- Email: [email protected]
Interdiffusion at the interface between Sn-based solders and Cu substrate
Microelectronics Reliability 53 (2013) 327–333
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Interdiffusion at the interface between Sn-based solders and Cu substrate Yang Yang a,⇑, Yongzhi Li a, Hao Lu a,b,⇑, Chun Yu a, Junmei Chen a a b
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China Key Lab of Shanghai Laser Manufacturing and Materials Modification, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2012 Received in revised form 12 August 2012 Accepted 13 August 2012 Available online 5 September 2012
a b s t r a c t The migration of interphase interfaces in Sn-based solder joints was investigated by using the upper surface of unreacted Cu substrate (the original reaction interface) as a reference. Then a model was built to study the interdiffusion of Cu and Sn in Cu6Sn5 and Cu3Sn layers. After being aged at 150 °C, both solder/Cu6Sn5 and Cu6Sn5/Cu3Sn interfaces moved toward the solder matrix, and the Cu3Sn/Cu interface migrated toward the Cu substrate. Compared with Sn, Cu was found to be the faster diffuser in Cu6Sn5 and Cu3Sn layers. The diffusion fluxes (DFs) of Cu and Sn in both intermetallic compound (IMC) layers decreased with the extension of aging period. The DF of Cu in Cu3Sn layer got close to that in Cu6Sn5 layer. So did the DF of Sn. The diffusion of Cu and Sn can be suppressed by the addition of alloying elements (Ag 3.5 wt.% or Cu 0.7 wt.%), which can be indicated by various descending rates of the DFs of Cu and Sn in different joints. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Sn-based solders are widely used in the industry of microelectronic packaging. The interfacial reaction between the solder and under bump metallurgy (UBM) has attracted much attention of researchers due to the reliability concerns, top of which is the thick IMC layers universally existing at the reaction interface. Though the formation and growth of the IMCs during the reflow process [1–3] and subsequent solid-state aging state [4–7] have been studied in detail, the interdiffusion of Cu and Sn in IMC layers at the Sn-based solder/Cu interface during the solid-state aging remains an important issue needing further exploring. In recent decades, researchers have focused on the interdiffusion of component elements in Sn/Cu couple, especially the main diffusing species in Cu6Sn5 and Cu3Sn layers. It was reported that Cu was the dominant diffusing species at lower temperatures (20 °C to 70 °C) [8], while Sn diffusion was dominant at higher temperatures (>170 °C) [9–11]. The diffusivity of Cu in Cu3Sn and Cu6Sn5 at 155 °C was obtained by Onishi and Fujibuchi [9], 1.88 1012 cm2/s and 3.28 1013 cm2/s, respectively, which was inconsistent with the results by Liu et al. [12], 2.57 1013 cm2/s and 3.51 1013 cm2/s, respectively. Paul et al. [4] investigated the diffusion properties of Cu and Sn in Cu6Sn5 and Cu3Sn layers at 215 °C, and determined the ratio of tracer diffusivities of the elements in Cu6Sn5 layer as DCu/DSn 0.65 and that in Cu3Sn layer as DCu/DSn 1.1. That is, Sn was the dominant diffusing species in ⇑ Corresponding author at: School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel./fax: +86 21 34202548. E-mail address: [email protected] (Y. Yang). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.08.013
Cu6Sn5 layer, while Cu in Cu3Sn layer. Recently, a systematic study was also performed by Paul et al. [13], and the ratio values were updated, for Cu6Sn5 to DCu/DSn 0.35 and for Cu3Sn to DCu/DSn 30. However, Kumar et al. [14] came to a different conclusion that Cu was the faster diffusing species both in Cu6Sn5 and Cu3Sn layers, the ratio of intrinsic diffusivities of the component elements in Cu6Sn5 layer was DCu/DSn 3.23 and that in Cu3Sn layer was DCu/DSn 1.89. Solder joint is different from the diffusion couple for a layer of IMC will be developed during the reflow process. The diffusion of component elements at the Sn-rich solder/Cu interface has also been studied by many researchers, and Cu was also considered to be the dominant diffusing species [15,16]. However, few detailed works were reported on the diffusion of Cu and Sn in Cu6Sn5 and Cu3Sn layers [17]. In this study, a simple method was proposed to investigate the migration of interphase interfaces and the diffusion properties of Cu and Sn in IMC layers at the Sn-rich solder/Cu interface. 2. Experimental procedures The solders used in this work were Sn (99.99 wt.%), Sn0.7Cu (wt.%) and Sn3.5Ag (wt.%). The former two are commercially available, and Sn3.5Ag was fabricated from pure Sn (99.99 wt.%) and Ag (99.99 wt.%). The substrate was Cu foil of high purity (99.99 wt.%, 10 mm 10 mm 0.1 mm). The solder joints were prepared by melting solders on the square pieces at 260 °C. Prior to the reflow process, these pieces were deoxidized and degreased in 5 wt.% NaOH and 5 vol.% HCl solutions sequentially, rinsed in de-ionized water after each step, and finally, treated with flux. The isothermal
328
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
aging for the as-reflowed samples was conducted at 150 °C with different aging periods. Then these samples were mounted in epoxy and metallurgically polished. The interfacial microstructure of IMC layers was observed by using the optical microscope (OM) and scanning electron microscope (SEM). 3. Modeling The cross-sectional SEM micrographs of the corner of solder joints after isothermal aging at 150 °C are shown in Figs. 1 and 2. It can be observed in the different joints that the consumption of Cu increased with the extension of aging period. The upper surface of unreacted Cu substrate in the corner of joints can act as a natural reference, making the observation of the migration of interphase interfaces much easier. In order to quantitatively investigate the migration of interphase interfaces, a single coordinate axis perpendicular to the Cu substrate was introduced. The coordinate value of the original interface of joints was set as zero. Accordingly, the coordinate values of the interphase interfaces, Cu3Sn/Cu, Cu6Sn5/Cu3Sn and solder/Cu6Sn5, can be obtained through the average thickness of Cu consumption, Cu3Sn and Cu6Sn5 layers which can be derived from the SEM and OM micrographs. The coordinate value of Cu3Sn/Cu interface was negative for its relatively low position. These coordinate values of each interface with different aging time can be fitted by using the exponential functions, exponential growth or exponential decay. The exponential growth function f(t) can be expressed as:
f ðtÞ ¼ f0 þ A expðt=tc Þ
ð1Þ
where t is the aging time. f0 is the initial coordinate value of the interface, A and tc are constants, which can be determined during
the fitting process. Similarly, the exponential decay function f(t) can be represented as:
f ðtÞ ¼ f0 þ A expðt=tc Þ
ð2Þ
According to Eq. (1) or Eq. (2), three curves can be obtained, f1(t) for Cu3Sn/Cu interface, f2(t) for Cu6Sn5/Cu3Sn interface and f3(t) for solder/Cu6Sn5 interface. Based on these fitted curves, the DFs of Cu and Sn in Cu6Sn5 and Cu3Sn layers can be calculated. In order to simplify the model, five assumptions were made here: (1) This model only considered the interfacial diffusion of Cu and Sn during the solid-state aging. While the consumption of Cu substrate and thickness of Cu6Sn5 layer during the reflow process would be used as the initial values for the following calculations. (2) The mobility of Cu in the pure Sn phase is extremely rapid by an interstitial diffusion mechanism, while that of Sn in Cu phase is much slower [17,18]. At 150 °C, the diffusion coefficient of Cu in Sn was 2.05 1011 m2/s, while that of Sn in Cu lied at the range of 4.61 1027 to 4.18 1023 m2/s [17]. Besides, Cu6Sn5 phase was observed in the solder matrix near the solder/Cu6Sn5 interface, and more Cu6Sn5 can be found with the prolonging of aging period. Therefore, Cu atoms at the solder/Cu6Sn5 interface can diffuse into the solder matrix, participating in the reaction, while the diffusion of Sn in Cu substrate was considered to be negligible at the Cu3Sn/Cu interface. (3) During the thermal aging, the formation of IMC layers at the interface will cause a significant volume contraction [19,20]. When pure Cu and Sn react and form g0 -Cu6Sn5, a net 5.44% contraction in volume will happen; there will be a net 9.97%
Fig. 1. BSE images of the corner of solder joints after aging at 150 °C for 240 h. (a) Sn/Cu; (b) Sn3.5Ag/Cu and (c) Sn0.7Cu/Cu.
