- Email: [email protected]
Effects of rapid solidification on the microstructure and microhardness of a lead-free Sn-3.5Ag solder
RARE METALS Vol. 25, No. 4, Aug 2006, p . 365
Effects of rapid solidification on the microstructure and microhardness of a lead-free Sn-3.5Ag solder SHENJun”, LIU Yongchung2’, Hun Yajing2’,and GAO Houxiu2’ I) College of Mechanical Engineering,Chongqing University, Chongqing 4ooo44, China
2) College of Materials Science and Engineering, ‘Iianjin University, Tianjin 300072. China (Received 2005-08-14)
Abstract: A lead-& Sn-3.5Ag solder was prepared by rapid solidification technology. The high solidification rate, o b tained by rapid cooling, promotes nucleation, and suppresses the growth of Ag$n intermetallic compounds W C s ) in Ag-rich zone, yielding fine Ag3Sn nanoparticuiates with spherical morphology in the matrix of the solder. The large amount of tough homogeneously-dispersed IMCs helps to improve the surface area per unit volume and obstructs the dislocation lines passing through the solder, which fits with the dispersion-strengthening theory. Hence, the rapidly-solidified Sn-3.5Ag solder exhibits a higher microhardness when compared with a slowly-solidifiedSn-3.5Ag solder. Key words: lead-free solder; nanoparticulate; rapid solidification;intermetalliccompounds; dispersion-strengthening effect
[This work was financially supported by the National Natural Science Foundation of China (No. 50401003), the Natural Science Foundation of i’ianjin Cify (No. 033608811)and Fok Hnng Tong Education Foundation (No. 104015).]
1. Introduction Under the legislation and regulations passed by governments to create a lead-free environment, the global trend toward using lead-free solders as a substitute for traditional lead-bearing solders in electronic industry has emerged. The lead-free solder, to become acceptable, should function as a suitable alternative in terms of electrical interconnectablity, structural integrity, and reliability [l]. Till now, among the new generation of lead-free solders, Sn-Ag alloys with higher strength, superior resistance to creep, and thermal fatigue compared with an eutectic Sn-Pb solder was preferred as it also had cost-effectiveness compared with other lead-free solder alternatives. Therefore, they have been recognized as one of the best choices to replace conventional Sn-Pb eutectic solders [2-4]. At the same time, because of the practical difficulties in soldering technology such as different cooling rates achieved with solder alloys, the effects Corresponding author: SHEN Jun
of applied cooling rate on the microstructure, and mechanical behavior of solder alloys were studied intensively. Ochoa et al. [5-61 found that the applied cooling rate had a significant effect on a size of intermetallic compounds, microstructures, and mechanical properties of Sn-Ag alloys. At relatively fast cooling rate (24 Ws), a distribution of Ag3Sn spherical particles was observed. The intermetallic compounds exhibit a rod-like morphology at lower cooling rates (0.08 Ws). Kim et al. [2] studied the solidified microstructure of Sn-Ag-Cu alloys at cooling rates from 0.012 to 8.3 Ws, and as a result, different tensile properties were obtained in these alloys. Dutta et al. [3]explored the impression creep characterization of rapidly-cooled Sn-Ag solders, but the cooling rate was still only about 31 Ws between 553 and 463 K, and 20 W Sbetween 463 and 333 K at the bottom of the sample. Since laser welding is widely used in the soldering industry and a very fast cooling rate is achieved in solder alloys by this technology, the relationship between the mi-
E-mail: [email protected]
RARE METALS, Vol. 25, No. 4, Aug 2006
366
crostructure and mechanical properties of a rapidly-solidified Sn-Ag solder (about lo4 Ws) is still an area to be investigated. In this experiment, an Sn-3.5%Ag eutectic alloy was used as a sample for investigating the influences of rapid cooling rate on the microstructure and microhardness of Sn-3.5Ag solders. In addition, a slowly-solidified Sn-3.5Ag solder was prepared as a reference.
