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Cu solder joint during solid state aging
Journal of Alloys and Compounds 634 (2015) 94–98
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
In situ quantitative study of microstructural evolution at the interface of Sn3.0Ag0.5Cu/Cu solder joint during solid state aging Hailong Li a, Rong An b,⇑, Chunqing Wang a, Zhi Jiang a a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, PR China Key Laboratory of Micro-systems and Micro-structures Manufacturing, Harbin Institute of Technology, Ministry of Education, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 9 January 2015 Received in revised form 9 February 2015 Accepted 11 February 2015 Available online 16 February 2015 Keywords: Interfacial reaction Solder joint Quantitative Microstructure Nanoindentation
a b s t r a c t In situ microstructural evolution at the interface of Sn3.0Ag0.5Cu/Cu solder joint during solid state aging was quantitatively studied by nanoindentation. The morphology of Cu6Sn5 gradually altered from scallop type to layer type. Though the growth of IMCs was diffusion-controlled, the consumption of Cu substrate was not linear with the square root of aging time. At the initial stage of solid state aging, the Cu atoms essential to the growth of IMCs were mainly from the supersaturated solder matrix. When the Cu atoms from supersaturated solder matrix were exhausted, the Cu atoms for the growth of IMCs were primarily from the Cu substrate. In addition, the IMCs formed at this state were principally used to fill up the gaps between scallops. After the gaps disappeared, the consumption of Cu substrate slowed down. Furthermore, the growth of Cu3Sn layer in Sn3.0Ag0.5Cu/Cu solder joint was on the both sides with layer type. Since the Sn atoms were inhibited to diffuse into the Cu substrate by the alloying elements of Ag and Cu, the thickness of Cu3Sn layer in SnAgCu/Cu solder joint was much thinner than that in pure Sn/Cu solder joint. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Interconnect technology based on the reactions between Cu and Sn plays a key role in the manufacturing process of electronics. During reflowing, the reactions between molten solder and bond pad have a significant influence on the quality of solder joints. However, the subsequent solid state interfacial reactions are closely related to the reliability of solder joints in the field. There are lots of studies on the reflowing process [1–4]. Tu et al. has established the growth model of scallops in SnPb/Cu solder joints during reflowing [5]. Gong et al. studied the evolution of SnCu IMCs between a molten SnAgCu solder and the Cu under bump metallization (UBM) throughout reflowing based on interruption of soldering reactions in experiments by removing the liquid solder from the substrate [6]. Nevertheless, most of the methods used to study the solid state aging just remained in the level of qualitative and statistical analysis due to the slower reaction rate and the difficulty of precise measurements of interfacial movement. Tu and Thompson deposited a flash of W film between the Cu and Sn thin films and studied the interfacial reactions by mean of Rutherford backscattering, finding that Cu is the dominant diffusing species [7]. Yang et al. quantitatively studied the migration of interphase interfaces in Sn-based solder joints using the upper surface of ⇑ Corresponding author. Tel.: +86 451 86418725; fax: +86 451 86416186. E-mail address: [email protected] (R. An). http://dx.doi.org/10.1016/j.jallcom.2015.02.088 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
unreacted Cu substrate as a reference [8], but the quantitatively measurements with aging time were not for the same sample. Ouyang and Kao studied the in situ thermomigration of Sn3.0Ag0.5Cu solder using nanoindentation marks [9]. However, these nanoindentation marks migrated due to the small size and the impact of thermal gradient, and were not able to be used in quantitative calculation. In this article, the in situ microstructural evolution of Sn3.0Ag0.5Cu/Cu solder joint during solid state aging was studied. By making the nanoindentation marks as the reference points, the in situ interfacial movement and IMC thickness were calculated precisely. 2. Experimental 2.1. Materials and soldering The Cu block with 10 mm 10 mm 10 mm in size was used as substrate. Then, a layer of solder paste of Sn3.0A0.5Cu was dispensed through a metal mask with an opening of 6 mm diameter onto one of the six surfaces of the Cu block. After that, the Cu block was placed on the hot plate and heated at 250 °C for 2 min. After soldering, the solder cap formed on the Cu surface. 2.2. Cross sectioning and polishing When the solder joint of Sn3.0Ag0.5Cu/Cu was ready, the solder cap was cross sectioned. To facilitate making marks, the cross section and the opposite surface of the specimen were ground to remain parallel. When the two surfaces were ground
H. Li et al. / Journal of Alloys and Compounds 634 (2015) 94–98 to remain parallel, the cross section was ground sequentially with 240, 800, 1500, 2000 and 3000 grits SiC abrasive papers, respectively. Finally, the specimen was polished through 0.5 lm diamond paste. 2.3. Marking by nanoindentation In order to illustrate the interfacial movement quantitatively, marks used as reference points made by nanoindentation were carved on the polished cross section of the solder cap. Nanoindentation was selected to make marks at the interface of Sn3.0Ag0.5Cu/Cu, since the nanoindentation tester equipped with a 400 magnifying lens can make marks precisely after calibration. The indenter is the shape of triangular pyramid, and the load is 25 g. After the nanoindentation test, a clearly triangular pyramid mark can be found at the interface clearly, as shown in Fig. 1(a)-1. 2.4. Thermal aging To promote the growth of the IMCs, the marked specimen was aged at 150 °C. In addition, to prevent the oxidation of the cross section, vacuum oven was used. To further reduce the oxidation of the surface, after the first evacuation, the argon gas was used to fill the oven, and the second evacuation was performed. After the second evacuation, the pressure of oven was about 100 Pa. 2.5. SEM observation and sample preparing While the specimen was aged at 150 °C, it was fetched out to take photos at specified time. The interfacial microstructures were taken photos with a scanning electron microscope (SEM) equipped with EDX. The specified time was chosen at 250 h, 500 h, 750 h and 1000 h, respectively. When the specimen was aged at 150 °C, the IMCs at the solder joint would grow, which inevitably caused the movement of the interface. By processing these images, we can calculate the interfacial movement quantitatively. After the specimen was taken out from the vacuum oven to take photos, though vacuum and inert gas were applied, some oxidation would happen at the cross section of the solder cap. To remove the oxidation on the surface of the solder joint, the sample was lightly polished. These actions may destroy the marks. To go on the observation, some other new marks need to be made before the next steps, as shown in Fig. 1(b)-3 and (c)-3. 2.6. Measurement of IMC thickness and the interfacial movement Measurement of IMC thickness and the interfacial movement were done by the software of Photoshop to digital images taken by SEM. The average IMC thicknesses were determined by dividing the cross-sectional area (A) by the linear length (L), as shown in the following equation:
X ¼ A=L
ð1Þ
As for the interfacial movement, the marks were used as the reference point. The average distance between the reference point and the Cu/Cu3Sn interface was also determined by dividing the cross-sectional area (A) by the linear length (L).