329
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
Fig. 2. BSE images of the corner of solder joints after aging at 150 °C for 720 h. (a) Sn/Cu; (b) Sn3.5Ag/Cu and (c) Sn0.7Cu/Cu.
decrease in volume during the formation of e-Cu3Sn. However, even a 10% contraction in volume can 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 [21]. In addition, the calculation and analysis in this model only involved the thickness of IMC layers and consumption thickness of Cu substrate. Therefore, the volume contraction was not taken into consideration here. (4) At the Cu6Sn5/Cu3Sn interface, it was assumed that once a certain volume of one phase was formed, the other one with the same volume would be consumed. (5) The calculation of the DFs of Cu and Sn started from the Cu3Sn/Cu interface. 3.1. Chemical reactions at the interface Chemical reaction equations at three different interphase interfaces can be expressed as follows [22]. At the Cu3Sn/Cu interface (the elements in square brackets denote the diffusing species):
3Cu þ ½Sn ! Cu3 Sn
ð3Þ
At the Cu6Sn5/Cu3Sn interface (the interface moves toward the solder matrix):
Cu6 Sn5 þ 9½Cu ! 5Cu3 Sn
ð4Þ
Cu6 Sn5 ! 3½Sn þ 2Cu3 Sn
ð5Þ
Table 1 Densities and molar volumes (Vx, x denotes the different phases) of b-Sn, e-Cu3Sn, g0 Cu6Sn5.
Density (g/cm3) Vx (cm3/mol)
Cu
b-Sn
e-Cu3Sn
g0 -Cu6Sn5
8.96 7.09
7.28 16.31
9.14 33.846
8.28 117.733
At the solder/Cu6Sn5 interface:
5Sn þ 6½Cu ! Cu6 Sn5
ð8Þ
In fact, the reactions in Eqs. (4) and (5) can proceed in both directions at the same time. Their reversible processes are reactions in Eqs. (6) and (7), respectively. When the reactions in Eqs. (4) and (5) move forward, the Cu6Sn5/Cu3Sn interface shifts toward the solder matrix, while for the reactions in Eqs. (6) and (7), the interface migrate toward the Cu substrate. In addition, there were some differences between the reaction equations here and those proposed by Paul et al. [22]. According to Paul et al., Cu atoms needed at the solder/Cu6Sn5 interface and Sn atoms demanded at the Cu3Sn/Cu interface were totally provided by the decomposition of Cu3Sn and Cu6Sn5, respectively. However, in this work, the supply of Cu and Sn atoms to their corresponding interfaces was composed of two parts, released from Cu substrate or solder, and the decomposition of IMC phase. There was no certain relationship between Eqs. (3) and (5). So was Eqs. (6) and (8).
At the Cu6Sn5/Cu3Sn interface (the interface migrates toward the Cu substrate):
3.2. DF of Cu in Cu3Sn layer
5Cu3 Sn ! 9½Cu þ Cu6 Sn5
ð6Þ
2Cu3 Sn þ 3½Sn ! Cu6 Sn5
ð7Þ
The density and molar volume data used in the model is listed in Table 1, among which Vx denotes the molar volume of different phases.
330
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
During the aging period Dt, the variation of f1(t) (the growth of Cu3Sn layer) is given by
Df1 ¼ f1 ðtÞ f1 ðt DtÞ
0 0 J Cu Cu6 Sn5 ðtÞ ¼ ð3=V Cu3 Sn 1=V Cu Þ f1 3ð1=V Cu3 Sn 2=V Cu6 Sn5 Þ f2
ð19Þ
ð9Þ
The total consumption of Cu per unit area (mol/cm2) can be expressed as
cCu ¼ Df1 =V Cu
ð10Þ
At the interface of Cu3Sn/Cu, not all the Cu atoms from Cu substrate will pass into other interfaces, part of which will be consumed at this interface. The consumption of Cu per unit area due to the growth of Cu3Sn can be determined by
cCu3 Sn ¼ 3Df1 =V Cu3 Sn
ð11Þ 2
3.4. DF of Sn in Cu3Sn layer According to the Eq. (3), the consumption of Sn at the Cu3Sn/Cu interface is
sCu3 Sn ¼ Df1 =V Cu3 Sn
ð20Þ
Therefore, the DF of Sn (mol/(cm2 s)) across the Cu3Sn layer is
J Sn Cu3 Sn ¼ sCu3 Sn =Dt ¼ Df1 =ðV Cu3 Sn DtÞ
ð21Þ
Within Dt, the DF of Cu (mol/(cm s)) across the Cu3Sn layer is
J Cu Cu3 Sn ¼ ðc Cu c Cu3 Sn Þ=Dt ¼ ð3=V Cu3 Sn 1=V Cu Þ Df1 =Dt
ð12Þ
0 J Sn Cu3 Sn ¼ f1 =V Cu3 Sn
Namely 0 J Cu Cu3 Sn ¼ ð3=V Cu3 Sn 1=V Cu Þ f1
where f is the derivative of f in Eq. (1) or Eq. (2), which can be written as 0
f ¼ A1 expðt=t 1 Þ=t1
ð14-1Þ
Or
f 0 ¼ A1 expðt=t1 Þ=t 1
ð14-2Þ
Similarly, during the aging period Dt, the variation of f2(t)is given by
ð15Þ
If Df2 > 0, the reactions in Eqs. (4) and (5) will happen, Cu atoms coming from the Cu3Sn/Cu interface will be consumed and Sn atoms will be generated. On the contrary, if Df2 < 0, the reactions in Eqs. (6) and (7) will occur, Cu atoms will be generated and Sn atoms coming from the solder/Cu6Sn5 interface will be consumed. At the Cu6Sn5/Cu3Sn interface, let x mol Cu3Sn from Eq. (4) and y mol from Eq. (5). According to the assumption (4), one phase (Cu3Sn or Cu6Sn5) with the same volume will be formed at the expense of the other one (Cu6Sn5 or Cu3Sn). Thus, two equations can be obtained
x þ y ¼ Df2 =V Cu3 Sn
ð16-1Þ
x=5 þ y=2 ¼ Df2 =V Cu6 Sn5
ð16-2Þ
The solutions of Eqs. (16-1) and (16-2) are
x ¼ 5ð1=V Cu3 Sn 2=V Cu6 Sn5 Þ Df2 =3
ð17-1Þ
y ¼ 2ð5=V Cu6 Sn5 1=V Cu3 Sn Þ Df2 =3
ð17-2Þ
The consumption or production of component elements can be derived from Eqs. (17-1) and (17-2). If Df2 > 0, the consumption of Cu atoms is 9x/5, and the production of Sn atoms is 3y/2; reversely, if Df2 < 0, the production of Cu atoms is 9x/5, and the consumption of Sn atoms is 3y/2. Within Dt, the DF of Cu (mol/(cm2 s)) across the Cu6Sn5 layer is
J Cu Cu6 Sn5 ¼ ðcCu c Cu3 Sn 9x=5Þ=Dt ¼ ð3=V Cu3 Sn 1=V Cu Þ Df1 =Dt 3ð1=V Cu3 Sn 2=V Cu6 Sn5 Þ Df2 =Dt
3.5. DF of Sn in Cu6Sn5 layer In the same way, the DF of Sn (mol/(cm2 s)) across the Cu6Sn5 layer can be written as
J Sn Cu6 Sn5 ¼ ðsCu3 Sn 3y=2Þ=Dt ¼ Df1 =ðV Cu3 Sn DtÞ ð5=V Cu6 Sn5 1=V Cu3 Sn Þ Df2 =Dt
ð23Þ
Namely
3.3. DF of Cu in Cu6Sn5 layer
Df2 ¼ f2 ðtÞ f2 ðt DtÞ
ð22Þ
ð13Þ
0
Namely
Namely
ð18Þ
0 0 J Sn Cu3 Sn ¼ f1 =V Cu3 Sn ð5=V Cu6 Sn5 1=V Cu3 Sn Þ f2
ð24Þ
4. Results According to the model above, the fitted curves of the interphase interfaces in Sn/Cu, Sn3.5Ag/Cu and Sn0.7Cu/Cu joints can be obtained, as presented in Fig. 3a–c, respectively. The parameters estimated by the curve fitting are listed in Table 2. It can be found that both solder/Cu6Sn5 and Cu6Sn5/Cu3Sn interfaces in all three kinds of joints moved toward the solder, and the Cu3Sn/Cu interface migrated toward the Cu substrate. The migration of these interfaces was quite fast at the initial stage, and then became slow with the prolonging of aging period. In the case of Cu6Sn5/Cu3Sn interface, their relative positions varied with different kinds of joints. The position of Cu6Sn5/Cu3Sn interface in the Sn/Cu joints was the highest, that in the Sn3.5Ag/Cu joints the second and that in the Sn0.7Cu/Cu joints the lowest. The DFs of Cu and Sn in the IMC layers can be obtained from the relative positions of these interphase interfaces. The relationship between DFs and aging time in different kinds of joints is illustrated in Fig. 4a–c. On the whole, for three kinds of solder joints, Cu diffused faster than Sn both in Cu6Sn5 and Cu3Sn layers during isothermal aging, and the DFs of Cu and Sn decreased with the extension of aging period. Besides, the DF of Cu in Cu3Sn layer gradually got close to that in Cu6Sn5 layer. So did the DF of Sn. For Sn/Cu and Sn3.5Ag/Cu joints, the DFs of Cu and Sn in Cu6Sn5 layer rose sharply at the initial stage of aging treatment, then became stable gradually, and finally decreased with the prolonging of aging period. As for Sn0.7Cu/Cu joint, the DFs of both elements in Cu6Sn5 and Cu3Sn layers decreased monotonically with the extension of aging period, which was not as complicated as that in Sn/Cu and Sn3.5Ag/Cu joints. In addition, the DFs of Cu and Sn at the interface in Sn3.5Ag/Cu and Sn0.7Cu/Cu joints declined faster than that in Sn/Cu joints.