2. Experimental Sn-3.5Ag alloys (wt.%) were prepared from bulk rods of pure Sn, Ag right in eutectic composition (as shown in Fig. 1). The melting process was canied out in a vacuum arc furnace under a high purity argon atmosphere to produce button-like samples with a diameter of approximate 3.5 cm. The ingot was remelted four times to get a homogeneous composition within the ingots. After that, the melts were cooled down to ambient temperature with two different cooling rates. The slowly-solidified sample was obtained by keeping it in an A1203 crucible with a cooling rate of about 0.083 Ws. The rapidly-solidifiedsample was achieved by casting into a water-cooled copper mold and obtaining a 5 mm diameter rod with a cooling rate of about 104US. The applied slow cooling rate simulated the soldering process of large components or printed wiring boards with high heat capacity, and the rapid cooling rate simulated the soldering process of laser welding.
Ag
Mass fraction of tin 1 %
Sn
Fig. 1. Binary Sn-Ag phase diagram 171.
The microstructures of two Sn-3.5Ag samples
were prepared by standard metallographic procedures. They were mechanically polished with 1 pm diamond paste and etched with a solution of 5 v01.%HN03+ 95 vol.%C2H50H.An optical microscope was used for the observation'of microstructures of the samples. X-ray diffraction (XRD) was adopted for the determination of phase structure. Scanning electron microscopy (SEM) and transmission electron microscope (TEM)were also used for the observation of fine microstructure in the rapidly-solidified sample. Additionally, energy diffraction spectrometer (EDS) was used to determine the phase in the rapidly-solidified sample. Vickers hardness measurement was performed to clarify the relationship of microstructure and microhardness of the two samples. The corresponding Vickers hardness values were obtained as Hv =
2 ~ s i n g
d2 where 4 is the indenter apex angle, P is the applied load and d is the average length of diagonals. Here the applied load and loading period are 98 mN and 5 s, respectively. For each sample, five points were tested and the arithmetical mean values were obtained.
3. Results and discussion 3.1. Microstructural observation Fig. 2(a) illustrates that the slowly-solidified sample, with a cooling rate of 0.083 Ws(it nearly follows the equilibrium solidification process), exhibits a full eutectic structure consisting of gray and white needle-like phases. According to the corresponding X-ray diffraction spectrum (cf. Fig. 3) and the predication of binary Sn-Ag phase diagram, it is clear that the eutectic structure consists of a mixture of needle-like Ag3Sn IMCs and p-Sn phase. To determine the distance of Ag3Sn IMCs in solder alloys, twenty locations were chosen randomly in the samples and the arithmetical mean values were evaluated as the results. The distance of Ag3Sn phases in slowly-solidified solder alloys is about 0.28 p. Different morphologies appear in the microstruc-
M e n J. et al., Effects of rapid solidification on the microstructureand microhardnessof ...
ture of the rapidly-solidified sample (Fig. 2(b)). Contrary to the fine eutectic structure in the slowly-solidified sample, this sample exhibits clearly fine dendritic structure. The typical microstructure of this sample shows two kinds of white and black areas. According to the X-ray diffraction spectrum in Fig. 3, the white dendrites are primary p-Sn phases surrounded by a net-like Ag-rich zone of the Ag3Sn Ih4Cs and p-Sn phases.
367
tion. It is amazing that hundreds of nanoparticulates form in Ag-rich zone. According to the EDS analysis results shown in Fig. 5 and Table 1, these nanoparticulates contain Ag element. Since the solder is Sn-Ag and Ag3Sn is the only precipitate phase containing Ag element in the solder, these nanoparticulates are Ag3Sn IMCs.
P
P Sn
a Ag,Sn
I
10
30
50 281
I
I
70
90
(0)
Fig. 3. X-Ray diffraction spectrum of the Sn-35Ag solders.
Fig. 4. Typical microstructure of A e n nanoparticulates in the rapidly-solidified Sn-3.5Ag alloy.
Fig. 2. Optical images of the investigated Sn-35Ag alloy: (a) the slowly-solidifiedand (b)rapidly-solidified.
Fig. 3 shows that only in the rapidly-solidified sample, three relatively low diffraction peaks and a halo peak corresponding to Ag3Sn phase are observed. This halo peak indicates that a nanocrystalline structure forms in the rapidly-solidified sample. Hence, SEM was used to observe the detailed structures. Fig. 4 shows the image under high magnifica-
To determine the distribution of Ag3Sn nanoparticulates in the solder matrix, the size, and spacing of Ag3Sn nanoparticulates were explored using TEM. Fig. 6 shows the fine homogeneously-dispersed Ag3Sn IMCs in the Ag-rich zone being spherical but not quite round. To obtain their size, twenty Ag3Sn nanoparticulates were chosen randomly in the samples and the arithmetical mean value was evaluated. The average size of Ag3Sn nanoparticulates was
RARE METALS, VoL 25, No. 4, Aug 2006
368
about 68 nm, and the interparticulatespacing, A, was determined using the following relation, which assumes an ordered arrangement of nanoparticulates
PI: f
- \x
3.2. Formation of A@n nanoparticulates
"="id 500
2 .
