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Study of interfacial reactions between Sn3.5Ag0.5Cu composite alloys and Cu substrate
Microelectronic Engineering 88 (2011) 2964–2969
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Study of interfacial reactions between Sn3.5Ag0.5Cu composite alloys and Cu substrate L.C. Tsao ⇑, C.P. Chu, S.F. Peng Department of Materials Engineering, National Pingtung University of Science & Technology, Neipu, Pingtung 91201, Taiwan
a r t i c l e
i n f o
Article history: Received 12 June 2010 Accepted 9 April 2011 Available online 28 April 2011 Keywords: TiO2 nanopowder Sn3.5Ag0.5Cu composite solder Kinetics analysis
a b s t r a c t For development of a lead-free composite solder for advance electrical components, lead-free Sn3.5Ag0.5Cu solder was produced by mechanically mixing 0.5 wt.% TiO2 nanopowder with Sn3.5Ag0.5Cu solder. The morphology and growth kinetics of the intermetallic compounds that formed during the soldering reactions between Sn3.5Ag0.5Cu solder with intermixed TiO2 nanopowder and Cu substrates at various temperatures ranging from 250 to 325 °C were investigated. A scanning electron microscope (SEM) was used to quantify the interfacial microstructure at each processing condition. The thickness of interfacial intermetallic layers was quantitatively evaluated from SEM micrographs using imaging software. Experimental results show that a discontinuous layer of scallop-shaped Cu–Sn intermetallic compounds formed during the soldering. Kinetics analysis shows that the growth of such interfacial Cu–Sn intermetallic compounds is diffusion controlled with an activation energy of 67.72 kJ/mol. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The solder is an important part of the microelectronic interconnection for the transfer of both electrons and thermal energy. Due to health and legislative pressures, the electronics industry is moving to adoption of lead-free solders. Many lead free solders have been studied as replacements for Sn–Pb solders. Among the various lead-free solder alloys, such Sn–Ag, Sn–Cu, Sn–Zn and Sn– Ag–Cu eutectic system, Sn–Ag–Cu solder (SAC) has been proposed as the most promising substitute for lead-containing solders because of its relatively low melting temperature compared with Sn–Ag system, for it provides better mechanical properties than those of eutectic Sn37Pb solder [1–3]. During the soldering process, the metallurgical reaction between liquid solder and copper or Ni/Au metallization result in a intermetallic compound (IMC) layer at the solder/metallization interface. The interfacial reactions of Sn-base solders with Cu pads have been investigated by a number of researchers [4–6]. They all showed that the intermetallic compounds formed during these interfacial reactions below 350 °C were of the Cu3Sn (e) and Cu6Sn5 (g) phases [7]. Because these intermetallics are quite brittle, excessive thickness may decrease solder joint ductility and strength [8]. Recently, the composite solders have been identified as potential materials that may provide higher strength as compared to the conventional solders. ⇑ Corresponding author. Address: Department of Materials Engineering, National Pingtung University of Science & Technology, 1, Hseuhfu Road, Neipu, Pingtung 91201, Taiwan. Tel.: +886 8 7703202x7560, fax: +886 8 7740552. E-mail address: [email protected] (L.C. Tsao). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.04.034
Especially, the addition of dioxide nanopowder enhances the strength of the conventional solder. Tsao and Chang added TiO2 nanoparticles to Sn3.5Ag0.25Cu lead-free solder alloy [9]. The mechanical properties (microhardness, 0.2%YS and UTS) increase with the increasing presence of reinforcement. Tsao et al. added nanopowder of Al2O3 to a conventional Sn3.5Ag0.5Cu solder [10]. Microhardness improved with the addition of Al2O3 nanopowders. This improved mechanical property was due to the composite microstructure, which is close to the theoretical prediction from dispersion strengthening theory. Shen et al. [11] investigated the Sn–3.5Ag–ZrO2 composite solders and found that the size of primary b-Sn dendrites and Ag3Sn phases was refined. Since information on the growth kinetics of such intermetallic compounds in the liquid SAC composite solder/solid Cu interface is scarce, this study focuses on that area. The present work investigates the effect of TiO2 nanopowder addition to the Sn3.5Ag0.5Cu solder on the interfacial reaction between solder and Cu substrate. The morphologies of Cu–Sn IMC formed at various temperatures were observed and their thicknesses measured as a function of reaction time. Through analysis of the kinetics, the activation energy for the growth of these intermetallic compounds can be obtained. 2. Experimental Pure Sn, Ag and Cu were melted in a vacuum furnace at 650 °C for 2.5 h to produce solder alloys of eutectic Sn3.5Ag0.5Cu (wt.%). The purity of all the materials was 99.99 wt.%. The lead-free
L.C. Tsao et al. / Microelectronic Engineering 88 (2011) 2964–2969
Fig. 1. (a) FEG-SEM image of the TiO2 nanopowders and (b) X-ray diffraction spectra of the TiO2 nanopowders.
