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Cu joint interface
Materials Letters 57 (2003) 3361 – 3365 www.elsevier.com/locate/matlet
Development of Cu–Sn intermetallic compound at Pb-free solder/Cu joint interface Xin Ma a,*, Fengjiang Wang b, Yiyu Qian b, Fusahito Yoshida a b
a Department of Mechanical System Engineering, Hiroshima University, Higashihiroshima 739-8527, Japan National Key Laboratory of Welding, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
Received 28 October 2002; received in revised form 17 January 2003; accepted 20 January 2003
Abstract The development of Cu – Sn intermetallic compound (IMC) at the solder/Cu joint interface had been studied using two Pbfree solders, Sn – 3.8Ag – 0.7Cu and Sn – 2Ag – 0.8Cu – 0.6Sb alloys. Meanwhile, 100Sn/Cu joint was applied for comparison. Both Pb-free solder joints were found with thinner Cu – Sn IMC layers at as-soldered state due to the slower dissolution rate of intermetallic compound into the liquid Pb-free solders during reflow and, consequently, slower growth rates of Cu – Sn IMC during the solid-state thermal aging at 125 jC, where Sn – 2Ag – 0.8Cu – 0.6Sb solder joint gave the minimum value. Thermodynamic analysis showed that such phenomena could be attributed to the reduction of the driving force for Cu – Sn IMC formation due to the existence of Ag and Sb atoms. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Lead-free solder; Intermetallic compound; Dissolution kinetics; Thermodynamic
1. Introduction Leaded solders, mainly Sn63 –Pb37 eutectic alloy, have been widely used in electronics packaging and assembly for more than 50 years. However, lead is a toxic metal element and is harmful to individual health. Nowadays, requirement for banning the use of lead in electronic industry, at least in consumer electronics, keeps on increasing all over the world. Therefore, development of environmental-friendly solder, namely,
* Corresponding author. Venture Business Laboratory, Hiroshima University, 2-313, Kagamiyama, Higashihiroshima 739-8527, Japan. Tel.: +81-824-24-7540; fax: +81-824-24-7881. E-mail address: [email protected] (X. Ma).
Pb-free solder, has been greatly concerned in recent years [1– 4]. It is well recognized that the upcoming Pb-free solders should be Sn-based alloys. As the metallurgic basic of soldering technology, when molten Sn-contained solder wets the Cu pads on the printed wiring board (PWB), Cu –Sn intermetallic compound (IMC) will form at the solder/Cu interface and thus the connection is made. Furthermore, such Cu – Sn IMC layer will continue growing at solid state under certain thermal loading, e.g., the operating of electronic devices or environmental temperature cycling (e.g., Refs. [5– 7]). While recent investigations had reported that the growth of Cu –Sn IMC layer had a degraded effect on the solder joint reliability, with increasing thickness of the Cu – Sn IMC layer, the thermal fatigue life,
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00075-2
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tensile strength and fracture toughness of solder joints will decrease (e.g., Refs. [8– 11]). Generally, the Sn composition of most lead-free solders is much more than that of eutectic Sn – Pb solder. Therefore, study on the development of Cu –Sn IMC layer at Pb-free solder joint interface is also essential to characterize the upcoming lead-free solders. Two Pb-free solders, Sn – 3.8Ag – 0.7Cu and Sn – 2Ag – 0.8Cu –0.6Sb, had been used in this work. These two solders were chosen because the former one is recommended as the best candidate of ‘drop-in’ leadfree solder [1,2] and the latter one exhibits outstanding thermal fatigue property [12]. Meanwhile, 100Sn was applied for comparison. Thermal aging tests were conducted to accelerate the solid-state growth of Cu – Sn IMC layer. The corresponding development was analyzed based on dissolution kinetic and thermodynamic models.