300
Energy I keV Fig. 5. EDX analysis spectrum of nanoparticulatesin the rapidly-solidified Sn-35Ag alloy. Table 1. EDX analysis results of nanoparticulates in the rapidly-solidified Sn-35Ag alloy Phase Nanouarticulates
Composition /at.% Sn As
35.75
IMCs. The Ag3Sn was only present in the Ag-rich zone, and the Ag-rich zone makes up half of the total microstructure, so the average interparticulate spacing is about 150 nm.
64.25
Phase identification
Aedn
When comparing the microstructures of two samples with different solidification rates, it Seems that the rapid solidification condition brings about the formation of Ag3Sn nanoparticulates in Sn-Ag alloy. The formation of Ag3Sn nanoparticulates in Sn-Ag alloy is divided into two stages such as nucleation and growth to explain the rapid solidification condition and it also plays an important role in all stages to produce Ag3Sn nanoparticulates. In the Sn-Ag binary system, although the nucleation of Ag3Sn may occur under a smaller undercooling and the p-Sn phase requires a significantly larger undercooling to induce nucleation and final solid& cation. In rapid solidification condition, the obtained kinetic undercooling degree is much stronger than the required undercooling one of PSn and the intrinsically-rapid growth kinetics of Sn, and the solidification of the PSn phase is very rapid [9]. Hence, this leads to the formation of p-Sn dendritic structure and the remaining liquid is rich in Ag element and the Ag-rich liquid helps the nucleation of Ag$n, which can be explained by the classic nucleation theory [103: I = n:mLnexp( - s ) e x p (
Fig. 6. "EM images depicting the Ag-rich zone containingAg3Sn nanoparticulates.
where d is the average diameter of particles andfis the volume fraction of Ag3Sn nanoparticulates. Since the silver shows negligible solid solubility in tin (cf. Fig. l), all silver in the melt must form Ag3Sn
-E)
(3)
where I is the nucleation rate, n: is the number of atoms on the surface of the critical nucleus, n is the number of atoms per unit volume of the liquid, k is the Boltzman's constant, T is the thermodynamic temperature, &and depend on the activity of atoms, AG* is an activation energy for the nucleation of a critical number of the clustered atoms, and Q is an activation free energy for diffusion across solid-liquid interface. According to Eq. (3), the rich Ag element in the remaining liquid means that a large number of Ag atoms exist in per unit volume of the liquid. Hence, it promotes the nucleation of Ag3Sn and results in a large amount of Ag3Sn crystal nuclei forming in the
Shen J. et aL, Effects of rapid solidification on the microstructureand microhardness of.
Ag-rich zone. At the same time, the rapid solidification also leads to the quick dissipation of heat during solidification, which makes the long distance diffusion of Sn and Ag atoms improbable. So, the Ag3Sn IMCs were easily nucleated but the growth of Ag3Sn in Ag-rich zone was totally suppressed,yielding a large number of very fine Ag3Sn with a spherical morphology. Hence, the high cooling rate promotes the nucleation and suppresses the growth of Ag3Sn IMCs in the rapidly-solidified Sn-Ag alloy, and it results in the formation of Ag3Sn nanoparticulates.
..