3. Results and discussion 3.1. Microstructural morphology The in situ interfacial microstructural evolution over aging time is shown in Fig. 1. After reflowing, a layer of scallop-like Cu6Sn5 was found at the interface of joint, adjacent to which was a thin layer of Cu3Sn, as shown in Fig. 1(a)-1. Then the solder joint was aged at 150 °C for 250 h. After aging treatment, it can be found that the original layer of Cu6Sn5 was covered with a new generated layer of Cu6Sn5, as shown in Fig. 1(b)-1. For this new generated layer of Cu6Sn5, it can be inferred that the Cu atoms were mainly from the supersaturated solder matrix [10], which can be verified from the consumption of Cu substrate in the following discussion. At the same time, the gaps between Cu6Sn5 scallops were blocked by the growth of Cu6Sn5, as shown in Fig. 1(b)-2 and (b)-3. After the solder joint was annealed at 150 °C for 500 h, the gaps between scallops were further blocked and the Cu6Sn5 gradually changed from scallop type to layer type, as shown from Fig. 1(c)-1–(c)-3. When the aging time exceeded 500 h, the gaps between scallops disappeared, and the layer type Cu6Sn5 had a moderate increase in thickness with aging time increasing, as shown from Fig. 1(d)-
95
1–(e)-2. As for the Cu3Sn layer, during the whole solid state aging, it grew in layer type at a slow rate. After soldering, a lot of tiny Ag3Sn particles were dispersed in the solder bulk. When the specimen was aged at 150 °C for a certain time, these tiny Ag3Sn particles would coalesce and grow up. Some of these Ag3Sn particles gathered at the interface of Cu6Sn5/ Sn3.0Ag0.5Cu, a few even went into the gaps between Cu6Sn5 scallops. With the growth of IMCs, these particles could be embedded into the bulk of Cu6Sn5 layer. Fig. 1(c)-2 showed an Ag3Sn particle were encapsulated by the Cu6Sn5 layer. These encapsulated Ag3Sn particles indicated that the growth of Cu6Sn5 was imbalance, which had a preferential growth at the gaps between Cu6Sn5 scallops to fill up the gaps. From Fig. 1(a)-1, we could also find an interesting phenomenon. After reflowing, the interface at Cu3Sn/Cu was wavy. Below the scallops, the Cu/Cu3Sn interface was concave downward. While below the gaps, the Cu/Cu3Sn interface was curved upward. This indicated that the Cu atoms dissolved into liquid solder during reflowing were uneven, and mainly from the Cu substrate below the gaps. These gaps between scallops formed during reflowing would be the fast diffusion path and go on working at the initial stage of solid state aging. Since the gaps were not blocked at the initial stage of solid state aging, the Cu atoms would preferentially diffuse into the solder matrix from the gaps between scallops to form IMCs and flatten the IMC layers [11]. 3.2. Growth behavior of IMC layers and interfacial movement The IMC thickness can be easily obtained by the Eq. (1). But to gain the interfacial movement, a fixed reference point is needed. We use the central point of mark 1 as reference point 1 (RP 1). The distance between RP 1 and the interface can be calculated like the IMC thickness. When distance between RP 1 and the interface at different aging time was obtained, the interfacial movement over aging time is also acquired. To study the interfacial movement, some assumptions need to be made: (1) This experiment only studied the interfacial diffusion during solid state aging. While the consumption of Cu substrate and the thicknesses of Cu6Sn5 and Cu3Sn layer during reflowing would be used as the initial values for the subsequent calculations. (2) The marks made by nanoindentation are assumed to be fixed during solid state aging. Due to the interdiffusion between Cu substrate and the solder, the marks inevitably have a slight movement. But this slight movement caused by interdiffusion is within the uncertainty of measurements and has no effect on the measurement results. At the same time, due to the big size of mark, the slight movement caused by interdiffusion is also negligible. Therefore, this assumption stands. (3) During solid state aging, the volume concentration caused by the formation of IMCs at the interface is neglected. When pure Cu and Sn react and form g0 -Cu6Sn5, there is a net 5.44% concentration, while a net 9.97% decrease in volume will also occur during the formation of e-Cu3Sn [12,13]. However, even a 10% volume contraction could only lead to an approximately 3% decrease in the linear growth rate of a relaxed planar layer, which is within the accuracy of measurement of the growth of IMC layers in practice [14]. Therefore, the volume concentration has not been taken into the calculation here. Under these assumptions, we can calculate the distance between the reference point 1 and the Cu/Cu3Sn interface. During the experiment, RP 1 was ground off after 500 h, as shown in Fig. 1(c)-2. To go
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H. Li et al. / Journal of Alloys and Compounds 634 (2015) 94–98
Fig. 1. In situ interfacial microstructures after aging at 150 °C for (a) 0 h, (b) 250 h, (c) 500 h, (d) 750 h and (e) 1000 h.