2. Experimental aspects The solder joint samples were prepared by reflowing solder balls on Cu pads on a BT substrate in the presence of a water-soluble flux. Reflows were carried out in a belt oven, and for each solder, the reflowing profile was the same except for the peak reflow temperature, which was set as 40 jC higher than its liquidus temperature, namely, 260 jC for Sn – 3.8Ag– 0.7Cu and Sn – 2Ag –0.8Cu –0.6Sb, and 270 jC for 100Sn. The consequent thermal aging of the as-soldered joints was performed at the temperature of 125 jC for 24, 48, 72, 96 and 120 h. Samples were cross-sectioned, polished and lightly etched for scanning electron microscopy (SEM) observation both before and after aging. Energy-dispersive X-ray (EDX) was applied to characterize the Cu – Sn IMC and its average thickness was determined according to multipoint measuring results. Fig. 1. SEM images of as-soldered joints: (a) 100Sn/Cu; (b) Sn – 3.8Ag – 0.7Cu/Cu; (c) Sn – 2Ag – 0.8Cu – 0.6Sb/Cu.
3. Results and discussions 3.1. As-soldered joints The SEM images of as-soldered joints are shown in Fig. 1, where the upper, bottom and middle materials are solder alloy, copper and interfacial Cu –Sn IMC,
respectively. It is obvious that the average thickness of Cu – Sn IMC layer in the Pb-free solder/Cu joints is much less than that of 100Sn/Cu joint. Such large difference cannot be attributed to the slight higher peak reflow temperature in the former case. Here we consider the following.
X. Ma et al. / Materials Letters 57 (2003) 3361–3365
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Fig. 2. Development of Cu – Sn IMC layer of Sn – 2Ag – 0.8Cu – 0.6Sb/Cu joint thermally aged at 125 jC: (a) as-soldered; (b) aged for 48 h; (c) aged for 120 h.
During reflow, although Cu can directly react with Sn in the liquid solder to form Cu – Sn compound, the latter will soon dissolve into atoms of Cu and Sn until the liquid solder is saturated with Cu. Therefore, dissolution of the intermetallic compound plays an important role on the intermetallic growth [13]. According to Dybkov’s analysis, the dissolution rate of Cu – Sn IMC into the molten solder maybe expressed as [13,14]: dc S ¼ k ðcs cÞ dt V
3.2. Thermally aged solder joints The formed Cu – Sn IMC layer in as-soldered joint will develop during the solid-state thermal aging; simply speaking, its thickness increases with time under certain temperature. The SEM images of thermally aged Sn – 2Ag – 0.8Cu – 0.6Sb/Cu joint are shown in Fig. 2 as a representative case in this work. Meanwhile, as the results of the EDX analysis shown in Fig. 3, the Cu – Sn IMC can be characterized as
ð1Þ
where k is the dissolution rate constant, S is the surface area of the IMC in contact with the solder, V is the volume of the liquid solder and t is the time. The above parameters are the same for all the cases in this work. What we are concern for is the term (cs c). cs, the solubility of Cu in the liquid solder at the reflow temperature, is almost the same, namely 1.54 wt.%, for the cases of pure Sn and the present Pb-free solders [15]. While c, the concentration of Cu in the liquid solder, is zero for the case of pure Sn and 0.7– 0.8 for the cases of Pb-free solders used in this work. It should be noted that the concentration of Cu in the liquid Pb-free solder adjacent to the solder/IMC interface is more than the above value due to the formation of Cu – Sn compound. Therefore, in the case of pure Sn/Cu joint, the liquid solder will be saturated with Cu sooner due to the faster dissolution rate of Cu –Sn IMC. The consequent growth of intermetallic compound will lead to scallop morphology of Cu – Sn IMC layer, as shown in Fig. 1(a), due to the Gibbs – Thomson effect and Ostwald ripening as reported in Refs. [16,17]. On the other hand, the slower dissolution rate of intermetallic compound left little time for the scallop growth during reflow and planar Cu – Sn IMC layer formed as shown in Fig. 1(b) and (c).
Fig. 3. EDX analysis of the Cu – Sn IMC layer: (a) Sn – 3.8Ag – 0.7Cu/Cu joint; (b) Sn – 2Ag – 0.8Cu – 0.6Sb/Cu joint.
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Fig. 4. Cu – Sn IMC thickness versus solid-state thermal aging time.