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
331
Fig. 3. The fitting curves of the interphase interfaces in (a) Sn/Cu joints, (b) Sn3.5Ag/Cu joints and (c) Sn0.7Cu/Cu joints.
Table 2 Parameters obtained by the curve fitting. Joint
Interface
f0
A
tc
Sn/Cu
Cu3Sn/Cu Cu6Sn5/Cu3Sn Solder/Cu6Sn5
5.772 0.339 3.333
4.799 1.29714 2.059
799.356 39.123 749.092
Sn3.5Ag/Cu
Cu3Sn/Cu Cu6Sn5/Cu3Sn Solder/Cu6Sn5
3.620 0.322 1.248
2.580 0.632 0.359
492.104 36.803 646.622
Sn0.7Cu/Cu
Cu3Sn/Cu Cu6Sn5/Cu3Sn Solder/Cu6Sn5
3.614 0.415 2.372
2.491 0.464 0.891
385.158 298.199 547.726
5. Discussion Since both Cu6Sn5 and Cu3Sn phases are relatively rich in Cu, the formation of two phases requires more Cu than Sn [21]. Furthermore, part of Cu atoms can pass through the IMC layers and diffuse into the solder matrix, while only a small amount of Sn atoms can diffuse into the Cu substrate. In addition, some Sn atoms can be produced at the Cu6Sn5/Cu3Sn interface during the thermal aging and then diffuse into the Cu3Sn/Cu interface, which would reduce the demand for Sn atoms from the solder matrix. Therefore, Cu was dominant diffusing species at the reaction interface, which was in accordance with the results obtained by Kumar et al. [14].
The growth of IMC layer during the thermal aging required a continuous supply of Cu and Sn, which came from Cu substrate and solder, respectively. Consequently, the consumption of substrate and solder naturally led to the migration of interfaces (solder/Cu6Sn5 and Cu3Sn/Cu) on the both sides of IMC layer. With the extension of aging period, the IMC layer grew thicker and thicker. The thick layer would act as the diffusion barrier, retarding the interdiffusion of Cu and Sn [1]. Thus, the DFs of Cu and Sn were reduced simultaneously, as seen in Fig. 4. Accordingly, the growth of IMC layer was suppressed, and the move of interphase interfaces was hindered. Mei et al. [21] constructed a multiphase diffusion model to investigate the growth of Cu6Sn5 and Cu3Sn layers in Sn/Cu couples. They predicted that the Cu6Sn5/Cu3Sn interface would move toward Cu at high temperature (P180 °C) and the original Sn/Cu interface would locate within the Cu6Sn5 layer after diffusion, which was in accordance with the experimental observations [9]. However, in this work, the Cu6Sn5/Cu3Sn interface moved toward solder. This may relate to the critical temperature (170 °C) mentioned in the introduction section. When the aging temperature was higher than 170 °C, the diffusion of Sn was dominant at the interface, which would promote the reaction in Eq. (7) and made the Cu6Sn5/Cu3Sn interface shift toward Cu. Nevertheless, all the experiments here were carried out at 150 °C which was lower than 170 °C, and the diffusion of Cu was dominant at the interface. Therefore, the Cu6Sn5/Cu3Sn interface migrated toward the opposite direction. Shang et al. [6,7] studied the growth of Cu3Sn layer
332
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
Fig. 4. Diffusion fluxes of Cu and Sn in the Cu3Sn and Cu6Sn5 layers of (a) Sn/Cu joints, (b) Sn3.5Ag/Cu joints and (c) Sn0.7Cu/Cu joints.
Table 3 Diffusivities of Cu and Sn in the IMC layers of solder joints after aging at 150 °C with different aging periods. The unit for diffusivity is in 1013 cm2/s. Joint
IMC
120 h
240 h
480 h
Cu
Sn
Cu
Sn
Cu
Sn
Sn/Cu
Cu3Sn Cu6Sn5
1.64 1.85
0.94 1.54
1.91 2.04
1.09 1.54
1.85 2.38
1.06 1.79
Sn3.5Ag/Cu
Cu3Sn Cu6Sn5
0.91 2.71
0.52 2.14
0.94 2.24
0.54 1.69
0.75 1.53
0.43 1.15
Sn0.7Cu/Cu
Cu3Sn Cu6Sn5
0.98 2.88
0.56 2.32
0.92 2.61
0.52 2.09
0.66 1.48
0.38 1.17
in the SnBi/Cu (polycrystalline substrate) joints at 120 °C. They also found that the Cu6Sn5/Cu3Sn interface moved toward the solder at a lower temperature. The different positions of Cu6Sn5/Cu3Sn interfaces revealed that the addition of alloying elements (3.5 wt.% Ag or 0.7 wt.% Cu) affected the interdiffusion of Cu and Sn, especially Cu. In the three kinds of joints, Ag or Cu had minor effect on the total thickness of IMC layer, but had great effect on the average thickness ratio of Cu3Sn layer to Cu6Sn5 layer. In Sn/Cu joints, the thickness ratio was 1.438, while the values in Sn3.5Ag/Cu and Sn0.7Cu/Cu joints were 0.604 and 0.372, respectively, which were much smaller than the former one and agreed well with other experimental results [23–26]. The ratio values in different joints were consistent with the relative positions of Cu6Sn5/Cu3Sn interface, that is, the smaller the thickness ratio was, the lower the relative position of Cu6Sn5/ Cu3Sn interface, as illustrated in Fig. 3a, b and c. Besides, Cu3Sn phase contains more Cu than Cu6Sn5 phase. Therefore, the growth of Cu3Sn layer was depressed at the interface with alloying addition, and the diffusion of Cu was suppressed obviously, which
could also be indicated by various descending rates of the DF of Cu in the different joints. Ag can react with Sn, forming Ag3Sn both at the solder/IMC interface and in the solder, which might have a certain effect on the diffusion of Cu and Sn. The addition of Cu to Sn solder can lower the Cu concentration gradient at the reaction interface, suppressing the diffusion of Cu from substrate to solder [27]. In Sn/Cu and Sn3.5Ag/Cu joints, the complicated evolution of the DFs of Cu and Sn was closely related to the rapid migration of Cu6Sn5/Cu3Sn interface towards solder matrix at the initial stage of aging treatment, as shown in Fig. 3a and b. During the interfacial reaction, a portion of Cu atoms released from the Cu substrate would be consumed at the Cu6Sn5/Cu3Sn interface, and the rest would diffuse into the Cu6Sn5 layer. Meanwhile, some Sn atoms would be produced at this interface, which would reduce the supply of Sn atoms from solder matrix. Hence, the diffusion of Cu and Sn in Cu6Sn5 layer was suppressed at beginning. With the continuing decrease in the growth rate of Cu3Sn phase, less Cu atoms would be consumed at the Cu6Sn5/Cu3Sn interface, and less Sn
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
atoms would be produced. That is, more both kinds of atoms would pass through the Cu6Sn5 layer, and the DFs of them increased. Then the diffusion of Cu and Sn in Cu6Sn5 layer was gradually suppressed due to the thickening of IMC layer. Therefore, the evolution of the DFs of Cu and Sn was rather complicated in the Cu6Sn5 layer. However, the migration of Cu6Sn5/Cu3Sn interface was not obvious in Sn0.7/Cu joints, as seen in Fig. 3c. Thus, there was no noticeable change in the DFs of Cu and Sn in Cu6Sn5 layer. According to the Fick’s First Law, J ¼ D @C , the D (diffusivity) of @x Cu and Sn in IMC layers can be estimated by the J (diffusion flux) and @C (concentration gradient). Actually, the ratio of Cu to Sn is @x almost constant in each IMC layer. Thus, a hypothetical concentration gradient was applied to the IMC layer. Cu3Sn layer is located between Cu substrate and Cu6Sn5 layer, and the concentration gradient of Cu in this layer is approximately 0.09/dCu3Sn mol/cm4 (dCu3Sn is the thickness of Cu3Sn layer); Cu6Sn5 layer is situated between Cu3Sn layer and solder, and the concentration gradient of Cu in it is approximately 0.0886/dCu6Sn5 mol/cm4. In the same way, the concentration gradient of Sn in Cu3Sn and Cu6Sn5 layers are 0.0425/dCu3Sn and 0.0318/dCu6Sn5 mol/cm4, respectively. Based on this method, the diffusivities of Cu and Sn in IMC layers were calculated and the results were listed in Table 3. It can be found that the estimated diffusivities of Cu in both IMC layers were close to the other experimental results [9,12,13], and they were all roughly in the same order of magnitude, 1013 cm2/s. Though the diffusivities of Cu and Sn in this work were much lower than those obtained by Kumar et al. [14], both results showed the same trend that Cu diffused faster than Sn both in Cu6Sn5 and Cu3Sn layers.