on the experiment, mark 2 was made after 250 h, as shown in Fig. 1(b)-3. The distance between RP 1 and RP 2 is 5.65 lm. When RP 1 was gone, we can use RP 2 as the reference point to go on with the measurement. By adding the distance between RP 1 and RP 2, we could also get the distance of the interface of Cu/Cu3Sn relative to the RP 1. As for the interface of Cu3Sn/Cu6Sn5 and Cu6Sn5/ Sn3.0Ag0.5Cu, the distance between them and RP 1 could be obtained by adding the average layer thicknesses of Cu3Sn and Cu6Sn5 to the coordinate of Cu/Cu3Sn interface, respectively. The IMC thickness and interfacial movement was shown in Table 1.
3.2.1. Growth behavior of IMC layers The relationship between IMC thickness and aging time is generally considered to be diffusion-controlled. The IMC thickness is plotted as a function of the square root of aging time in Fig. 2. From Fig. 2, it can be seen that the mean thickness of the interfacial Cu– Sn IMC layer increases linearly with the square root of aging time, indicating that the growth of IMCs is an ordinary diffusion-controlled reaction described by classic kinetic theory and the rate is controlled by the interdiffusion of constitutive species (i.e. Cu and Sn).
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H. Li et al. / Journal of Alloys and Compounds 634 (2015) 94–98 Table 1 The thickness of IMCs and interfacial movement*. Aging time
0h
250 h
500 h
750 h
1000 h
Thick of Cu6Sn5 (lm)
3.62
5.29 5.32
6.06 5.77
6.44
6.24
Thick of Cu3Sn (lm)
0.19
0.44 0.42
0.74 0.74
1.06
1.28
Distance between RP 1 and the Cu/Cu3Sn interface (lm)
8.07
8.21
8.82
8.93
8.97
8.23
8.79
*
There are two measurement values at the time of 250 h and 500 h due to the new marks were made to continue the experiment.
Fig. 2. The linear fit of IMC thicknesses versus square root of aging time.
there would be about 0.21 lm Cu6Sn5 formed at the interface, the density and molar volume data used in the calculation is listed in Table 2. However, there are actually about 1.685 lm Cu6Sn5, much greater than the 0.21 lm. Therefore, at the initial stage of solid state aging, the Cu atoms to form Cu6Sn5 were not mainly from the Cu substrate, and primarily from the supersaturated solder matrix, which also verified the previous inference. From 250 h to 500 h, the Cu substrate had a large consumption. At this stage, the Cu atoms from supersaturated solder matrix were almost exhausted, so the consumed Cu substrate was principally used to form Cu6Sn5. The consumed Cu thickness from 250 h to 500 h was 0.585 lm. If these Cu atoms were completely converted to Cu6Sn5, there would be about 0.82 lm Cu6Sn5 to form at the interface. Actually, the formed Cu6Sn5 thickness was 0.755 lm, close to the 0.82 lm. So, we can conclude that the consumed Cu substrate was mainly used to form Cu6Sn5. From Fig. 1(c)-2, we can find that the gaps between scallops were blocked. Therefore, the consumed Cu substrate was mainly used to form Cu6Sn5 to fill up the gaps between scallops. When aging time exceeds 500 h, the gaps disappeared. Due to the lack of fast diffusion paths, the consumed rate of Cu substrate slowed down. From Fig. 3, we can also find that the layer of Cu3Sn grew on both sides. Though at 500 h, there was a sharp drop of Cu3Sn layer on the side of solder matrix, which is mainly due to the fast consumption of Cu substrate. But with the aging time increasing, the Cu3Sn layer will rise up again on the side of solder matrix. The consumed Cu thicknesses from 500 h to 750 h and from 750 h to 1000 h were 0.125 lm and 0.04 lm, respectively. If these Cu atoms were completely converted to Cu3Sn, there would be 0.15 lm and 0.05 lm Cu3Sn to form, respectively. But the actually Cu3Sn thicknesses were 0.32 lm and 0.22 lm, greater than the calculated values. So we can conclude that the Cu6Sn5 were converted to Cu3Sn. The mobility of Cu atoms in the pure Sn phase is extremely rapid by an interstitial diffusion mechanism [8,15,16]. At 150 °C, the diffusion coefficient of Cu in Sn was 2.05 1011 m2/s [15]. Therefore, the following reaction occurred at the Cu6Sn5/Cu3Sn interface (the elements in square brackets denote the diffusing species):
Cu6 Sn5 þ 9½Cu 5Cu3 Sn
Fig. 3. The interfacial position as a function of aging time.