Cu6Sn5 compound, while the Cu3Sn compound, which needs much higher activation energy for formation [18], is not found in all the joints after aging for 120 h. Fig. 4 shows the corresponding measured thicknesses of IMC layers in each case after aging for different hours. This diffusion-controlled solid-state Cu –Sn IMC growth can be expressed as the following one-dimensional empirical equation: pffiffiffiffiffi X ðtÞ X0 ¼ Dt ð2Þ where X(t) is the IMC thickness at aging time t, X0 is the initial thickness in as-soldered joint and D is the diffusion coefficient, which is a measure of the growth rate of Cu –Sn IMC layer. Calculated results give D values of 4.7 10 13 cm2/s for Sn– 3.8Ag– 0.7Cu/Cu joint, 1.6 10 13 cm2/s for Sn– 2Ag – 0.8Cu –0.6Sb/ Cu joint and 6.6 10 13 cm2/s for 100Sn/Cu joint. From the above results, it can be seen that (1) both Pb-free solder/Cu joints have lower growth rates of
Fig. 5. Effect of the local mole fraction of Ag on the Gibbs energy of Cu – Sn system.
Cu – Sn IMC than 100Sn/Cu joint, and (2) doped addition of Sb in Sn – Ag –Cu solder further depresses the Cu – Sn IMC growth. The same result was also reported in Ref. [12]. By marker analysis, Tu and Thompson [5] had pointed out that the Cu – Sn IMC forms by the transport of Cu atoms across the compound, i.e., the reaction between Cu and Sn occurs at the solder side. That is, the present experimental results indicate that the existence of Ag and Sb atoms has an influence on the Cu –Sn IMC formation. Thermodynamic analysis has been carried out to understand such effect.
Table 1 Parameters used in thermodynamic calculation Element n1/3a U ws (V)
V 2/3 l (cm3)
Constant Value Constant Value
Sn Cu Ag Sb
6.4 3.7 4.7 6.6
PSn – Cu PSn – Ag PAg – Cu PSn – Sb PSb – Cu
1.24 1.47 1.39 1.26
4.15 4.55 4.45 4.4
0.04 0.07 0.07 0.04
12.3 12.3 14.1 12.3 12.3
bSn – Cu bSn – Ag bAg – Cu bSn – Sb bSb – Cu
0.63 0.315 0 0 0.69
a Electron density is the electron numbers in every (0.529 10 10m)3 volume.
Fig. 6. Effect of the local mole fraction of Sb on the Gibbs energy of Cu – Sn system.
X. Ma et al. / Materials Letters 57 (2003) 3361–3365
Thermodynamic models, the Miedema model [19] and the Tanaka modification for a binary system [20] and the Chou geometric model for a ternary system [21], are used in this work. Previous works had illustrated their applicability to nonferrous metal systems [22], and by using them, several thermodynamic phenomena of Sn –Pb solder alloy had been clarified [11,23]. The details of these models could be found in Refs. [11,22]. All of the thermodynamic parameters used in this work are listed in Table 1. Figs. 5 and 6 show the calculated Gibbs energy, DG, of Cu –Sn system at 125 jC and the effect of Ag and Sb atoms. It can be seen that, for Cu – Sn binary system, Cu6Sn5 phase will spontaneously form with priority since it has the minimum negative Gibbs energy. On the other hand, if some Ag or Sb atoms exist in the local reaction region, the abstract value of DG decreases, i.e., the driving force for Cu6Sn5 IMC formation is reduced. Although no calculation for quaternary system is performed here since the present thermodynamic models are only effective for binary and ternary systems, such reduction should be considered as a reason for the slower solid-state growth rate of Cu –Sn IMC in Pb-free solder joints.
References [1] B.P. Richards, C.L. Levoguer, C.P. Hunt, K. Nimmo, S. Peters, P. Cusack, An Analysis of the Current Status of LeadFree Soldering, NPL and ITRI Report, 1999.