6. Conclusions In this work, a simple reference method was applied to study the migration of interphase interfaces in Sn-rich solder/Cu joints after aging at 150 °C with different aging periods. The solder/Cu6Sn5 and Cu6Sn5/Cu3Sn interfaces migrated toward the solder matrix with the extension of aging period, and the Cu3Sn/Cu interface moved toward the Cu substrate. A model was built to calculate the DFs of Cu and Sn in the IMC layers. The results showed that Cu diffused faster than Sn both in Cu6Sn5 and Cu3Sn layers. The diffusion of both elements was gradually suppressed with the growth of IMC layers. The various relative positions of Cu6Sn5/ Cu3Sn interface and different descending rates of the DFs of Cu and Sn implied that the addition of alloying element (Ag 3.5 wt.% or Cu 0.7 wt.%) could suppress the diffusion of Cu. In Sn/Cu and Sn3.5Ag/ Cu joints, the complicated evolution of the DFs of Cu and Sn was relevant to the rapid migration of Cu6Sn5/Cu3Sn interface at the initial stage of aging treatment.
Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant Nos. 50975176 and 51105251).
333
References [1] Tu KN, Lee TY, Jang JW, Li L, Frear DR, Zeng K, et al. Wetting reaction versus solid state aging of eutectic SnPb on Cu. J Appl Phys 2001;89:4843–9. [2] Görlich J, Schmitz G, Tu KN. On the mechanism of the binary Cu/Sn solder reaction. Appl Phys Lett 2005;86:053106. [3] Gong JC, Liu CQ, Conway PP, Silberschmidt VV. Evolution of CuSn intermetallics between molten SnAgCu solder and Cu substrate. Acta Mater 2008;56:4291–7. [4] Paul A, Kodentsov AA, Van Loo FJJ. Intermetallic growth and Kirkendall effect manifestations in Cu/Sn and Au/Sn diffusion couples. Z Metallkd 2004;95:913–20. [5] Li JF, Mannan SH, Clode MP, Whalley DC, Hutt DA. Interfacial reactions between molten Sn-Bi-X solders and Cu substrates for liquid solder interconnects. Acta Mater 2006;54:2907–22. [6] Shang PJ, Liu ZQ, Li DX, Shang JK. TEM observations of the growth of intermetallic compounds at the SnBi/Cu interface. J Electron Mater 2009;38:2579–84. [7] Shang PJ, Liu ZQ, Pang XY, Li DX, Shang JK. Growth mechanisms of Cu3Sn on polycrystalline and single crystalline Cu substrates. Acta Mater 2009;57:4697–706. [8] Tu KN, Thompson RD. Kinetic of interfacial reaction in bimetallic Cu–Sn thin films. Acta Metall 1982;30:947–52. [9] Onishi M, Fujibuchi H. Reaction-diffusion in the Cu–Sn system. Trans Jpn Inst Met 1975;16:539–47. [10] Oikawa H, Hosoi A. Interdiffusion in Cu–Sn solid solutions. confirmation of anomalously large Kirkendall effect. Scripta Metall 1975;9:823–8. [11] Hoshino K, Lijima Y, Hirano K. Interdiffusion and Kirkendall effect in Cu–Sn alloys. Trans Jpn Inst Met 1980;21:674–82. [12] Liu CY, Ke L, Chuang YC, Wang SJ. Study of electromigration-induced Cu consumption in the flip-chip Sn/Cu solder bumps. J Appl Phys 2006;100:083702. [13] Paul A, Ghosh C, Boettinger WJ. Diffusion parameters and growth mechanism of phases in the Cu–Sn system. Metall Mater Trans A 2011;42:952–63. [14] Kumar S, Handwerker CA, Dayananda MA. Intrinsic and interdiffusion in Cu– Sn system. J Phase Equilib Diff 2011;32:309–19. [15] Zeng K, Stierman R, Chiu TC, Edwards D, Ano K, Tu KN. Kirkendall void formation in eutectic SnPb solder joints on bare Cu and its effect on joint reliability. J Appl Phys 2005;97:024508. [16] Zeng K, Tu KN. Six cases of reliability study of Pb-free solder joints in electronic packaging technology. Mater Sci Eng R 2002;38:55–105. [17] Chao B, Chae SH, Zhang XF, Lu KH, Im J, Ho PS. Investigation of diffusion and electromigration parameters for Cu–Sn intermetallic compounds in Pb-free solders using simulated annealing. Acta Mater 2007;55:2805–14. [18] Dvson BF, Anthony TR, Turnbull D. Interstitial diffusion of copper in tin. J Appl Phys 1967;38:3408. [19] Song JY, Yu J, Lee TY. Effects of reactive diffusion on stress evolution in Cu–Sn films. Scripta Mater 2004;51:167–70. [20] Buchovecky E, Jadhav N, Bower AF, Chason E. Finite element modeling of stress evolution in Sn films due to growth of the Cu6Sn5 intermetallic compound. J Electron Mater 2009;38:2676–84. [21] Mei Z, Sunwoo AJ, Morris Jr JW. Analysis of low-temperature intermetallic growth in copper-tin diffusion couples. Metall Trans A 1992;23:857–64. [22] Paul A. The Kirkendall effect in solid state diffusion. Doctoral Dissertation, Technical University of Eindhoven, Eindhoven; 2004. [23] Flanders DR, Jacobs EG, Pinizzotto RF. Activation energies of intermetallic growth of Sn–Ag eutectic solder on copper substrates. J Electron Mater 1997;26:883–7. [24] Choi S, Bieler TR, Lucas JP, Subramanlan KN. Characterization of the growth of intermetallic interfacial layers of Sn–Ag and Sn–Pb eutectic solders and their composite solders on Cu substrate during isothermal long-term aging. J Electron Mater 1999;28:1209–15. [25] Lee TY, Choi WJ, Tu KN, Jang JW, Kuo SM, Lin JK, et al. Morphology, kinetics, and thermodynamics of solid-state aging of eutectic SnPb and Pb-free solders. J Mater Res 2002;17:291–301. [26] Deng X, Piotrowski G, Williams JJ, Chawla N. Influence of initial morphology and thickness of Cu6Sn5 and Cu3Sn intermetallics on growth and evolution during thermal aging of Sn–Ag solder/Cu joints. J Electron Mater 2003;32:1403–13. [27] Wang YW, Lin YW, Kao CR. Inhibiting the formation of microvoids in Cu3Sn by additions of Cu to solders. J Alloys Compd 2010;493:233–9.