3.2.2. The interfacial movement Since the distance between RP 1 and the Cu3Sn/Cu interface and the thicknesses of IMCs were obtained, we choose the interface of Cu3Sn/Cu as the origin of the coordinate, thus we can draw the interfacial position as a function of aging time, as shown in Fig. 3. From Fig. 3, we find that at the initial stage of solid state aging, such as at 250 h, the Cu substrate was consumed only a little, about 0.15 lm. If these Cu atoms were completely converted to Cu6Sn5,
ð2Þ
This reaction would lead to the Cu3Sn layer grow on both sides. It is well known that the thickness of Cu3Sn layer in pure Sn/Cu solder joint is thicker than that in SnAgCu/Cu solder joint, and the alloying elements of Ag and Cu would inhibit the growth of Cu3Sn layer [8]. This difference may be caused by the diffusion of Sn atoms. WQ Peng considered that though the dominant diffusing species is Cu at the interface of solder joint, there are still a certain amount of Sn atoms diffusing into the Cu substrate [17]. For the pure Sn solder, these diffusing Sn atoms would strengthen the diffusion of Cu atoms, namely strengthen the Eq. (2), which would lead to a thicker Cu3Sn layer. For the SnAgCu solder, due to the combination of Sn atoms with Ag atoms and Cu atoms, the Sn atoms diffusing into the Cu substrate was negligible, which would reduce the diffusion of Cu atoms and hence weaken the Eq. (2), leading to a thinner Cu3Sn layer. Furthermore, to supply Sn atoms to form Cu3Sn at the interface of Cu3Sn/Cu, the following reaction would happen at the interface of Cu6Sn5/Cu3Sn:
Cu6 Sn5 Cu3 Sn þ 3½Sn
ð3Þ
Table 2 Densities and molar volumes of Cu, b-Sn, e-Cu3Sn, g0 -Cu6Sn5.
Density (g/cm3) Molar volume (cm3/mol)
Cu
b-Sn
e-Cu3Sn
g0 -Cu6Sn5
8.96 7.09
7.28 16.29
9.14 33.83
8.28 117.66
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H. Li et al. / Journal of Alloys and Compounds 634 (2015) 94–98
This reaction also grow Cu3Sn layer on the both sides. However, it dramatically weakens the Eq. (2) duo to inhibit the diffusion of Cu atoms by the combination of Sn atoms with Cu atoms. Therefore, the thickness of Cu3Sn layer in SnAgCu solder is much less than that in pure Sn solder. 4. Conclusions The in situ interfacial microstructural evolution of the Sn3.0Ag0.5Cu/Cu solder joint during solid state aging was quantitatively studied using the marks made by nanoindentation as the reference points. The following conclusions are obtained: (1) The wavy interface of Cu3Sn/Cu after reflowing indicated that the dissolution of Cu substrate during reflowing was uneven, which mainly occurred under the gaps between scallops. (2) Though the growth of IMCs was diffusion-controlled, the consumption of Cu substrate was not linear with the square root of aging time. At the initial stage of solid state aging, the Cu atoms to form IMCs were primarily from the supersaturated solder matrix. During the subsequent solid state aging, the gaps between scallops were gradually blocked with the IMCs growing. For these IMCs, the Cu atoms were primarily from the Cu substrate. When the gaps disappeared, the consumption of Cu substrate slowed down. (3) The growth of Cu3Sn layer in Sn3.0Ag0.5Cu/Cu solder joint was on the both sides according to the quantitative calculation. (4) The thickness of Cu3Sn layer in Sn3.0Ag0.5Cu/Cu solder joint was much thinner than that in pure Sn/Cu solder joint, which was due to the inhibiting effect of alloying elements of Ag and Cu on the diffusion of Sn atoms into the Cu substrate.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51005055) and Fundamental Research Funds for the Central Universities (No. HIT.NSRIF.2015066).
References [1] K.N. Tu, T.Y. Lee, J.W. Jang, L. Li, D.R. Frear, K. Zeng, J.K. Kivilahti, Wetting reaction versus solid state aging of eutectic SnPb on Cu, J. Appl. Phys. 89 (2001) 4843–4849. [2] H.K. Kim, H.K. Liou, K.N. Tu, Three-dimensional morphology of a very rough interface formed in the soldering reaction between eutectic SnPb and Cu, Appl. Phys. Lett. 66 (1995) 2337–2339. [3] S.K. Kang, R.S. Rai, S. Purushothaman, Interfacial reactions during soldering with lead–tin eutectic and lead (Pb)-free, tin-rich solders, J. Electron. Mater. 25 (1996) 1113–1120. [4] K.N. Tu, K. Zeng, Tin-lead (SnPb) solder reaction in flip chip technology, Mater. Sci. Eng., R 34 (2001) 1–58. [5] H.K. Kim, K.N. Tu, Kinetic analysis of the soldering reaction between eutectic SnPb alloy and Cu accompanied by ripening, Phys. Rev. B 53 (1996) 16027– 16034. [6] J.C. Gong, C.Q. Liu, P.P. Conway, V.V. Silberschmidt, Evolution of CuSn intermetallics between molten SnAgCu solder and Cu substrate, Acta Mater. 56 (2008) 4291–4297. [7] K.N. Tu, R.D. Thompson, Kinetics of interfacial reaction in bimetallic Cu/Sn thin films, Acta Metall. 30 (1982) 947–952. [8] Y. Yang, Y.Z. Li, H. Lu, C. Yu, J.M. Chen, Interdiffusion at the interface between Sn-based solders and Cu substrate, Microelectron. Reliab. 