369
where zis the stress at the particle surface, n is the piled-up dislocation number, and is the yielding stress of the alloy. Meanwhile, the piled-up dislocation number is x ( 1 - v ) Lzo n= Gb where L is the average distance between the second particles, b is the Burgers Vector, v is the Poisson's ratio, and G is the shear elastic modulus of the substrate. Therefore, x (1- v) Lzo' z= Gb and the yielding stress of the alloy becomes
33. Microhardness measurement The measurement of microhardness is normally adopted to characterize the mechanical properties of alloys. The microhardness of slowly-solidified and rapidly-solidified Sn-3.5Ag solders were measured. As shown in Fig. 7, an increase in microhardness in the presence of Ag3Sn nanoparticulates is observed in the solder matrix. This could be accounted by the dispersion-strengtheningeffect.
zo=J
Gbz x(1-v)L
(7)
If we assume that zis the fracture stress of second particles and v as a constant, the yield stress of the alloy increases when the average distance between the secondary particles decreases. Therefore, the fine-dispersion of secondary particles can enhance the strength of alloys. In our study, the dispersion-strengthening effect is dominated by Ag3Sn. The average distance between Ag3Sn particles in the Sn-3.5Ag solder alloy is 0.28 pn, while it is 0.15 pn in the rapidly-solidified. Therefore, LIIb = 1.88 leads to clq = 1.37. In our microhardness measurement, the microhardness of solder alloys is Hv21HV1= 18.74114.40= 1.30. Within the acceptable range of tolerance, the results of microhardness measurement are in accordance with the theoretical calculation result of the dispersion strengtheningtheory.
Fig. 7. Measured Vickers hardness of Sn-35Ag alloys at different cooling rates.
4.
As reported previously [ll], the hardness of Ag3Sn is about 15 times more than pure Sn and 12 times more than Ag-rich zone in the Sn-3.5%Ag solder. Therefore, the Ag3Sn is a reinforcing phase and the stress acting on the particle's surface can be expressed with the piling-up of linear dislocation, that is [ 121 z = nz, (4)
Comparing the microstructure and microhardness of two Sn-3.5Ag solders solidified under different conditions, the rapidly-solidified Sn-3.5Ag solder with Ag3Sn nanoparticulates exhibits a higher microhardness. This could be attributed to the rapid cooling condition, which promotes the nucleation and suppresses the growth of Ag3Snin Ag-rich zone, and yields fine Ag3Sn nanoparticulates with a spherical morphology in the solder matrix. The large
Conclusions
RARE METALS, Vol. 25, No. 4, Aug 2006
370
amount homogeneously-dispersed Ag3Sn nanoparticulates helps to improve the surface area per unit volume and can limit the dislocation lines passing through the solder, which makes the rapidly-solidified Sn-3.5Ag solder exhibit a higher microhardness.
References Laurila T., Vuorinen V., and Kivilahti J.K., Interfacial reactions between lead-free solders and common base materials, Mater. Sci. Eng. R, 2005,49 1. Kim K.S., Huh S.H., and Suganuma K., Effects of cooling speed on microstructure and tensile properties of Sn-Ag-Cu alloys, Mater. Sci. Eng. A, 2002, 333: 106. Dutta I., Park C., and Choi S., Impression creep charactehtion of rapidly cooled Sn-3.5Ag solders, Mater. Sci. Eng. A, 2004,379: 401. Kim K.S., Huh S.H.,and Suganuma K., Effects of intermetallic compounds on properties of Sn-Ag-Cu lead-free solderedjoints, J. Alloys Compd, 2003,352: 226. Ochoa F., Williams J.J., and Chawla N., Effect of
cooling rate on the microstructure and mechanical behavior of Sn-3.5Ag solder, JOM, 2003,55: 56. Ochoa F., Williams J.J., and Chawla N., Effects of cooling rate on the microstructure and tensile behavior of an Sn-3.5 wt.% Ag solder, J . Electron. Mater., 2003,32: 1414. Chevalier P.Y., A thermodynamic evaluation of the Ag-Sn system, Thermochim.Actu, 1988,136: 45. Meyen M.A. and Chawla K.K., Mechanical Behavior ofMaterials, Prentice-Hall, Upper Saddle River, NJ, 1998: 135. Shen J.,Liu Y.C., Gao H.X., Wei C., and Yang Y.Q., Formation of bulk Ag3Sn intermetallic compounds in Sn-Ag lead-free solders in solidification,J . Electron. Muter., 2005,M 1591. [lo] Chalmers B., Principles of Solidijication, John Wiley & Sons Inc., New York, 1964: 6. [ 111 Deng X., Chawla N., Chawla K.K., and Koopman M., Deformation behavior of (Cu,Ag)-Sn intermetallics by nanoindentation,Acta Mater., 2004,52: 4291. [12] Kuanfu H., Mico-Theory of Metallic Mechanical P r o p e q (in Chinese), Science Press, Beijing, 1983: 122.