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[2] IPC Roadmap, A Guide for Assembly of Lead-Free Electronics, 2000. [3] M. Abtew, G. Selvaduray, Mater. Sci. Eng., Rep. 27 (2000) 95. [4] K. Suganuma, Curr. Opin. Solid State Mater. Sci. 5 (2001) 55. [5] K.N. Tu, R.D. Thompson, Acta Metall. 30 (1982) 947. [6] H.K. Kim, K.N. Tu, Phys. Rev., B 53 (1996) 16027. [7] B.J. Lee, N.M. Hwang, H.M. Lee, Acta Mater. 45 (1997) 1867. [8] P.L. Tu, Y.C. Chan, J.K.L. Lai, IEEE Trans. Compon. Packag. Manuf. Technol., Part B 20 (1997) 87. [9] L. Quan, D.R. Frear, D. Grivas, J.W. Morris, J. Electron. Mater. 16 (1987) 203. [10] R.E. Pratt, E.I. Stromswold, D.J. Quensnel, IEEE Trans. Compon. Packaging Manuf. Technol., Part A 19 (1996) 134. [11] X. Ma, Y.Y. Qian, F. Yoshida, J. Alloys Compd. 334 (2002) 224. [12] K. Seeling, D. Suraski, Circuits Assem. 9 (2000) 56. [13] D. Ma, W.D. Wang, S.K. Lahiri, J. Appl. Phys. 91 (2002) 3312. [14] V.I. Dybkov, Growth Kinetics of Chemical Compound Layers, Cambridge International Science, Cambridge, 1998. [15] K. Zeng, K.N. Tu, Mater. Sci. Eng., Rep. 38 (2002) 55. [16] H.K. Kim, H.K. Liou, K.N. Tu, Appl. Phys. Lett. 66 (1995) 2337. [17] G. Ghosh, J. Appl. Phys. 88 (2000) 6887. [18] K.N. Tu, T.Y. Lee, J.W. Jang, L. Li, D.R. Frear, K. Zeng, J.K. Kivilahti, J. Appl. Phys. 89 (2001) 4843. [19] F.R. De Boer, R. Boom, W.C.M. Mattens, A.R. Miedema, A.K. Niessen, Cohesion in Metals, North-Holland, Amsterdam, 1998. [20] T. Tanaka, N.A. Gokcen, Z. Morita, Z. Metallkd. 81 (1990) 49. [21] K.C. Chou, CALPHAD 17 (1995) 315. [22] X. Ma, F. Yoshida, Mater. Lett. 56 (2002) 441. [23] X. Ma, Y.Y. Qian, F. Yoshida, J. Alloys Compd. 327 (2001) 263.
Development of Cu–Sn intermetallic compound at Pb-free solder/Cu joint interface Xin Ma a,*, Fengjiang Wang b, Yiyu Qian b, Fusahito Yoshida a b
a Department of Mechanical System Engineering, Hiroshima University, Higashihiroshima 739-8527, Japan National Key Laboratory of Welding, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
Received 28 October 2002; received in revised form 17 January 2003; accepted 20 January 2003
Abstract The development of Cu – Sn intermetallic compound (IMC) at the solder/Cu joint interface had been studied using two Pbfree solders, Sn – 3.8Ag – 0.7Cu and Sn – 2Ag – 0.8Cu – 0.6Sb alloys. Meanwhile, 100Sn/Cu joint was applied for comparison. Both Pb-free solder joints were found with thinner Cu – Sn IMC layers at as-soldered state due to the slower dissolution rate of intermetallic compound into the liquid Pb-free solders during reflow and, consequently, slower growth rates of Cu – Sn IMC during the solid-state thermal aging at 125 jC, where Sn – 2Ag – 0.8Cu – 0.6Sb solder joint gave the minimum value. Thermodynamic analysis showed that such phenomena could be attributed to the reduction of the driving force for Cu – Sn IMC formation due to the existence of Ag and Sb atoms. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Lead-free solder; Intermetallic compound; Dissolution kinetics; Thermodynamic
1. Introduction Leaded solders, mainly Sn63 –Pb37 eutectic alloy, have been widely used in electronics packaging and assembly for more than 50 years. However, lead is a toxic metal element and is harmful to individual health. Nowadays, requirement for banning the use of lead in electronic industry, at least in consumer electronics, keeps on increasing all over the world. Therefore, development of environmental-friendly solder, namely,
* Corresponding author. Venture Business Laboratory, Hiroshima University, 2-313, Kagamiyama, Higashihiroshima 739-8527, Japan. Tel.: +81-824-24-7540; fax: +81-824-24-7881. E-mail address: [email protected] (X. Ma).