53 (2013) 327–333. [9] F.Y. Ouyang, C.L. Kao, In situ observation of thermomigration of Sn atoms to the hot end of 96.5Sn–3Ag–0.5Cu flip chip solder joints, J. Appl. Phys. 110 (2011) 123525. [10] M.L. Huang, T. Loeher, A. Ostmann, H. Reichl, Role of Cu in dissolution kinetics of Cu metallization in molten Sn-based solders, Appl. Phys. Lett. 86 (2005) 181908. [11] J. Shen, M.L. Zhao, P.P. He, Y.Y. Pu, Growth behaviors of intermetallic compounds at Sn–3Ag–0.5Cu/Cu interface during isothermal and nonisothermal aging, J. Alloys Comp. 574 (2013) 451–458. [12] J.Y. Song, J. Yu, T.Y. Lee, Effects of reactive diffusion on stress evolution in Cu– Sn films, Scr. Mater. 51 (2004) 167–170. [13] E. Buchovecky, N. Jadhav, A.F. Bower, E. Chason, Finite element modeling of stress evolution in Sn films due to growth of the Cu6Sn5 intermetallic compound, J. Electron. Mater. 38 (2009) 2676–2684. [14] Z. Mei, A.J. Sunwoo, J.W. Morris, Analysis of low-temperature intermetallic growth in copper–tin diffusion couples, Metall. Trans. A 23 (1992) 857–864. [15] B. Chao, S.H. Chae, X.F. Zhang, K.H. Lu, J. Im, P.S. Ho, Investigation of diffusion and electromigration parameters for Cu–Sn intermetallic compounds in Pbfree solders using simulated annealing, Acta Mater. 55 (2007) 2805–2814. [16] B.F. Dyson, T.R. Anthony, D. Turnbull, Interstitial diffusion of copper in tin, J. Appl. Phys. 38 (1967). 3408–3408. [17] W.Q. Peng, E. Monlevade, M.E. Marques, Effect of thermal aging on the interfacial structure of SnAgCu solder joints on Cu, Microelectron. Reliab. 47 (2007) 2161–2168.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
In situ quantitative study of microstructural evolution at the interface of Sn3.0Ag0.5Cu/Cu solder joint during solid state aging Hailong Li a, Rong An b,⇑, Chunqing Wang a, Zhi Jiang a a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, PR China Key Laboratory of Micro-systems and Micro-structures Manufacturing, Harbin Institute of Technology, Ministry of Education, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 9 January 2015 Received in revised form 9 February 2015 Accepted 11 February 2015 Available online 16 February 2015 Keywords: Interfacial reaction Solder joint Quantitative Microstructure Nanoindentation
a b s t r a c t In situ microstructural evolution at the interface of Sn3.0Ag0.5Cu/Cu solder joint during solid state aging was quantitatively studied by nanoindentation. The morphology of Cu6Sn5 gradually altered from scallop type to layer type. Though the growth of IMCs was diffusion-controlled, the consumption of Cu substrate was not linear with the square root of aging time. At the initial stage of solid state aging, the Cu atoms essential to the growth of IMCs were mainly from the supersaturated solder matrix. When the Cu atoms from supersaturated solder matrix were exhausted, the Cu atoms for the growth of IMCs were primarily from the Cu substrate. In addition, the IMCs formed at this state were principally used to fill up the gaps between scallops. After the gaps disappeared, the consumption of Cu substrate slowed down. Furthermore, the growth of Cu3Sn layer in Sn3.0Ag0.5Cu/Cu solder joint was on the both sides with layer type. Since the Sn atoms were inhibited to diffuse into the Cu substrate by the alloying elements of Ag and Cu, the thickness of Cu3Sn layer in SnAgCu/Cu solder joint was much thinner than that in pure Sn/Cu solder joint. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Interconnect technology based on the reactions between Cu and Sn plays a key role in the manufacturing process of electronics. During reflowing, the reactions between molten solder and bond pad have a significant influence on the quality of solder joints. However, the subsequent solid state interfacial reactions are closely related to the reliability of solder joints in the field. There are lots of studies on the reflowing process [1–4]. Tu et al. has established the growth model of scallops in SnPb/Cu solder joints during reflowing [5]. Gong et al. studied the evolution of SnCu IMCs between a molten SnAgCu solder and the Cu under bump metallization (UBM) throughout reflowing based on interruption of soldering reactions in experiments by removing the liquid solder from the substrate [6]. Nevertheless, most of the methods used to study the solid state aging just remained in the level of qualitative and statistical analysis due to the slower reaction rate and the difficulty of precise measurements of interfacial movement. Tu and Thompson deposited a flash of W film between the Cu and Sn thin films and studied the interfacial reactions by mean of Rutherford backscattering, finding that Cu is the dominant diffusing species [7]. Yang et al. quantitatively studied the migration of interphase interfaces in Sn-based solder joints using the upper surface of ⇑ Corresponding author. Tel.: +86 451 86418725; fax: +86 451 86416186. E-mail address: [email protected] (R. An). http://dx.doi.org/10.1016/j.jallcom.2015.02.088 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