Effects of rapid solidification on the microstructure and microhardness of a lead-free Sn-3.5Ag solder SHENJun”, LIU Yongchung2’, Hun Yajing2’,and GAO Houxiu2’ I) College of Mechanical Engineering,Chongqing University, Chongqing 4ooo44, China
2) College of Materials Science and Engineering, ‘Iianjin University, Tianjin 300072. China (Received 2005-08-14)
Abstract: A lead-& Sn-3.5Ag solder was prepared by rapid solidification technology. The high solidification rate, o b tained by rapid cooling, promotes nucleation, and suppresses the growth of Ag$n intermetallic compounds W C s ) in Ag-rich zone, yielding fine Ag3Sn nanoparticuiates with spherical morphology in the matrix of the solder. The large amount of tough homogeneously-dispersed IMCs helps to improve the surface area per unit volume and obstructs the dislocation lines passing through the solder, which fits with the dispersion-strengthening theory. Hence, the rapidly-solidified Sn-3.5Ag solder exhibits a higher microhardness when compared with a slowly-solidifiedSn-3.5Ag solder. Key words: lead-free solder; nanoparticulate; rapid solidification;intermetalliccompounds; dispersion-strengthening effect
[This work was financially supported by the National Natural Science Foundation of China (No. 50401003), the Natural Science Foundation of i’ianjin Cify (No. 033608811)and Fok Hnng Tong Education Foundation (No. 104015).]
1. Introduction Under the legislation and regulations passed by governments to create a lead-free environment, the global trend toward using lead-free solders as a substitute for traditional lead-bearing solders in electronic industry has emerged. The lead-free solder, to become acceptable, should function as a suitable alternative in terms of electrical interconnectablity, structural integrity, and reliability [l]. Till now, among the new generation of lead-free solders, Sn-Ag alloys with higher strength, superior resistance to creep, and thermal fatigue compared with an eutectic Sn-Pb solder was preferred as it also had cost-effectiveness compared with other lead-free solder alternatives. Therefore, they have been recognized as one of the best choices to replace conventional Sn-Pb eutectic solders [2-4]. At the same time, because of the practical difficulties in soldering technology such as different cooling rates achieved with solder alloys, the effects Corresponding author: SHEN Jun
of applied cooling rate on the microstructure, and mechanical behavior of solder alloys were studied intensively. Ochoa et al. [5-61 found that the applied cooling rate had a significant effect on a size of intermetallic compounds, microstructures, and mechanical properties of Sn-Ag alloys. At relatively fast cooling rate (24 Ws), a distribution of Ag3Sn spherical particles was observed. The intermetallic compounds exhibit a rod-like morphology at lower cooling rates (0.08 Ws). Kim et al. [2] studied the solidified microstructure of Sn-Ag-Cu alloys at cooling rates from 0.012 to 8.3 Ws, and as a result, different tensile properties were obtained in these alloys. Dutta et al. [3]explored the impression creep characterization of rapidly-cooled Sn-Ag solders, but the cooling rate was still only about 31 Ws between 553 and 463 K, and 20 W Sbetween 463 and 333 K at the bottom of the sample. Since laser welding is widely used in the soldering industry and a very fast cooling rate is achieved in solder alloys by this technology, the relationship between the mi-
E-mail: [email protected]
RARE METALS, Vol. 25, No. 4, Aug 2006
366
crostructure and mechanical properties of a rapidly-solidified Sn-Ag solder (about lo4 Ws) is still an area to be investigated. In this experiment, an Sn-3.5%Ag eutectic alloy was used as a sample for investigating the influences of rapid cooling rate on the microstructure and microhardness of Sn-3.5Ag solders. In addition, a slowly-solidified Sn-3.5Ag solder was prepared as a reference.
2. Experimental Sn-3.5Ag alloys (wt.%) were prepared from bulk rods of pure Sn, Ag right in eutectic composition (as shown in Fig. 1). The melting process was canied out in a vacuum arc furnace under a high purity argon atmosphere to produce button-like samples with a diameter of approximate 3.5 cm. The ingot was remelted four times to get a homogeneous composition within the ingots. After that, the melts were cooled down to ambient temperature with two different cooling rates. The slowly-solidified sample was obtained by keeping it in an A1203 crucible with a cooling rate of about 0.083 Ws. The rapidly-solidifiedsample was achieved by casting into a water-cooled copper mold and obtaining a 5 mm diameter rod with a cooling rate of about 104US. The applied slow cooling rate simulated the soldering process of large components or printed wiring boards with high heat capacity, and the rapid cooling rate simulated the soldering process of laser welding.
