Ni–CNTs) was chosen to carry out the electromigration test. The fabrication process of the Cu line/solder ball/Cu line samples is reported elsewhere [18]. Finally the fabricated samples were currently stressed with a direct current of 10 A (the current density is about 1 104 A/cm2) for 260 h, 640 h and 960 h respectively. The microstructural evolution of the solder joints was examined by a scanning electron microscope (SEM), and the chemical compositions of the IMC were identified by an energy dispersive X-ray spectroscopy (EDX).
3. Results and discussion The representative SEM images showing the IMC layers formed between the solders (SAC and SAC/1Ni–CNTs) and Cu substrates after soldering are exhibited in Fig. 1. Scallop-type Cu6Sn5 layer with an average thickness of 2.10 lm can be found at the SAC/Cu interface, as shown in Fig. 1(a). Generally, Cu6Sn5 is the main reaction product between most Sn-based solder alloys and Cu at low temperature [19]. A markedly different interfacial reaction was observed in composite solder joints, as illustrated in Fig. 1(b). The morphology of IMC layer was transformed from scallop-type to thin layer-type after introducing Ni–CNTs to the solder. Thecompound was confirmed to be (Cu,Ni)6Sn5 by EDS analysis, whose compositions were 47.55 at.% Cu, 3.99 at.% Ni and 48.46 at.% Sn. During the reflow, most Ni atoms coated on CNTs arrived at the solder/Cu interface, inducing the formation of the Ni-containing Cu6Sn5 ((Cu,Ni)6Sn5) IMC. In the solder matrix Ni atoms accounted for 0.12% by the EDS mapping analysis. As also can be seen from Fig. 1(b), the thickness of (Cu,Ni)6Sn5, which is 2.93 lm, is bigger than that of Cu6Sn5. This phenomenon can be explain by the fact that the activation energy of (Cu,Ni)6Sn5 was smaller than that of Cu6Sn5 [20], which resulted in the increasing in high growth rate constants of Cu6Sn5 after introducing Ni into the SAC solder despite of low Ni content. Fig. 2 shows the microstructure of SAC solder joints with Cu pads after current stressing of 0 h, 260 h and 640 h, with Fig. 2a, c, and e showing the cathode side and Fig. 2b and d, and f showing the anode side. It is obvious that the polarity effect, which the current enhanced the growth rate of IMC at the anode whilst limiting
Fig. 1. Representative SEM images showing the IMC layers formed at the interfaces after reflow: (a) SAC/Cu and (b) SAC-1CNT/Cu.
Fig. 2. SEM images of the cross-sectioned SAC/Cu after current stressing for 0 h; 260 h and 640 h: (a, c and e) the cathode side; (b, d and f) the anode side.
the rate at the cathode, was clearly produced in the specimen. Cu is expected to be the dominant electromigration-diffused specie in SAC solder joints [21]. When the interconnection was exposed to current, the Cu atoms decomposed from the Cu6Sn5 intermetallics as well as Cu pad (the cathode side) and moved withelectron flow. The Sn atoms were driven back to the cathode interface by backstress. As the EM time increasing, Cu6Sn5 at the cathode was almost consumed and replaced with Sn, as shown in Fig. 2(e). At the anode side, IMC cliffs developed on the surface of the IMC layer along the anode side after EM for 260 h, as shown in Fig. 2(d). With
Fig. 3. SEM images of the cross-sectioned SAC-1CNT /Cu after current stressing for 0 h; 260 h and 640 h: (a, c and e) the cathode side; (b, d and f) the anode side.
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Fig. 4. Thickness of IMC at SAC/Cu and SAC-1CNT/Cu interfaces of anode side after current stressing for 0 h; 260 h; 640 h and 960 h.