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composite solders were prepared by mechanically dispersing 0.5 wt.% of TiO2 nanopowder into the eutectic Sn3.5Ag0.5Cu (wt.%) solder with subsequent remelting in vacuum furnace at 650 °C for 2.5 h and casting in a mold. The solder was melted in a crucible and chill cast in a water–cooled copper mold to form square ingots of 8 10 20 mm. The average size of the nominally spherical TiO2 nanopowder was 20 nm diameter (Nanostructured & Amorphous Materials, USA). Fig. 1a shows a field-emission gun scanning electron microscope (FEG-SEM) image of the TiO2 nanopowders taken by a LEO 1530. X-ray diffraction (XRD, D/max 2500 V/PC) analysis was performed to determine the phase composition of the TiO2 nanopowders (Fig. 1b). The ingot was rolled into 1 mm thick foil. Cu substrates with a dimension of 8 mm 12 mm were cut from a 1 mm-thick Cu plate (3N5), ground with SiC paper, and polished with 1 lm and 0.3 lm Al2O3 powders. Before the soldering reaction, substrates were dipped in a rosin mildly activated (RAM) type flux. For the study of interfacial reactions, the SAC solder foil was placed on the Cu substrate and heated in a furnace under a vacuum of 103 torr. Through a water cooling system installed within the furnace, the specimens were quickly cooled to room temperature. Soldering reactions were conducted at temperatures between 250 and 325 °C for various heating times. For the observations of the morphology of intermetallic compounds formed at the Sn3.5Ag0.5Cu composite solder/Cu interface after soldering reactions, scanning electron microscopy (SEM) was used. The specimens were cross-sectioned, ground with SiC paper, polished with 1 lm and 0.3 lm Al2O3 powders, and etched with 5% HCl and 95% H2O solution. The chemical compositions of intermetallic compounds were analyzed by energy-dispersion spectroscopy (EDS). For growth kinetics analysis, the thickness of intermetallic compounds that formed with various temperatures and time periods were calculated through dividing the total area
Fig. 2. The as-cast Sn3.5Ag0.5Cu composite solder: (a) microstructure; (b) DSC; (c) EDS analysis to show the eutectic area in (a).
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Fig. 3. X-ray diffraction scan of the Sn3.5Ag0.5Cu composite solder.
of intermetallic cells spread out on the micrograph by the width of these intermetallics. 3. Results and discussion The microstructure of the as-cast Sn3.5Ag0.5Cu composite solder is shown in Fig. 2a. The melting temperature of the solder as
Fig. 4. Typical morphology of intermetallic compounds formed at the SAC composite solder/Cu interface after soldering reactions at 250 °C for various times of (a) 30 min, (b) 45 min, and (c) 60 min.
measured by DSC was 224.1 °C, as shown in Fig. 2b, which is slightly higher than the eutectic point of Sn3.5Ag0.5Cu alloy (223.6 °C). The microstructure of the lead-free Sn3.5Ag0.5Cu solder was composed of dendritic b-Sn with a size of 15–36 lm, small Ag3Sn particles, and an eutectic area where the needle-like Ag3Sn phase is finely dispersed. The needle-like Ag3Sn grains are in average 12.1 lm long and 1.21 lm wide. However, large particles of Ag3Sn and Cu6Sn5 were not observed in the rapidly-cooled leadfree Sn3.5Ag0.5Cu composite solder. According to the EDS analysis results shown in Fig. 2c, the eutectic areas were found to contain Ti, O, Cu, Sn and Ag. Since the solders used were SAC solder intermixing TiO2 nanopowder and Ag3Sn was the only precipitate phase containing Ag element in the solder, it can be concluded that these network eutectic areas are submicro-Ag3Sn and TiO2 nanopowders. Cu element is solution in the matrix. Fig. 3 presents the X-ray diffraction scan of SAC composite solder, indicating that the TiO2 nanopowders were successfully blended with the SAC solder. As observed from the microstructural observation, both the reinforcing TiO2 nanopowder and the refined IMC of submicro-Ag3Sn can be considered the main reasons for the improved microhardness of the SAC composite solders. As reported in Refs. [12,11], this results in greatly-refined Ag3Sn IMC in the microstructure of the solder. Furthermore, surface absorption theory also can be used to explain the refinement of Ag3Sn particles in the prepared Sn3.5Ag0.5Cu composite solder [13].