Ag
Mass fraction of tin 1 %
Sn
Fig. 1. Binary Sn-Ag phase diagram 171.
The microstructures of two Sn-3.5Ag samples
were prepared by standard metallographic procedures. They were mechanically polished with 1 pm diamond paste and etched with a solution of 5 v01.%HN03+ 95 vol.%C2H50H.An optical microscope was used for the observation'of microstructures of the samples. X-ray diffraction (XRD) was adopted for the determination of phase structure. Scanning electron microscopy (SEM) and transmission electron microscope (TEM)were also used for the observation of fine microstructure in the rapidly-solidified sample. Additionally, energy diffraction spectrometer (EDS) was used to determine the phase in the rapidly-solidified sample. Vickers hardness measurement was performed to clarify the relationship of microstructure and microhardness of the two samples. The corresponding Vickers hardness values were obtained as Hv =
2 ~ s i n g
d2 where 4 is the indenter apex angle, P is the applied load and d is the average length of diagonals. Here the applied load and loading period are 98 mN and 5 s, respectively. For each sample, five points were tested and the arithmetical mean values were obtained.
3. Results and discussion 3.1. Microstructural observation Fig. 2(a) illustrates that the slowly-solidified sample, with a cooling rate of 0.083 Ws(it nearly follows the equilibrium solidification process), exhibits a full eutectic structure consisting of gray and white needle-like phases. According to the corresponding X-ray diffraction spectrum (cf. Fig. 3) and the predication of binary Sn-Ag phase diagram, it is clear that the eutectic structure consists of a mixture of needle-like Ag3Sn IMCs and p-Sn phase. To determine the distance of Ag3Sn IMCs in solder alloys, twenty locations were chosen randomly in the samples and the arithmetical mean values were evaluated as the results. The distance of Ag3Sn phases in slowly-solidified solder alloys is about 0.28 p. Different morphologies appear in the microstruc-
M e n J. et al., Effects of rapid solidification on the microstructureand microhardnessof ...
ture of the rapidly-solidified sample (Fig. 2(b)). Contrary to the fine eutectic structure in the slowly-solidified sample, this sample exhibits clearly fine dendritic structure. The typical microstructure of this sample shows two kinds of white and black areas. According to the X-ray diffraction spectrum in Fig. 3, the white dendrites are primary p-Sn phases surrounded by a net-like Ag-rich zone of the Ag3Sn Ih4Cs and p-Sn phases.
367
tion. It is amazing that hundreds of nanoparticulates form in Ag-rich zone. According to the EDS analysis results shown in Fig. 5 and Table 1, these nanoparticulates contain Ag element. Since the solder is Sn-Ag and Ag3Sn is the only precipitate phase containing Ag element in the solder, these nanoparticulates are Ag3Sn IMCs.
P
P Sn
a Ag,Sn
I
10
30
50 281
I
I
70
90
(0)
Fig. 3. X-Ray diffraction spectrum of the Sn-35Ag solders.
Fig. 4. Typical microstructure of A e n nanoparticulates in the rapidly-solidified Sn-3.5Ag alloy.
Fig. 2. Optical images of the investigated Sn-35Ag alloy: (a) the slowly-solidifiedand (b)rapidly-solidified.
Fig. 3 shows that only in the rapidly-solidified sample, three relatively low diffraction peaks and a halo peak corresponding to Ag3Sn phase are observed. This halo peak indicates that a nanocrystalline structure forms in the rapidly-solidified sample. Hence, SEM was used to observe the detailed structures. Fig. 4 shows the image under high magnifica-
To determine the distribution of Ag3Sn nanoparticulates in the solder matrix, the size, and spacing of Ag3Sn nanoparticulates were explored using TEM. Fig. 6 shows the fine homogeneously-dispersed Ag3Sn IMCs in the Ag-rich zone being spherical but not quite round. To obtain their size, twenty Ag3Sn nanoparticulates were chosen randomly in the samples and the arithmetical mean value was evaluated. The average size of Ag3Sn nanoparticulates was
RARE METALS, VoL 25, No. 4, Aug 2006
368
about 68 nm, and the interparticulatespacing, A, was determined using the following relation, which assumes an ordered arrangement of nanoparticulates
PI: f
- \x
3.2. Formation of A@n nanoparticulates
"="id 500
2 .