unreacted Cu substrate as a reference [8], but the quantitatively measurements with aging time were not for the same sample. Ouyang and Kao studied the in situ thermomigration of Sn3.0Ag0.5Cu solder using nanoindentation marks [9]. However, these nanoindentation marks migrated due to the small size and the impact of thermal gradient, and were not able to be used in quantitative calculation. In this article, the in situ microstructural evolution of Sn3.0Ag0.5Cu/Cu solder joint during solid state aging was studied. By making the nanoindentation marks as the reference points, the in situ interfacial movement and IMC thickness were calculated precisely. 2. Experimental 2.1. Materials and soldering The Cu block with 10 mm 10 mm 10 mm in size was used as substrate. Then, a layer of solder paste of Sn3.0A0.5Cu was dispensed through a metal mask with an opening of 6 mm diameter onto one of the six surfaces of the Cu block. After that, the Cu block was placed on the hot plate and heated at 250 °C for 2 min. After soldering, the solder cap formed on the Cu surface. 2.2. Cross sectioning and polishing When the solder joint of Sn3.0Ag0.5Cu/Cu was ready, the solder cap was cross sectioned. To facilitate making marks, the cross section and the opposite surface of the specimen were ground to remain parallel. When the two surfaces were ground
H. Li et al. / Journal of Alloys and Compounds 634 (2015) 94–98 to remain parallel, the cross section was ground sequentially with 240, 800, 1500, 2000 and 3000 grits SiC abrasive papers, respectively. Finally, the specimen was polished through 0.5 lm diamond paste. 2.3. Marking by nanoindentation In order to illustrate the interfacial movement quantitatively, marks used as reference points made by nanoindentation were carved on the polished cross section of the solder cap. Nanoindentation was selected to make marks at the interface of Sn3.0Ag0.5Cu/Cu, since the nanoindentation tester equipped with a 400 magnifying lens can make marks precisely after calibration. The indenter is the shape of triangular pyramid, and the load is 25 g. After the nanoindentation test, a clearly triangular pyramid mark can be found at the interface clearly, as shown in Fig. 1(a)-1. 2.4. Thermal aging To promote the growth of the IMCs, the marked specimen was aged at 150 °C. In addition, to prevent the oxidation of the cross section, vacuum oven was used. To further reduce the oxidation of the surface, after the first evacuation, the argon gas was used to fill the oven, and the second evacuation was performed. After the second evacuation, the pressure of oven was about 100 Pa. 2.5. SEM observation and sample preparing While the specimen was aged at 150 °C, it was fetched out to take photos at specified time. The interfacial microstructures were taken photos with a scanning electron microscope (SEM) equipped with EDX. The specified time was chosen at 250 h, 500 h, 750 h and 1000 h, respectively. When the specimen was aged at 150 °C, the IMCs at the solder joint would grow, which inevitably caused the movement of the interface. By processing these images, we can calculate the interfacial movement quantitatively. After the specimen was taken out from the vacuum oven to take photos, though vacuum and inert gas were applied, some oxidation would happen at the cross section of the solder cap. To remove the oxidation on the surface of the solder joint, the sample was lightly polished. These actions may destroy the marks. To go on the observation, some other new marks need to be made before the next steps, as shown in Fig. 1(b)-3 and (c)-3. 2.6. Measurement of IMC thickness and the interfacial movement Measurement of IMC thickness and the interfacial movement were done by the software of Photoshop to digital images taken by SEM. The average IMC thicknesses were determined by dividing the cross-sectional area (A) by the linear length (L), as shown in the following equation:
X ¼ A=L
ð1Þ
As for the interfacial movement, the marks were used as the reference point. The average distance between the reference point and the Cu/Cu3Sn interface was also determined by dividing the cross-sectional area (A) by the linear length (L).
3. Results and discussion 3.1. Microstructural morphology The in situ interfacial microstructural evolution over aging time is shown in Fig. 1. After reflowing, a layer of scallop-like Cu6Sn5 was found at the interface of joint, adjacent to which was a thin layer of Cu3Sn, as shown in Fig. 1(a)-1. Then the solder joint was aged at 150 °C for 250 h. After aging treatment, it can be found that the original layer of Cu6Sn5 was covered with a new generated layer of Cu6Sn5, as shown in Fig. 1(b)-1. For this new generated layer of Cu6Sn5, it can be inferred that the Cu atoms were mainly from the supersaturated solder matrix [10], which can be verified from the consumption of Cu substrate in the following discussion. At the same time, the gaps between Cu6Sn5 scallops were blocked by the growth of Cu6Sn5, as shown in Fig. 1(b)-2 and (b)-3. After the solder joint was annealed at 150 °C for 500 h, the gaps between scallops were further blocked and the Cu6Sn5 gradually changed from scallop type to layer type, as shown from Fig. 1(c)-1–(c)-3. When the aging time exceeded 500 h, the gaps between scallops disappeared, and the layer type Cu6Sn5 had a moderate increase in thickness with aging time increasing, as shown from Fig. 1(d)-
95