EM progressing, more Cu atoms were driven to the anode side and a large amount of Cu6Sn5 formed along the electron flowing direction. When the Cu atoms were driven to the interface (the anode) they would react with Sn to form Cu6Sn5. Owing to the advent of Cu atoms there had not enough space to accept the new production, which resulted in Cu6Sn5 developed upwards along the anode side. Moreover, it is noted that some IMC hillocks generated on the surface of matrix during current stress, which was attributed to stress relaxation behavior owing to the ambulation of atoms [22]. Fig. 3 shows the cross-sectioned Cu/SAC-1CNTs/Cu SEM images of the anode and the cathode after current stressing for 0 h, 260 h and 640 h. There were less IMC grains scattering near the anode region and none IMC hillocks produced on the surface of matrix. The average thickness of the IMCs at the anode side just increased by 0.35 lm after exposure to current for 260 h while that in unreinforced solder joints increased by 3.21 lm, which was 9.2 times thicker than that of the composite sample, as shown in Fig. 4. Accordingly, the addition of Ni-CNTs as reinforcement in the solder suppressed EM effects effectively. The presence of CNTs in the composite solder matrix after fine polishing and etched is shown in Fig. 5(a) and (b) respectively. It can be seen that CNTs distributed randomly in the solder matrix. Several CNTs joined with each other and had a good interfacial bonding with the matrix. After various measures during experiment, Ni–CNTs could bond with solder singly. With the addition of increasing amounts of CNTs in the solder, there would be areas where CNTs come into contact with each other to form networks. Thus CNTs in the composite solder was believed to be present in the forms of singly dispersed CNTs and networks randomly
Fig. 6. Electrical resistivity results of monolithic and composite solders.
buildup by CNTs. It is well known that CNT was provided with excellent conductivity. Electrical resistivity results of the composite solders also revealed that the incorporation of CNTs lowered the electrical resistivity of such solders, as can be seen from Fig. 6. In the solders alloyed with CNTs, some high conductivity paths (the networks) may be built. When the solder joint was exposed to current, electrons would go through the high conductivity paths preferentially rather than collided with atoms in the solder. As a result, a relatively small number of atoms moved from cathode to anode and the effect of EM was suppressed. Hence, it was proposed that the presence of these CNTs played a role of fast channel for electron to pass through in this study. In order to get a better understanding of the different EM behaviors, two-dimensional finite element simulation was performed to obtain the current distribution in the both joints, as shown in Fig. 7. A current density of about 1 104 A/cm2 is powered through the sample in the simulation. CNTs are intentional distributed in two region of composite solder to get a better observation of CNTs effect. Obviously, the electrical current accumulates at the CNT networks and its vicinity in composite solder joint. Moreover, when the CNTs join with each other as Fig. 5(b), the electrical current will accumulate entirely at the intermediate CNTs among the network, where the current density reaches to 106 A/cm2. The current density at region B in composite solder matrix is about 6 103 A/cm2, which is obviously lower than that at region A in its monolithic counterpart. Since the resistivity of CNTs is lower than that of solder, the electrical current will primarily choose the CNTs as the flowing path rather than the solder matrix. So when entering the solder, the electrical current will head to and accumulate at the CNT networks, resulting in the reduction of current density in solder matrix. As is well-known, CNT has excellent
Fig. 5. SEM images showing the presence of Ni–MWCNTs in solder matrix after: (a) Fine polishing and (b) etched.
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composite solder joints showed that the presence of reinforcement can suppress the effect of EM effectively. According to the finite element simulation, the CNT networks were chosen primarily as the path for current flux, which resulted in the reduction of the current density in solder matrix. Electrical resistivity results of the composite solders also revealed that the incorporation of CNTs degraded the electrical resistivity of such solders. Acknowledgments This work was supported by the National Natural Science Foundation of China (51074112) (11247224), the Key Program of Tianjin Natural Science Foundation (11JCZDJC22100). References
Fig. 7. Current density distribution in the: (a) SAC solder joint and (b) SAC/1Ni–CNT solder joint (the resistivity of Cu, Sn, Cu6Sn5 and CNT is 1.7 lX cm, 11 lX cm, 18 lX cm and 0.01 lX cm, respectively).
current carrying capacity [10]. Thus current crowding in CNTs does not make damage to solder joint but improves of electromigration resistance of composite solders. 4. Conclusions The addition of Ni–MWCNTs in solder altered the morphology of the IMC layer after reflow. At melting state the Ni coated on CNTs reacted with Sn and Cu in the solder matrix and at the interface forming (Cu,Ni)6Sn5. EM analysis of the monolithic and
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