Pb-free solder, has been greatly concerned in recent years [1– 4]. It is well recognized that the upcoming Pb-free solders should be Sn-based alloys. As the metallurgic basic of soldering technology, when molten Sn-contained solder wets the Cu pads on the printed wiring board (PWB), Cu –Sn intermetallic compound (IMC) will form at the solder/Cu interface and thus the connection is made. Furthermore, such Cu – Sn IMC layer will continue growing at solid state under certain thermal loading, e.g., the operating of electronic devices or environmental temperature cycling (e.g., Refs. [5– 7]). While recent investigations had reported that the growth of Cu –Sn IMC layer had a degraded effect on the solder joint reliability, with increasing thickness of the Cu – Sn IMC layer, the thermal fatigue life,
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00075-2
3362
X. Ma et al. / Materials Letters 57 (2003) 3361–3365
tensile strength and fracture toughness of solder joints will decrease (e.g., Refs. [8– 11]). Generally, the Sn composition of most lead-free solders is much more than that of eutectic Sn – Pb solder. Therefore, study on the development of Cu –Sn IMC layer at Pb-free solder joint interface is also essential to characterize the upcoming lead-free solders. Two Pb-free solders, Sn – 3.8Ag – 0.7Cu and Sn – 2Ag – 0.8Cu –0.6Sb, had been used in this work. These two solders were chosen because the former one is recommended as the best candidate of ‘drop-in’ leadfree solder [1,2] and the latter one exhibits outstanding thermal fatigue property [12]. Meanwhile, 100Sn was applied for comparison. Thermal aging tests were conducted to accelerate the solid-state growth of Cu – Sn IMC layer. The corresponding development was analyzed based on dissolution kinetic and thermodynamic models.
2. Experimental aspects The solder joint samples were prepared by reflowing solder balls on Cu pads on a BT substrate in the presence of a water-soluble flux. Reflows were carried out in a belt oven, and for each solder, the reflowing profile was the same except for the peak reflow temperature, which was set as 40 jC higher than its liquidus temperature, namely, 260 jC for Sn – 3.8Ag– 0.7Cu and Sn – 2Ag –0.8Cu –0.6Sb, and 270 jC for 100Sn. The consequent thermal aging of the as-soldered joints was performed at the temperature of 125 jC for 24, 48, 72, 96 and 120 h. Samples were cross-sectioned, polished and lightly etched for scanning electron microscopy (SEM) observation both before and after aging. Energy-dispersive X-ray (EDX) was applied to characterize the Cu – Sn IMC and its average thickness was determined according to multipoint measuring results. Fig. 1. SEM images of as-soldered joints: (a) 100Sn/Cu; (b) Sn – 3.8Ag – 0.7Cu/Cu; (c) Sn – 2Ag – 0.8Cu – 0.6Sb/Cu.
3. Results and discussions 3.1. As-soldered joints The SEM images of as-soldered joints are shown in Fig. 1, where the upper, bottom and middle materials are solder alloy, copper and interfacial Cu –Sn IMC,
respectively. It is obvious that the average thickness of Cu – Sn IMC layer in the Pb-free solder/Cu joints is much less than that of 100Sn/Cu joint. Such large difference cannot be attributed to the slight higher peak reflow temperature in the former case. Here we consider the following.
X. Ma et al. / Materials Letters 57 (2003) 3361–3365
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Fig. 2. Development of Cu – Sn IMC layer of Sn – 2Ag – 0.8Cu – 0.6Sb/Cu joint thermally aged at 125 jC: (a) as-soldered; (b) aged for 48 h; (c) aged for 120 h.