300
Energy I keV Fig. 5. EDX analysis spectrum of nanoparticulatesin the rapidly-solidified Sn-35Ag alloy. Table 1. EDX analysis results of nanoparticulates in the rapidly-solidified Sn-35Ag alloy Phase Nanouarticulates
Composition /at.% Sn As
35.75
IMCs. The Ag3Sn was only present in the Ag-rich zone, and the Ag-rich zone makes up half of the total microstructure, so the average interparticulate spacing is about 150 nm.
64.25
Phase identification
Aedn
When comparing the microstructures of two samples with different solidification rates, it Seems that the rapid solidification condition brings about the formation of Ag3Sn nanoparticulates in Sn-Ag alloy. The formation of Ag3Sn nanoparticulates in Sn-Ag alloy is divided into two stages such as nucleation and growth to explain the rapid solidification condition and it also plays an important role in all stages to produce Ag3Sn nanoparticulates. In the Sn-Ag binary system, although the nucleation of Ag3Sn may occur under a smaller undercooling and the p-Sn phase requires a significantly larger undercooling to induce nucleation and final solid& cation. In rapid solidification condition, the obtained kinetic undercooling degree is much stronger than the required undercooling one of PSn and the intrinsically-rapid growth kinetics of Sn, and the solidification of the PSn phase is very rapid [9]. Hence, this leads to the formation of p-Sn dendritic structure and the remaining liquid is rich in Ag element and the Ag-rich liquid helps the nucleation of Ag$n, which can be explained by the classic nucleation theory [103: I = n:mLnexp( - s ) e x p (
Fig. 6. "EM images depicting the Ag-rich zone containingAg3Sn nanoparticulates.
where d is the average diameter of particles andfis the volume fraction of Ag3Sn nanoparticulates. Since the silver shows negligible solid solubility in tin (cf. Fig. l), all silver in the melt must form Ag3Sn
-E)
(3)
where I is the nucleation rate, n: is the number of atoms on the surface of the critical nucleus, n is the number of atoms per unit volume of the liquid, k is the Boltzman's constant, T is the thermodynamic temperature, &and depend on the activity of atoms, AG* is an activation energy for the nucleation of a critical number of the clustered atoms, and Q is an activation free energy for diffusion across solid-liquid interface. According to Eq. (3), the rich Ag element in the remaining liquid means that a large number of Ag atoms exist in per unit volume of the liquid. Hence, it promotes the nucleation of Ag3Sn and results in a large amount of Ag3Sn crystal nuclei forming in the
Shen J. et aL, Effects of rapid solidification on the microstructureand microhardness of.
Ag-rich zone. At the same time, the rapid solidification also leads to the quick dissipation of heat during solidification, which makes the long distance diffusion of Sn and Ag atoms improbable. So, the Ag3Sn IMCs were easily nucleated but the growth of Ag3Sn in Ag-rich zone was totally suppressed,yielding a large number of very fine Ag3Sn with a spherical morphology. Hence, the high cooling rate promotes the nucleation and suppresses the growth of Ag3Sn IMCs in the rapidly-solidified Sn-Ag alloy, and it results in the formation of Ag3Sn nanoparticulates.
..
369
where zis the stress at the particle surface, n is the piled-up dislocation number, and is the yielding stress of the alloy. Meanwhile, the piled-up dislocation number is x ( 1 - v ) Lzo n= Gb where L is the average distance between the second particles, b is the Burgers Vector, v is the Poisson's ratio, and G is the shear elastic modulus of the substrate. Therefore, x (1- v) Lzo' z= Gb and the yielding stress of the alloy becomes
33. Microhardness measurement The measurement of microhardness is normally adopted to characterize the mechanical properties of alloys. The microhardness of slowly-solidified and rapidly-solidified Sn-3.5Ag solders were measured. As shown in Fig. 7, an increase in microhardness in the presence of Ag3Sn nanoparticulates is observed in the solder matrix. This could be accounted by the dispersion-strengtheningeffect.