1–(e)-2. As for the Cu3Sn layer, during the whole solid state aging, it grew in layer type at a slow rate. After soldering, a lot of tiny Ag3Sn particles were dispersed in the solder bulk. When the specimen was aged at 150 °C for a certain time, these tiny Ag3Sn particles would coalesce and grow up. Some of these Ag3Sn particles gathered at the interface of Cu6Sn5/ Sn3.0Ag0.5Cu, a few even went into the gaps between Cu6Sn5 scallops. With the growth of IMCs, these particles could be embedded into the bulk of Cu6Sn5 layer. Fig. 1(c)-2 showed an Ag3Sn particle were encapsulated by the Cu6Sn5 layer. These encapsulated Ag3Sn particles indicated that the growth of Cu6Sn5 was imbalance, which had a preferential growth at the gaps between Cu6Sn5 scallops to fill up the gaps. From Fig. 1(a)-1, we could also find an interesting phenomenon. After reflowing, the interface at Cu3Sn/Cu was wavy. Below the scallops, the Cu/Cu3Sn interface was concave downward. While below the gaps, the Cu/Cu3Sn interface was curved upward. This indicated that the Cu atoms dissolved into liquid solder during reflowing were uneven, and mainly from the Cu substrate below the gaps. These gaps between scallops formed during reflowing would be the fast diffusion path and go on working at the initial stage of solid state aging. Since the gaps were not blocked at the initial stage of solid state aging, the Cu atoms would preferentially diffuse into the solder matrix from the gaps between scallops to form IMCs and flatten the IMC layers [11]. 3.2. Growth behavior of IMC layers and interfacial movement The IMC thickness can be easily obtained by the Eq. (1). But to gain the interfacial movement, a fixed reference point is needed. We use the central point of mark 1 as reference point 1 (RP 1). The distance between RP 1 and the interface can be calculated like the IMC thickness. When distance between RP 1 and the interface at different aging time was obtained, the interfacial movement over aging time is also acquired. To study the interfacial movement, some assumptions need to be made: (1) This experiment only studied the interfacial diffusion during solid state aging. While the consumption of Cu substrate and the thicknesses of Cu6Sn5 and Cu3Sn layer during reflowing would be used as the initial values for the subsequent calculations. (2) The marks made by nanoindentation are assumed to be fixed during solid state aging. Due to the interdiffusion between Cu substrate and the solder, the marks inevitably have a slight movement. But this slight movement caused by interdiffusion is within the uncertainty of measurements and has no effect on the measurement results. At the same time, due to the big size of mark, the slight movement caused by interdiffusion is also negligible. Therefore, this assumption stands. (3) During solid state aging, the volume concentration caused by the formation of IMCs at the interface is neglected. When pure Cu and Sn react and form g0 -Cu6Sn5, there is a net 5.44% concentration, while a net 9.97% decrease in volume will also occur during the formation of e-Cu3Sn [12,13]. However, even a 10% volume contraction could only lead to an approximately 3% decrease in the linear growth rate of a relaxed planar layer, which is within the accuracy of measurement of the growth of IMC layers in practice [14]. Therefore, the volume concentration has not been taken into the calculation here. Under these assumptions, we can calculate the distance between the reference point 1 and the Cu/Cu3Sn interface. During the experiment, RP 1 was ground off after 500 h, as shown in Fig. 1(c)-2. To go
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Fig. 1. In situ interfacial microstructures after aging at 150 °C for (a) 0 h, (b) 250 h, (c) 500 h, (d) 750 h and (e) 1000 h.
on the experiment, mark 2 was made after 250 h, as shown in Fig. 1(b)-3. The distance between RP 1 and RP 2 is 5.65 lm. When RP 1 was gone, we can use RP 2 as the reference point to go on with the measurement. By adding the distance between RP 1 and RP 2, we could also get the distance of the interface of Cu/Cu3Sn relative to the RP 1. As for the interface of Cu3Sn/Cu6Sn5 and Cu6Sn5/ Sn3.0Ag0.5Cu, the distance between them and RP 1 could be obtained by adding the average layer thicknesses of Cu3Sn and Cu6Sn5 to the coordinate of Cu/Cu3Sn interface, respectively. The IMC thickness and interfacial movement was shown in Table 1.
3.2.1. Growth behavior of IMC layers The relationship between IMC thickness and aging time is generally considered to be diffusion-controlled. The IMC thickness is plotted as a function of the square root of aging time in Fig. 2. From Fig. 2, it can be seen that the mean thickness of the interfacial Cu– Sn IMC layer increases linearly with the square root of aging time, indicating that the growth of IMCs is an ordinary diffusion-controlled reaction described by classic kinetic theory and the rate is controlled by the interdiffusion of constitutive species (i.e. Cu and Sn).
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H. Li et al. / Journal of Alloys and Compounds 634 (2015) 94–98 Table 1 The thickness of IMCs and interfacial movement*. Aging time
0h
250 h
500 h
750 h
1000 h
Thick of Cu6Sn5 (lm)
3.62
5.29 5.32
6.06 5.77
6.44
6.24
Thick of Cu3Sn (lm)
0.19
0.44 0.42
0.74 0.74
1.06
1.28
Distance between RP 1 and the Cu/Cu3Sn interface (lm)
8.07
8.21
8.82
8.93
8.97
8.23
8.79
*
There are two measurement values at the time of 250 h and 500 h due to the new marks were made to continue the experiment.
Fig. 2. The linear fit of IMC thicknesses versus square root of aging time.