During reflow, although Cu can directly react with Sn in the liquid solder to form Cu – Sn compound, the latter will soon dissolve into atoms of Cu and Sn until the liquid solder is saturated with Cu. Therefore, dissolution of the intermetallic compound plays an important role on the intermetallic growth [13]. According to Dybkov’s analysis, the dissolution rate of Cu – Sn IMC into the molten solder maybe expressed as [13,14]: dc S ¼ k ðcs cÞ dt V
3.2. Thermally aged solder joints The formed Cu – Sn IMC layer in as-soldered joint will develop during the solid-state thermal aging; simply speaking, its thickness increases with time under certain temperature. The SEM images of thermally aged Sn – 2Ag – 0.8Cu – 0.6Sb/Cu joint are shown in Fig. 2 as a representative case in this work. Meanwhile, as the results of the EDX analysis shown in Fig. 3, the Cu – Sn IMC can be characterized as
ð1Þ
where k is the dissolution rate constant, S is the surface area of the IMC in contact with the solder, V is the volume of the liquid solder and t is the time. The above parameters are the same for all the cases in this work. What we are concern for is the term (cs c). cs, the solubility of Cu in the liquid solder at the reflow temperature, is almost the same, namely 1.54 wt.%, for the cases of pure Sn and the present Pb-free solders [15]. While c, the concentration of Cu in the liquid solder, is zero for the case of pure Sn and 0.7– 0.8 for the cases of Pb-free solders used in this work. It should be noted that the concentration of Cu in the liquid Pb-free solder adjacent to the solder/IMC interface is more than the above value due to the formation of Cu – Sn compound. Therefore, in the case of pure Sn/Cu joint, the liquid solder will be saturated with Cu sooner due to the faster dissolution rate of Cu –Sn IMC. The consequent growth of intermetallic compound will lead to scallop morphology of Cu – Sn IMC layer, as shown in Fig. 1(a), due to the Gibbs – Thomson effect and Ostwald ripening as reported in Refs. [16,17]. On the other hand, the slower dissolution rate of intermetallic compound left little time for the scallop growth during reflow and planar Cu – Sn IMC layer formed as shown in Fig. 1(b) and (c).
Fig. 3. EDX analysis of the Cu – Sn IMC layer: (a) Sn – 3.8Ag – 0.7Cu/Cu joint; (b) Sn – 2Ag – 0.8Cu – 0.6Sb/Cu joint.
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Fig. 4. Cu – Sn IMC thickness versus solid-state thermal aging time.
Cu6Sn5 compound, while the Cu3Sn compound, which needs much higher activation energy for formation [18], is not found in all the joints after aging for 120 h. Fig. 4 shows the corresponding measured thicknesses of IMC layers in each case after aging for different hours. This diffusion-controlled solid-state Cu –Sn IMC growth can be expressed as the following one-dimensional empirical equation: pffiffiffiffiffi X ðtÞ X0 ¼ Dt ð2Þ where X(t) is the IMC thickness at aging time t, X0 is the initial thickness in as-soldered joint and D is the diffusion coefficient, which is a measure of the growth rate of Cu –Sn IMC layer. Calculated results give D values of 4.7 10 13 cm2/s for Sn– 3.8Ag– 0.7Cu/Cu joint, 1.6 10 13 cm2/s for Sn– 2Ag – 0.8Cu –0.6Sb/ Cu joint and 6.6 10 13 cm2/s for 100Sn/Cu joint. From the above results, it can be seen that (1) both Pb-free solder/Cu joints have lower growth rates of
Fig. 5. Effect of the local mole fraction of Ag on the Gibbs energy of Cu – Sn system.
Cu – Sn IMC than 100Sn/Cu joint, and (2) doped addition of Sb in Sn – Ag –Cu solder further depresses the Cu – Sn IMC growth. The same result was also reported in Ref. [12]. By marker analysis, Tu and Thompson [5] had pointed out that the Cu – Sn IMC forms by the transport of Cu atoms across the compound, i.e., the reaction between Cu and Sn occurs at the solder side. That is, the present experimental results indicate that the existence of Ag and Sb atoms has an influence on the Cu –Sn IMC formation. Thermodynamic analysis has been carried out to understand such effect.
Table 1 Parameters used in thermodynamic calculation Element n1/3a U ws (V)
V 2/3 l (cm3)
Constant Value Constant Value
Sn Cu Ag Sb
6.4 3.7 4.7 6.6
PSn – Cu PSn – Ag PAg – Cu PSn – Sb PSb – Cu
1.24 1.47 1.39 1.26
4.15 4.55 4.45 4.4
0.04 0.07 0.07 0.04
12.3 12.3 14.1 12.3 12.3
bSn – Cu bSn – Ag bAg – Cu bSn – Sb bSb – Cu
0.63 0.315 0 0 0.69
a Electron density is the electron numbers in every (0.529 10 10m)3 volume.