zo=J
Gbz x(1-v)L
(7)
If we assume that zis the fracture stress of second particles and v as a constant, the yield stress of the alloy increases when the average distance between the secondary particles decreases. Therefore, the fine-dispersion of secondary particles can enhance the strength of alloys. In our study, the dispersion-strengthening effect is dominated by Ag3Sn. The average distance between Ag3Sn particles in the Sn-3.5Ag solder alloy is 0.28 pn, while it is 0.15 pn in the rapidly-solidified. Therefore, LIIb = 1.88 leads to clq = 1.37. In our microhardness measurement, the microhardness of solder alloys is Hv21HV1= 18.74114.40= 1.30. Within the acceptable range of tolerance, the results of microhardness measurement are in accordance with the theoretical calculation result of the dispersion strengtheningtheory.
Fig. 7. Measured Vickers hardness of Sn-35Ag alloys at different cooling rates.
4.
As reported previously [ll], the hardness of Ag3Sn is about 15 times more than pure Sn and 12 times more than Ag-rich zone in the Sn-3.5%Ag solder. Therefore, the Ag3Sn is a reinforcing phase and the stress acting on the particle's surface can be expressed with the piling-up of linear dislocation, that is [ 121 z = nz, (4)
Comparing the microstructure and microhardness of two Sn-3.5Ag solders solidified under different conditions, the rapidly-solidified Sn-3.5Ag solder with Ag3Sn nanoparticulates exhibits a higher microhardness. This could be attributed to the rapid cooling condition, which promotes the nucleation and suppresses the growth of Ag3Snin Ag-rich zone, and yields fine Ag3Sn nanoparticulates with a spherical morphology in the solder matrix. The large
Conclusions
RARE METALS, Vol. 25, No. 4, Aug 2006
370
amount homogeneously-dispersed Ag3Sn nanoparticulates helps to improve the surface area per unit volume and can limit the dislocation lines passing through the solder, which makes the rapidly-solidified Sn-3.5Ag solder exhibit a higher microhardness.
References Laurila T., Vuorinen V., and Kivilahti J.K., Interfacial reactions between lead-free solders and common base materials, Mater. Sci. Eng. R, 2005,49 1. Kim K.S., Huh S.H., and Suganuma K., Effects of cooling speed on microstructure and tensile properties of Sn-Ag-Cu alloys, Mater. Sci. Eng. A, 2002, 333: 106. Dutta I., Park C., and Choi S., Impression creep charactehtion of rapidly cooled Sn-3.5Ag solders, Mater. Sci. Eng. A, 2004,379: 401. Kim K.S., Huh S.H.,and Suganuma K., Effects of intermetallic compounds on properties of Sn-Ag-Cu lead-free solderedjoints, J. Alloys Compd, 2003,352: 226. Ochoa F., Williams J.J., and Chawla N., Effect of
cooling rate on the microstructure and mechanical behavior of Sn-3.5Ag solder, JOM, 2003,55: 56. Ochoa F., Williams J.J., and Chawla N., Effects of cooling rate on the microstructure and tensile behavior of an Sn-3.5 wt.% Ag solder, J . Electron. Mater., 2003,32: 1414. Chevalier P.Y., A thermodynamic evaluation of the Ag-Sn system, Thermochim.Actu, 1988,136: 45. Meyen M.A. and Chawla K.K., Mechanical Behavior ofMaterials, Prentice-Hall, Upper Saddle River, NJ, 1998: 135. Shen J.,Liu Y.C., Gao H.X., Wei C., and Yang Y.Q., Formation of bulk Ag3Sn intermetallic compounds in Sn-Ag lead-free solders in solidification,J . Electron. Muter., 2005,M 1591. [lo] Chalmers B., Principles of Solidijication, John Wiley & Sons Inc., New York, 1964: 6. [ 111 Deng X., Chawla N., Chawla K.K., and Koopman M., Deformation behavior of (Cu,Ag)-Sn intermetallics by nanoindentation,Acta Mater., 2004,52: 4291. [12] Kuanfu H., Mico-Theory of Metallic Mechanical P r o p e q (in Chinese), Science Press, Beijing, 1983: 122.