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Interdiffusion at the interface between Sn-based solders and Cu substrate Yang Yang a,⇑, Yongzhi Li a, Hao Lu a,b,⇑, Chun Yu a, Junmei Chen a a b
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China Key Lab of Shanghai Laser Manufacturing and Materials Modification, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2012 Received in revised form 12 August 2012 Accepted 13 August 2012 Available online 5 September 2012
a b s t r a c t The migration of interphase interfaces in Sn-based solder joints was investigated by using the upper surface of unreacted Cu substrate (the original reaction interface) as a reference. Then a model was built to study the interdiffusion of Cu and Sn in Cu6Sn5 and Cu3Sn layers. After being aged at 150 °C, both solder/Cu6Sn5 and Cu6Sn5/Cu3Sn interfaces moved toward the solder matrix, and the Cu3Sn/Cu interface migrated toward the Cu substrate. Compared with Sn, Cu was found to be the faster diffuser in Cu6Sn5 and Cu3Sn layers. The diffusion fluxes (DFs) of Cu and Sn in both intermetallic compound (IMC) layers decreased with the extension of aging period. The DF of Cu in Cu3Sn layer got close to that in Cu6Sn5 layer. So did the DF of Sn. The diffusion of Cu and Sn can be suppressed by the addition of alloying elements (Ag 3.5 wt.% or Cu 0.7 wt.%), which can be indicated by various descending rates of the DFs of Cu and Sn in different joints. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Sn-based solders are widely used in the industry of microelectronic packaging. The interfacial reaction between the solder and under bump metallurgy (UBM) has attracted much attention of researchers due to the reliability concerns, top of which is the thick IMC layers universally existing at the reaction interface. Though the formation and growth of the IMCs during the reflow process [1–3] and subsequent solid-state aging state [4–7] have been studied in detail, the interdiffusion of Cu and Sn in IMC layers at the Sn-based solder/Cu interface during the solid-state aging remains an important issue needing further exploring. In recent decades, researchers have focused on the interdiffusion of component elements in Sn/Cu couple, especially the main diffusing species in Cu6Sn5 and Cu3Sn layers. It was reported that Cu was the dominant diffusing species at lower temperatures (20 °C to 70 °C) [8], while Sn diffusion was dominant at higher temperatures (>170 °C) [9–11]. The diffusivity of Cu in Cu3Sn and Cu6Sn5 at 155 °C was obtained by Onishi and Fujibuchi [9], 1.88 1012 cm2/s and 3.28 1013 cm2/s, respectively, which was inconsistent with the results by Liu et al. [12], 2.57 1013 cm2/s and 3.51 1013 cm2/s, respectively. Paul et al. [4] investigated the diffusion properties of Cu and Sn in Cu6Sn5 and Cu3Sn layers at 215 °C, and determined the ratio of tracer diffusivities of the elements in Cu6Sn5 layer as DCu/DSn 0.65 and that in Cu3Sn layer as DCu/DSn 1.1. That is, Sn was the dominant diffusing species in ⇑ Corresponding author at: School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel./fax: +86 21 34202548. E-mail address: [email protected] (Y. Yang). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.08.013
Cu6Sn5 layer, while Cu in Cu3Sn layer. Recently, a systematic study was also performed by Paul et al. [13], and the ratio values were updated, for Cu6Sn5 to DCu/DSn 0.35 and for Cu3Sn to DCu/DSn 30. However, Kumar et al. [14] came to a different conclusion that Cu was the faster diffusing species both in Cu6Sn5 and Cu3Sn layers, the ratio of intrinsic diffusivities of the component elements in Cu6Sn5 layer was DCu/DSn 3.23 and that in Cu3Sn layer was DCu/DSn 1.89. Solder joint is different from the diffusion couple for a layer of IMC will be developed during the reflow process. The diffusion of component elements at the Sn-rich solder/Cu interface has also been studied by many researchers, and Cu was also considered to be the dominant diffusing species [15,16]. However, few detailed works were reported on the diffusion of Cu and Sn in Cu6Sn5 and Cu3Sn layers [17]. In this study, a simple method was proposed to investigate the migration of interphase interfaces and the diffusion properties of Cu and Sn in IMC layers at the Sn-rich solder/Cu interface. 2. Experimental procedures The solders used in this work were Sn (99.99 wt.%), Sn0.7Cu (wt.%) and Sn3.5Ag (wt.%). The former two are commercially available, and Sn3.5Ag was fabricated from pure Sn (99.99 wt.%) and Ag (99.99 wt.%). The substrate was Cu foil of high purity (99.99 wt.%, 10 mm 10 mm 0.1 mm). The solder joints were prepared by melting solders on the square pieces at 260 °C. Prior to the reflow process, these pieces were deoxidized and degreased in 5 wt.% NaOH and 5 vol.% HCl solutions sequentially, rinsed in de-ionized water after each step, and finally, treated with flux. The isothermal
328
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
aging for the as-reflowed samples was conducted at 150 °C with different aging periods. Then these samples were mounted in epoxy and metallurgically polished. The interfacial microstructure of IMC layers was observed by using the optical microscope (OM) and scanning electron microscope (SEM). 3. Modeling The cross-sectional SEM micrographs of the corner of solder joints after isothermal aging at 150 °C are shown in Figs. 1 and 2. It can be observed in the different joints that the consumption of Cu increased with the extension of aging period. The upper surface of unreacted Cu substrate in the corner of joints can act as a natural reference, making the observation of the migration of interphase interfaces much easier. In order to quantitatively investigate the migration of interphase interfaces, a single coordinate axis perpendicular to the Cu substrate was introduced. The coordinate value of the original interface of joints was set as zero. Accordingly, the coordinate values of the interphase interfaces, Cu3Sn/Cu, Cu6Sn5/Cu3Sn and solder/Cu6Sn5, can be obtained through the average thickness of Cu consumption, Cu3Sn and Cu6Sn5 layers which can be derived from the SEM and OM micrographs. The coordinate value of Cu3Sn/Cu interface was negative for its relatively low position. These coordinate values of each interface with different aging time can be fitted by using the exponential functions, exponential growth or exponential decay. The exponential growth function f(t) can be expressed as:
f ðtÞ ¼ f0 þ A expðt=tc Þ
ð1Þ
where t is the aging time. f0 is the initial coordinate value of the interface, A and tc are constants, which can be determined during
the fitting process. Similarly, the exponential decay function f(t) can be represented as:
f ðtÞ ¼ f0 þ A expðt=tc Þ
ð2Þ
According to Eq. (1) or Eq. (2), three curves can be obtained, f1(t) for Cu3Sn/Cu interface, f2(t) for Cu6Sn5/Cu3Sn interface and f3(t) for solder/Cu6Sn5 interface. Based on these fitted curves, the DFs of Cu and Sn in Cu6Sn5 and Cu3Sn layers can be calculated. In order to simplify the model, five assumptions were made here: (1) This model only considered the interfacial diffusion of Cu and Sn during the solid-state aging. While the consumption of Cu substrate and thickness of Cu6Sn5 layer during the reflow process would be used as the initial values for the following calculations. (2) The mobility of Cu in the pure Sn phase is extremely rapid by an interstitial diffusion mechanism, while that of Sn in Cu phase is much slower [17,18]. At 150 °C, the diffusion coefficient of Cu in Sn was 2.05 1011 m2/s, while that of Sn in Cu lied at the range of 4.61 1027 to 4.18 1023 m2/s [17]. Besides, Cu6Sn5 phase was observed in the solder matrix near the solder/Cu6Sn5 interface, and more Cu6Sn5 can be found with the prolonging of aging period. Therefore, Cu atoms at the solder/Cu6Sn5 interface can diffuse into the solder matrix, participating in the reaction, while the diffusion of Sn in Cu substrate was considered to be negligible at the Cu3Sn/Cu interface. (3) During the thermal aging, the formation of IMC layers at the interface will cause a significant volume contraction [19,20]. When pure Cu and Sn react and form g0 -Cu6Sn5, a net 5.44% contraction in volume will happen; there will be a net 9.97%
Fig. 1. BSE images of the corner of solder joints after aging at 150 °C for 240 h. (a) Sn/Cu; (b) Sn3.5Ag/Cu and (c) Sn0.7Cu/Cu.