Fig. 6. Effect of the local mole fraction of Sb on the Gibbs energy of Cu – Sn system.
X. Ma et al. / Materials Letters 57 (2003) 3361–3365
Thermodynamic models, the Miedema model [19] and the Tanaka modification for a binary system [20] and the Chou geometric model for a ternary system [21], are used in this work. Previous works had illustrated their applicability to nonferrous metal systems [22], and by using them, several thermodynamic phenomena of Sn –Pb solder alloy had been clarified [11,23]. The details of these models could be found in Refs. [11,22]. All of the thermodynamic parameters used in this work are listed in Table 1. Figs. 5 and 6 show the calculated Gibbs energy, DG, of Cu –Sn system at 125 jC and the effect of Ag and Sb atoms. It can be seen that, for Cu – Sn binary system, Cu6Sn5 phase will spontaneously form with priority since it has the minimum negative Gibbs energy. On the other hand, if some Ag or Sb atoms exist in the local reaction region, the abstract value of DG decreases, i.e., the driving force for Cu6Sn5 IMC formation is reduced. Although no calculation for quaternary system is performed here since the present thermodynamic models are only effective for binary and ternary systems, such reduction should be considered as a reason for the slower solid-state growth rate of Cu –Sn IMC in Pb-free solder joints.
References [1] B.P. Richards, C.L. Levoguer, C.P. Hunt, K. Nimmo, S. Peters, P. Cusack, An Analysis of the Current Status of LeadFree Soldering, NPL and ITRI Report, 1999.
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[2] IPC Roadmap, A Guide for Assembly of Lead-Free Electronics, 2000. [3] M. Abtew, G. Selvaduray, Mater. Sci. Eng., Rep. 27 (2000) 95. [4] K. Suganuma, Curr. Opin. Solid State Mater. Sci. 5 (2001) 55. [5] K.N. Tu, R.D. Thompson, Acta Metall. 30 (1982) 947. [6] H.K. Kim, K.N. Tu, Phys. Rev., B 53 (1996) 16027. [7] B.J. Lee, N.M. Hwang, H.M. Lee, Acta Mater. 45 (1997) 1867. [8] P.L. Tu, Y.C. Chan, J.K.L. Lai, IEEE Trans. Compon. Packag. Manuf. Technol., Part B 20 (1997) 87. [9] L. Quan, D.R. Frear, D. Grivas, J.W. Morris, J. Electron. Mater. 16 (1987) 203. [10] R.E. Pratt, E.I. Stromswold, D.J. Quensnel, IEEE Trans. Compon. Packaging Manuf. Technol., Part A 19 (1996) 134. [11] X. Ma, Y.Y. Qian, F. Yoshida, J. Alloys Compd. 334 (2002) 224. [12] K. Seeling, D. Suraski, Circuits Assem. 9 (2000) 56. [13] D. Ma, W.D. Wang, S.K. Lahiri, J. Appl. Phys. 91 (2002) 3312. [14] V.I. Dybkov, Growth Kinetics of Chemical Compound Layers, Cambridge International Science, Cambridge, 1998. [15] K. Zeng, K.N. Tu, Mater. Sci. Eng., Rep. 38 (2002) 55. [16] H.K. Kim, H.K. Liou, K.N. Tu, Appl. Phys. Lett. 66 (1995) 2337. [17] G. Ghosh, J. Appl. Phys. 88 (2000) 6887. [18] K.N. Tu, T.Y. Lee, J.W. Jang, L. Li, D.R. Frear, K. Zeng, J.K. Kivilahti, J. Appl. Phys. 89 (2001) 4843. [19] F.R. De Boer, R. Boom, W.C.M. Mattens, A.R. Miedema, A.K. Niessen, Cohesion in Metals, North-Holland, Amsterdam, 1998. [20] T. Tanaka, N.A. Gokcen, Z. Morita, Z. Metallkd. 81 (1990) 49. [21] K.C. Chou, CALPHAD 17 (1995) 315. [22] X. Ma, F. Yoshida, Mater. Lett. 56 (2002) 441. [23] X. Ma, Y.Y. Qian, F. Yoshida, J. Alloys Compd. 327 (2001) 263.