there would be about 0.21 lm Cu6Sn5 formed at the interface, the density and molar volume data used in the calculation is listed in Table 2. However, there are actually about 1.685 lm Cu6Sn5, much greater than the 0.21 lm. Therefore, at the initial stage of solid state aging, the Cu atoms to form Cu6Sn5 were not mainly from the Cu substrate, and primarily from the supersaturated solder matrix, which also verified the previous inference. From 250 h to 500 h, the Cu substrate had a large consumption. At this stage, the Cu atoms from supersaturated solder matrix were almost exhausted, so the consumed Cu substrate was principally used to form Cu6Sn5. The consumed Cu thickness from 250 h to 500 h was 0.585 lm. If these Cu atoms were completely converted to Cu6Sn5, there would be about 0.82 lm Cu6Sn5 to form at the interface. Actually, the formed Cu6Sn5 thickness was 0.755 lm, close to the 0.82 lm. So, we can conclude that the consumed Cu substrate was mainly used to form Cu6Sn5. From Fig. 1(c)-2, we can find that the gaps between scallops were blocked. Therefore, the consumed Cu substrate was mainly used to form Cu6Sn5 to fill up the gaps between scallops. When aging time exceeds 500 h, the gaps disappeared. Due to the lack of fast diffusion paths, the consumed rate of Cu substrate slowed down. From Fig. 3, we can also find that the layer of Cu3Sn grew on both sides. Though at 500 h, there was a sharp drop of Cu3Sn layer on the side of solder matrix, which is mainly due to the fast consumption of Cu substrate. But with the aging time increasing, the Cu3Sn layer will rise up again on the side of solder matrix. The consumed Cu thicknesses from 500 h to 750 h and from 750 h to 1000 h were 0.125 lm and 0.04 lm, respectively. If these Cu atoms were completely converted to Cu3Sn, there would be 0.15 lm and 0.05 lm Cu3Sn to form, respectively. But the actually Cu3Sn thicknesses were 0.32 lm and 0.22 lm, greater than the calculated values. So we can conclude that the Cu6Sn5 were converted to Cu3Sn. The mobility of Cu atoms in the pure Sn phase is extremely rapid by an interstitial diffusion mechanism [8,15,16]. At 150 °C, the diffusion coefficient of Cu in Sn was 2.05 1011 m2/s [15]. Therefore, the following reaction occurred at the Cu6Sn5/Cu3Sn interface (the elements in square brackets denote the diffusing species):
Cu6 Sn5 þ 9½Cu 5Cu3 Sn
Fig. 3. The interfacial position as a function of aging time.
3.2.2. The interfacial movement Since the distance between RP 1 and the Cu3Sn/Cu interface and the thicknesses of IMCs were obtained, we choose the interface of Cu3Sn/Cu as the origin of the coordinate, thus we can draw the interfacial position as a function of aging time, as shown in Fig. 3. From Fig. 3, we find that at the initial stage of solid state aging, such as at 250 h, the Cu substrate was consumed only a little, about 0.15 lm. If these Cu atoms were completely converted to Cu6Sn5,
ð2Þ
This reaction would lead to the Cu3Sn layer grow on both sides. It is well known that the thickness of Cu3Sn layer in pure Sn/Cu solder joint is thicker than that in SnAgCu/Cu solder joint, and the alloying elements of Ag and Cu would inhibit the growth of Cu3Sn layer [8]. This difference may be caused by the diffusion of Sn atoms. WQ Peng considered that though the dominant diffusing species is Cu at the interface of solder joint, there are still a certain amount of Sn atoms diffusing into the Cu substrate [17]. For the pure Sn solder, these diffusing Sn atoms would strengthen the diffusion of Cu atoms, namely strengthen the Eq. (2), which would lead to a thicker Cu3Sn layer. For the SnAgCu solder, due to the combination of Sn atoms with Ag atoms and Cu atoms, the Sn atoms diffusing into the Cu substrate was negligible, which would reduce the diffusion of Cu atoms and hence weaken the Eq. (2), leading to a thinner Cu3Sn layer. Furthermore, to supply Sn atoms to form Cu3Sn at the interface of Cu3Sn/Cu, the following reaction would happen at the interface of Cu6Sn5/Cu3Sn:
Cu6 Sn5 Cu3 Sn þ 3½Sn
ð3Þ
Table 2 Densities and molar volumes of Cu, b-Sn, e-Cu3Sn, g0 -Cu6Sn5.
Density (g/cm3) Molar volume (cm3/mol)
Cu
b-Sn
e-Cu3Sn
g0 -Cu6Sn5
8.96 7.09
7.28 16.29
9.14 33.83
8.28 117.66
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This reaction also grow Cu3Sn layer on the both sides. However, it dramatically weakens the Eq. (2) duo to inhibit the diffusion of Cu atoms by the combination of Sn atoms with Cu atoms. Therefore, the thickness of Cu3Sn layer in SnAgCu solder is much less than that in pure Sn solder. 4. Conclusions The in situ interfacial microstructural evolution of the Sn3.0Ag0.5Cu/Cu solder joint during solid state aging was quantitatively studied using the marks made by nanoindentation as the reference points. The following conclusions are obtained: (1) The wavy interface of Cu3Sn/Cu after reflowing indicated that the dissolution of Cu substrate during reflowing was uneven, which mainly occurred under the gaps between scallops. (2) Though the growth of IMCs was diffusion-controlled, the consumption of Cu substrate was not linear with the square root of aging time. At the initial stage of solid state aging, the Cu atoms to form IMCs were primarily from the supersaturated solder matrix. During the subsequent solid state aging, the gaps between scallops were gradually blocked with the IMCs growing. For these IMCs, the Cu atoms were primarily from the Cu substrate. When the gaps disappeared, the consumption of Cu substrate slowed down. (3) The growth of Cu3Sn layer in Sn3.0Ag0.5Cu/Cu solder joint was on the both sides according to the quantitative calculation. (4) The thickness of Cu3Sn layer in Sn3.0Ag0.5Cu/Cu solder joint was much thinner than that in pure Sn/Cu solder joint, which was due to the inhibiting effect of alloying elements of Ag and Cu on the diffusion of Sn atoms into the Cu substrate.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51005055) and Fundamental Research Funds for the Central Universities (No. HIT.NSRIF.2015066).
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