329
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
Fig. 2. BSE images of the corner of solder joints after aging at 150 °C for 720 h. (a) Sn/Cu; (b) Sn3.5Ag/Cu and (c) Sn0.7Cu/Cu.
decrease in volume during the formation of e-Cu3Sn. However, even a 10% contraction in volume can 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 [21]. In addition, the calculation and analysis in this model only involved the thickness of IMC layers and consumption thickness of Cu substrate. Therefore, the volume contraction was not taken into consideration here. (4) At the Cu6Sn5/Cu3Sn interface, it was assumed that once a certain volume of one phase was formed, the other one with the same volume would be consumed. (5) The calculation of the DFs of Cu and Sn started from the Cu3Sn/Cu interface. 3.1. Chemical reactions at the interface Chemical reaction equations at three different interphase interfaces can be expressed as follows [22]. At the Cu3Sn/Cu interface (the elements in square brackets denote the diffusing species):
3Cu þ ½Sn ! Cu3 Sn
ð3Þ
At the Cu6Sn5/Cu3Sn interface (the interface moves toward the solder matrix):
Cu6 Sn5 þ 9½Cu ! 5Cu3 Sn
ð4Þ
Cu6 Sn5 ! 3½Sn þ 2Cu3 Sn
ð5Þ
Table 1 Densities and molar volumes (Vx, x denotes the different phases) of b-Sn, e-Cu3Sn, g0 Cu6Sn5.
Density (g/cm3) Vx (cm3/mol)
Cu
b-Sn
e-Cu3Sn
g0 -Cu6Sn5
8.96 7.09
7.28 16.31
9.14 33.846
8.28 117.733
At the solder/Cu6Sn5 interface:
5Sn þ 6½Cu ! Cu6 Sn5
ð8Þ
In fact, the reactions in Eqs. (4) and (5) can proceed in both directions at the same time. Their reversible processes are reactions in Eqs. (6) and (7), respectively. When the reactions in Eqs. (4) and (5) move forward, the Cu6Sn5/Cu3Sn interface shifts toward the solder matrix, while for the reactions in Eqs. (6) and (7), the interface migrate toward the Cu substrate. In addition, there were some differences between the reaction equations here and those proposed by Paul et al. [22]. According to Paul et al., Cu atoms needed at the solder/Cu6Sn5 interface and Sn atoms demanded at the Cu3Sn/Cu interface were totally provided by the decomposition of Cu3Sn and Cu6Sn5, respectively. However, in this work, the supply of Cu and Sn atoms to their corresponding interfaces was composed of two parts, released from Cu substrate or solder, and the decomposition of IMC phase. There was no certain relationship between Eqs. (3) and (5). So was Eqs. (6) and (8).
At the Cu6Sn5/Cu3Sn interface (the interface migrates toward the Cu substrate):
3.2. DF of Cu in Cu3Sn layer
5Cu3 Sn ! 9½Cu þ Cu6 Sn5
ð6Þ
2Cu3 Sn þ 3½Sn ! Cu6 Sn5
ð7Þ
The density and molar volume data used in the model is listed in Table 1, among which Vx denotes the molar volume of different phases.
330
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
During the aging period Dt, the variation of f1(t) (the growth of Cu3Sn layer) is given by
Df1 ¼ f1 ðtÞ f1 ðt DtÞ
0 0 J Cu Cu6 Sn5 ðtÞ ¼ ð3=V Cu3 Sn 1=V Cu Þ f1 3ð1=V Cu3 Sn 2=V Cu6 Sn5 Þ f2
ð19Þ
ð9Þ
The total consumption of Cu per unit area (mol/cm2) can be expressed as
cCu ¼ Df1 =V Cu
ð10Þ
At the interface of Cu3Sn/Cu, not all the Cu atoms from Cu substrate will pass into other interfaces, part of which will be consumed at this interface. The consumption of Cu per unit area due to the growth of Cu3Sn can be determined by
cCu3 Sn ¼ 3Df1 =V Cu3 Sn
ð11Þ 2
3.4. DF of Sn in Cu3Sn layer According to the Eq. (3), the consumption of Sn at the Cu3Sn/Cu interface is
sCu3 Sn ¼ Df1 =V Cu3 Sn
ð20Þ
Therefore, the DF of Sn (mol/(cm2 s)) across the Cu3Sn layer is
J Sn Cu3 Sn ¼ sCu3 Sn =Dt ¼ Df1 =ðV Cu3 Sn DtÞ
ð21Þ
Within Dt, the DF of Cu (mol/(cm s)) across the Cu3Sn layer is
J Cu Cu3 Sn ¼ ðc Cu c Cu3 Sn Þ=Dt ¼ ð3=V Cu3 Sn 1=V Cu Þ Df1 =Dt
ð12Þ
0 J Sn Cu3 Sn ¼ f1 =V Cu3 Sn
Namely 0 J Cu Cu3 Sn ¼ ð3=V Cu3 Sn 1=V Cu Þ f1
where f is the derivative of f in Eq. (1) or Eq. (2), which can be written as 0
f ¼ A1 expðt=t 1 Þ=t1
ð14-1Þ
Or
f 0 ¼ A1 expðt=t1 Þ=t 1
ð14-2Þ
Similarly, during the aging period Dt, the variation of f2(t)is given by
ð15Þ
If Df2 > 0, the reactions in Eqs. (4) and (5) will happen, Cu atoms coming from the Cu3Sn/Cu interface will be consumed and Sn atoms will be generated. On the contrary, if Df2 < 0, the reactions in Eqs. (6) and (7) will occur, Cu atoms will be generated and Sn atoms coming from the solder/Cu6Sn5 interface will be consumed. At the Cu6Sn5/Cu3Sn interface, let x mol Cu3Sn from Eq. (4) and y mol from Eq. (5). According to the assumption (4), one phase (Cu3Sn or Cu6Sn5) with the same volume will be formed at the expense of the other one (Cu6Sn5 or Cu3Sn). Thus, two equations can be obtained
x þ y ¼ Df2 =V Cu3 Sn
ð16-1Þ
x=5 þ y=2 ¼ Df2 =V Cu6 Sn5
ð16-2Þ
The solutions of Eqs. (16-1) and (16-2) are
x ¼ 5ð1=V Cu3 Sn 2=V Cu6 Sn5 Þ Df2 =3
ð17-1Þ
y ¼ 2ð5=V Cu6 Sn5 1=V Cu3 Sn Þ Df2 =3
ð17-2Þ
The consumption or production of component elements can be derived from Eqs. (17-1) and (17-2). If Df2 > 0, the consumption of Cu atoms is 9x/5, and the production of Sn atoms is 3y/2; reversely, if Df2 < 0, the production of Cu atoms is 9x/5, and the consumption of Sn atoms is 3y/2. Within Dt, the DF of Cu (mol/(cm2 s)) across the Cu6Sn5 layer is
J Cu Cu6 Sn5 ¼ ðcCu c Cu3 Sn 9x=5Þ=Dt ¼ ð3=V Cu3 Sn 1=V Cu Þ Df1 =Dt 3ð1=V Cu3 Sn 2=V Cu6 Sn5 Þ Df2 =Dt
3.5. DF of Sn in Cu6Sn5 layer In the same way, the DF of Sn (mol/(cm2 s)) across the Cu6Sn5 layer can be written as
J Sn Cu6 Sn5 ¼ ðsCu3 Sn 3y=2Þ=Dt ¼ Df1 =ðV Cu3 Sn DtÞ ð5=V Cu6 Sn5 1=V Cu3 Sn Þ Df2 =Dt
ð23Þ
Namely
3.3. DF of Cu in Cu6Sn5 layer
Df2 ¼ f2 ðtÞ f2 ðt DtÞ
ð22Þ
ð13Þ
0
Namely
Namely
ð18Þ
0 0 J Sn Cu3 Sn ¼ f1 =V Cu3 Sn ð5=V Cu6 Sn5 1=V Cu3 Sn Þ f2
ð24Þ
4. Results According to the model above, the fitted curves of the interphase interfaces in Sn/Cu, Sn3.5Ag/Cu and Sn0.7Cu/Cu joints can be obtained, as presented in Fig. 3a–c, respectively. The parameters estimated by the curve fitting are listed in Table 2. It can be found that both solder/Cu6Sn5 and Cu6Sn5/Cu3Sn interfaces in all three kinds of joints moved toward the solder, and the Cu3Sn/Cu interface migrated toward the Cu substrate. The migration of these interfaces was quite fast at the initial stage, and then became slow with the prolonging of aging period. In the case of Cu6Sn5/Cu3Sn interface, their relative positions varied with different kinds of joints. The position of Cu6Sn5/Cu3Sn interface in the Sn/Cu joints was the highest, that in the Sn3.5Ag/Cu joints the second and that in the Sn0.7Cu/Cu joints the lowest. The DFs of Cu and Sn in the IMC layers can be obtained from the relative positions of these interphase interfaces. The relationship between DFs and aging time in different kinds of joints is illustrated in Fig. 4a–c. On the whole, for three kinds of solder joints, Cu diffused faster than Sn both in Cu6Sn5 and Cu3Sn layers during isothermal aging, and the DFs of Cu and Sn decreased with the extension of aging period. Besides, the DF of Cu in Cu3Sn layer gradually got close to that in Cu6Sn5 layer. So did the DF of Sn. For Sn/Cu and Sn3.5Ag/Cu joints, the DFs of Cu and Sn in Cu6Sn5 layer rose sharply at the initial stage of aging treatment, then became stable gradually, and finally decreased with the prolonging of aging period. As for Sn0.7Cu/Cu joint, the DFs of both elements in Cu6Sn5 and Cu3Sn layers decreased monotonically with the extension of aging period, which was not as complicated as that in Sn/Cu and Sn3.5Ag/Cu joints. In addition, the DFs of Cu and Sn at the interface in Sn3.5Ag/Cu and Sn0.7Cu/Cu joints declined faster than that in Sn/Cu joints.
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
331
Fig. 3. The fitting curves of the interphase interfaces in (a) Sn/Cu joints, (b) Sn3.5Ag/Cu joints and (c) Sn0.7Cu/Cu joints.
Table 2 Parameters obtained by the curve fitting. Joint
Interface
f0
A
tc
Sn/Cu
Cu3Sn/Cu Cu6Sn5/Cu3Sn Solder/Cu6Sn5
5.772 0.339 3.333
4.799 1.29714 2.059
799.356 39.123 749.092
Sn3.5Ag/Cu
Cu3Sn/Cu Cu6Sn5/Cu3Sn Solder/Cu6Sn5
3.620 0.322 1.248
2.580 0.632 0.359
492.104 36.803 646.622
Sn0.7Cu/Cu
Cu3Sn/Cu Cu6Sn5/Cu3Sn Solder/Cu6Sn5
3.614 0.415 2.372
2.491 0.464 0.891
385.158 298.199 547.726
5. Discussion Since both Cu6Sn5 and Cu3Sn phases are relatively rich in Cu, the formation of two phases requires more Cu than Sn [21]. Furthermore, part of Cu atoms can pass through the IMC layers and diffuse into the solder matrix, while only a small amount of Sn atoms can diffuse into the Cu substrate. In addition, some Sn atoms can be produced at the Cu6Sn5/Cu3Sn interface during the thermal aging and then diffuse into the Cu3Sn/Cu interface, which would reduce the demand for Sn atoms from the solder matrix. Therefore, Cu was dominant diffusing species at the reaction interface, which was in accordance with the results obtained by Kumar et al. [14].
The growth of IMC layer during the thermal aging required a continuous supply of Cu and Sn, which came from Cu substrate and solder, respectively. Consequently, the consumption of substrate and solder naturally led to the migration of interfaces (solder/Cu6Sn5 and Cu3Sn/Cu) on the both sides of IMC layer. With the extension of aging period, the IMC layer grew thicker and thicker. The thick layer would act as the diffusion barrier, retarding the interdiffusion of Cu and Sn [1]. Thus, the DFs of Cu and Sn were reduced simultaneously, as seen in Fig. 4. Accordingly, the growth of IMC layer was suppressed, and the move of interphase interfaces was hindered. Mei et al. [21] constructed a multiphase diffusion model to investigate the growth of Cu6Sn5 and Cu3Sn layers in Sn/Cu couples. They predicted that the Cu6Sn5/Cu3Sn interface would move toward Cu at high temperature (P180 °C) and the original Sn/Cu interface would locate within the Cu6Sn5 layer after diffusion, which was in accordance with the experimental observations [9]. However, in this work, the Cu6Sn5/Cu3Sn interface moved toward solder. This may relate to the critical temperature (170 °C) mentioned in the introduction section. When the aging temperature was higher than 170 °C, the diffusion of Sn was dominant at the interface, which would promote the reaction in Eq. (7) and made the Cu6Sn5/Cu3Sn interface shift toward Cu. Nevertheless, all the experiments here were carried out at 150 °C which was lower than 170 °C, and the diffusion of Cu was dominant at the interface. Therefore, the Cu6Sn5/Cu3Sn interface migrated toward the opposite direction. Shang et al. [6,7] studied the growth of Cu3Sn layer
332
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
Fig. 4. Diffusion fluxes of Cu and Sn in the Cu3Sn and Cu6Sn5 layers of (a) Sn/Cu joints, (b) Sn3.5Ag/Cu joints and (c) Sn0.7Cu/Cu joints.
Table 3 Diffusivities of Cu and Sn in the IMC layers of solder joints after aging at 150 °C with different aging periods. The unit for diffusivity is in 1013 cm2/s. Joint
IMC
120 h
240 h
480 h
Cu
Sn
Cu
Sn
Cu
Sn
Sn/Cu
Cu3Sn Cu6Sn5
1.64 1.85
0.94 1.54
1.91 2.04
1.09 1.54
1.85 2.38
1.06 1.79
Sn3.5Ag/Cu
Cu3Sn Cu6Sn5
0.91 2.71
0.52 2.14
0.94 2.24
0.54 1.69
0.75 1.53
0.43 1.15
Sn0.7Cu/Cu
Cu3Sn Cu6Sn5
0.98 2.88
0.56 2.32
0.92 2.61
0.52 2.09
0.66 1.48
0.38 1.17
in the SnBi/Cu (polycrystalline substrate) joints at 120 °C. They also found that the Cu6Sn5/Cu3Sn interface moved toward the solder at a lower temperature. The different positions of Cu6Sn5/Cu3Sn interfaces revealed that the addition of alloying elements (3.5 wt.% Ag or 0.7 wt.% Cu) affected the interdiffusion of Cu and Sn, especially Cu. In the three kinds of joints, Ag or Cu had minor effect on the total thickness of IMC layer, but had great effect on the average thickness ratio of Cu3Sn layer to Cu6Sn5 layer. In Sn/Cu joints, the thickness ratio was 1.438, while the values in Sn3.5Ag/Cu and Sn0.7Cu/Cu joints were 0.604 and 0.372, respectively, which were much smaller than the former one and agreed well with other experimental results [23–26]. The ratio values in different joints were consistent with the relative positions of Cu6Sn5/Cu3Sn interface, that is, the smaller the thickness ratio was, the lower the relative position of Cu6Sn5/ Cu3Sn interface, as illustrated in Fig. 3a, b and c. Besides, Cu3Sn phase contains more Cu than Cu6Sn5 phase. Therefore, the growth of Cu3Sn layer was depressed at the interface with alloying addition, and the diffusion of Cu was suppressed obviously, which
could also be indicated by various descending rates of the DF of Cu in the different joints. Ag can react with Sn, forming Ag3Sn both at the solder/IMC interface and in the solder, which might have a certain effect on the diffusion of Cu and Sn. The addition of Cu to Sn solder can lower the Cu concentration gradient at the reaction interface, suppressing the diffusion of Cu from substrate to solder [27]. In Sn/Cu and Sn3.5Ag/Cu joints, the complicated evolution of the DFs of Cu and Sn was closely related to the rapid migration of Cu6Sn5/Cu3Sn interface towards solder matrix at the initial stage of aging treatment, as shown in Fig. 3a and b. During the interfacial reaction, a portion of Cu atoms released from the Cu substrate would be consumed at the Cu6Sn5/Cu3Sn interface, and the rest would diffuse into the Cu6Sn5 layer. Meanwhile, some Sn atoms would be produced at this interface, which would reduce the supply of Sn atoms from solder matrix. Hence, the diffusion of Cu and Sn in Cu6Sn5 layer was suppressed at beginning. With the continuing decrease in the growth rate of Cu3Sn phase, less Cu atoms would be consumed at the Cu6Sn5/Cu3Sn interface, and less Sn
Y. Yang et al. / Microelectronics Reliability 53 (2013) 327–333
atoms would be produced. That is, more both kinds of atoms would pass through the Cu6Sn5 layer, and the DFs of them increased. Then the diffusion of Cu and Sn in Cu6Sn5 layer was gradually suppressed due to the thickening of IMC layer. Therefore, the evolution of the DFs of Cu and Sn was rather complicated in the Cu6Sn5 layer. However, the migration of Cu6Sn5/Cu3Sn interface was not obvious in Sn0.7/Cu joints, as seen in Fig. 3c. Thus, there was no noticeable change in the DFs of Cu and Sn in Cu6Sn5 layer. According to the Fick’s First Law, J ¼ D @C , the D (diffusivity) of @x Cu and Sn in IMC layers can be estimated by the J (diffusion flux) and @C (concentration gradient). Actually, the ratio of Cu to Sn is @x almost constant in each IMC layer. Thus, a hypothetical concentration gradient was applied to the IMC layer. Cu3Sn layer is located between Cu substrate and Cu6Sn5 layer, and the concentration gradient of Cu in this layer is approximately 0.09/dCu3Sn mol/cm4 (dCu3Sn is the thickness of Cu3Sn layer); Cu6Sn5 layer is situated between Cu3Sn layer and solder, and the concentration gradient of Cu in it is approximately 0.0886/dCu6Sn5 mol/cm4. In the same way, the concentration gradient of Sn in Cu3Sn and Cu6Sn5 layers are 0.0425/dCu3Sn and 0.0318/dCu6Sn5 mol/cm4, respectively. Based on this method, the diffusivities of Cu and Sn in IMC layers were calculated and the results were listed in Table 3. It can be found that the estimated diffusivities of Cu in both IMC layers were close to the other experimental results [9,12,13], and they were all roughly in the same order of magnitude, 1013 cm2/s. Though the diffusivities of Cu and Sn in this work were much lower than those obtained by Kumar et al. [14], both results showed the same trend that Cu diffused faster than Sn both in Cu6Sn5 and Cu3Sn layers.
6. Conclusions In this work, a simple reference method was applied to study the migration of interphase interfaces in Sn-rich solder/Cu joints after aging at 150 °C with different aging periods. The solder/Cu6Sn5 and Cu6Sn5/Cu3Sn interfaces migrated toward the solder matrix with the extension of aging period, and the Cu3Sn/Cu interface moved toward the Cu substrate. A model was built to calculate the DFs of Cu and Sn in the IMC layers. The results showed that Cu diffused faster than Sn both in Cu6Sn5 and Cu3Sn layers. The diffusion of both elements was gradually suppressed with the growth of IMC layers. The various relative positions of Cu6Sn5/ Cu3Sn interface and different descending rates of the DFs of Cu and Sn implied that the addition of alloying element (Ag 3.5 wt.% or Cu 0.7 wt.%) could suppress the diffusion of Cu. In Sn/Cu and Sn3.5Ag/ Cu joints, the complicated evolution of the DFs of Cu and Sn was relevant to the rapid migration of Cu6Sn5/Cu3Sn interface at the initial stage of aging treatment.
Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant Nos. 50975176 and 51105251).
333
References [1] Tu KN, Lee TY, Jang JW, Li L, Frear DR, Zeng K, et al. Wetting reaction versus solid state aging of eutectic SnPb on Cu. J Appl Phys 2001;89:4843–9. [2] Görlich J, Schmitz G, Tu KN. On the mechanism of the binary Cu/Sn solder reaction. Appl Phys Lett 2005;86:053106. [3] Gong JC, Liu CQ, Conway PP, Silberschmidt VV. Evolution of CuSn intermetallics between molten SnAgCu solder and Cu substrate. Acta Mater 2008;56:4291–7. [4] Paul A, Kodentsov AA, Van Loo FJJ. Intermetallic growth and Kirkendall effect manifestations in Cu/Sn and Au/Sn diffusion couples. Z Metallkd 2004;95:913–20. [5] Li JF, Mannan SH, Clode MP, Whalley DC, Hutt DA. Interfacial reactions between molten Sn-Bi-X solders and Cu substrates for liquid solder interconnects. Acta Mater 2006;54:2907–22. [6] Shang PJ, Liu ZQ, Li DX, Shang JK. TEM observations of the growth of intermetallic compounds at the SnBi/Cu interface. J Electron Mater 2009;38:2579–84. [7] Shang PJ, Liu ZQ, Pang XY, Li DX, Shang JK. Growth mechanisms of Cu3Sn on polycrystalline and single crystalline Cu substrates. Acta Mater 2009;57:4697–706. [8] Tu KN, Thompson RD. Kinetic of interfacial reaction in bimetallic Cu–Sn thin films. Acta Metall 1982;30:947–52. [9] Onishi M, Fujibuchi H. Reaction-diffusion in the Cu–Sn system. Trans Jpn Inst Met 1975;16:539–47. [10] Oikawa H, Hosoi A. Interdiffusion in Cu–Sn solid solutions. confirmation of anomalously large Kirkendall effect. Scripta Metall 1975;9:823–8. [11] Hoshino K, Lijima Y, Hirano K. Interdiffusion and Kirkendall effect in Cu–Sn alloys. Trans Jpn Inst Met 1980;21:674–82. [12] Liu CY, Ke L, Chuang YC, Wang SJ. Study of electromigration-induced Cu consumption in the flip-chip Sn/Cu solder bumps. J Appl Phys 2006;100:083702. [13] Paul A, Ghosh C, Boettinger WJ. Diffusion parameters and growth mechanism of phases in the Cu–Sn system. Metall Mater Trans A 2011;42:952–63. [14] Kumar S, Handwerker CA, Dayananda MA. Intrinsic and interdiffusion in Cu– Sn system. J Phase Equilib Diff 2011;32:309–19. [15] Zeng K, Stierman R, Chiu TC, Edwards D, Ano K, Tu KN. Kirkendall void formation in eutectic SnPb solder joints on bare Cu and its effect on joint reliability. J Appl Phys 2005;97:024508. [16] Zeng K, Tu KN. Six cases of reliability study of Pb-free solder joints in electronic packaging technology. Mater Sci Eng R 2002;38:55–105. [17] Chao B, Chae SH, Zhang XF, Lu KH, Im J, Ho PS. Investigation of diffusion and electromigration parameters for Cu–Sn intermetallic compounds in Pb-free solders using simulated annealing. Acta Mater 2007;55:2805–14. [18] Dvson BF, Anthony TR, Turnbull D. Interstitial diffusion of copper in tin. J Appl Phys 1967;38:3408. [19] Song JY, Yu J, Lee TY. Effects of reactive diffusion on stress evolution in Cu–Sn films. Scripta Mater 2004;51:167–70. [20] Buchovecky E, Jadhav N, Bower AF, Chason E. Finite element modeling of stress evolution in Sn films due to growth of the Cu6Sn5 intermetallic compound. J Electron Mater 2009;38:2676–84. [21] Mei Z, Sunwoo AJ, Morris Jr JW. Analysis of low-temperature intermetallic growth in copper-tin diffusion couples. Metall Trans A 1992;23:857–64. [22] Paul A. The Kirkendall effect in solid state diffusion. Doctoral Dissertation, Technical University of Eindhoven, Eindhoven; 2004. [23] Flanders DR, Jacobs EG, Pinizzotto RF. Activation energies of intermetallic growth of Sn–Ag eutectic solder on copper substrates. J Electron Mater 1997;26:883–7. [24] Choi S, Bieler TR, Lucas JP, Subramanlan KN. Characterization of the growth of intermetallic interfacial layers of Sn–Ag and Sn–Pb eutectic solders and their composite solders on Cu substrate during isothermal long-term aging. J Electron Mater 1999;28:1209–15. [25] Lee TY, Choi WJ, Tu KN, Jang JW, Kuo SM, Lin JK, et al. Morphology, kinetics, and thermodynamics of solid-state aging of eutectic SnPb and Pb-free solders. J Mater Res 2002;17:291–301. [26] Deng X, Piotrowski G, Williams JJ, Chawla N. Influence of initial morphology and thickness of Cu6Sn5 and Cu3Sn intermetallics on growth and evolution during thermal aging of Sn–Ag solder/Cu joints. J Electron Mater 2003;32:1403–13. [27] Wang YW, Lin YW, Kao CR. Inhibiting the formation of microvoids in Cu3Sn by additions of Cu to solders. J Alloys Compd 2010;493:233–9.