Fig. 5. Typical morphology of intermetallic compounds formed at the Sn3.5Ag0.5Cu composite solder/Cu interface after soldering reactions at various temperatures for 40 min: (a) 275 °C, (b) 300 °C, and (c) 325 °C.
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Fig. 4 shows the microstructural evolutions of the Sn3.5Ag0.5Cu composite solder/Cu interface at lower temperatures such as 250 °C for different reaction times. It can be seen from Fig. 4a, that a discontinuous scallop-shaped Cu6Sn5 IMC layer was formed at the Sn3.5Ag0.5Cu composite solder/Cu interface after soldering reaction. EDS analysis showed that the intermetallic compounds comprised only one layer of Cu6Sn5 IMC. However, large particles of Ag3Sn and Cu6Sn5 were not observed in the Sn3.5Ag0.5Cu composite matrix. In addition, a large number of dot-shaped submicroAg3Sn precipitates can be observed around the eutectic network structure in Fig. 4c (Mark a). Fig. 5 shows the morphology of intermetallic compounds formed after interfacial reaction between liquid Sn3.5Ag0.5Cu composite solder and Cu substrate at various temperatures. The EDS analysis indicated that the inner layer adjacent to the Cu substrate the inner layer adjacent to the Cu substrate is Cu3Sn and the outer layer is Cu6Sn5, whereas only Cu6Sn5 is observed in Fig. 5c. Another, the dot scallop-shaped Ag3Sn IMC became widely distributed over the Sn3.5Ag0.5Cu composite solder matrix (Fig. 5c). It can therefore be expected that the mechanical properties of soldered joints are improved. Previous studies [14,15] declared that the formation of Cu3Sn was inhibited because of the prevention of Sn diffusion through the Cu6Sn5 layer, which forms at soldering. Notably, the growth of the dale-IMC site (Mark b) of the Sn3.5Ag0.5Cu composite solder/Cu specimens was not significantly influenced by temperature (Fig. 5) after soldering for 40 min. Compare to the low temperature for 30 min specimen (Fig. 4a), the dale-IMC site is more similar after high temperature solder. Fig. 6 shows the top surface morphology of the IMC formed at the interfacial between liquid Sn3.5Ag0.5Cu composite solder/Cu interface soldered. The scallops-type grains appear rounded, and there are deep channels between them (Fig. 6a). The surface of the Cu6Sn5 grains at the Sn3.5Ag0.5Cu composite solder/Cu interface reaction is rough, and light-colored particles are visible on the surface, as shown in Fig. 6a. With increased soldering time, the size of the nano-Ag3Sn particles increased (Fig. 6b–d). According to the EDS analysis, the scallop-type grains contained Cu, Sn, Ti,
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O and Ag. It is clear that the lighted-colored nano-particles were to confirmed to be Ag3Sn and miner TiO2 nanoparticles, which is adsorbed and filled on the surface of Cu6Sn5 IMC layer soldered. These results that the presence of these TiO2 nanoparticles and submicro-Ag3Sn IMC are a good barrier for preventing the interdiffusion between Cu and Sn (Fig. 7). The growth of interfacial IMC layer was thus delayed [16,17]. The relationship between IMC thickness and soldering time of SAC composite solder/Cu is show in Fig. 8. The intermetallic growth model by diffusion mechanism used to evaluate the diffusion constants of the Cu–Sn phases is the so-called parabolic growth law [18]: 1
Xðt; TÞ ¼ X 0 þ ðDtÞ2
ð1Þ
where D is the interdiffusion coefficient Arrhenius expression, X is the thickness of the IMC layer after soldering, t is the interface reaction time, and T is the absolute temperature. In the case of isothermal liquid-state reaction, the following boundary condition must be satisfied:
Xð0; TÞ ¼ X 0 ¼ 0
ð2Þ
The thickness variation of the Cu–Sn IMC layer of SAC composite solder/Cu at different reaction temperatures (Fig. 8) shows a linear relationship between the thickness (x2) of the IMC and the reaction time (t). The result indicates that the growth of Cu–Sn IMC during the interfacial reactions between liquid SAC composite solder and Cu substrates is diffusion-controlled. The temperature variations of D for the IMC layer growth can be expressed by the following Arrhenius equation in terms of the interdiffusion coefficient [18]:
D ¼ D0 exp ðQ =RTÞ
ð3Þ
where D0 is the interdiffusion constant, Q is activation energy for growth of the interfacial IMC layer, R is the gas constant, and T is the absolute temperature. The growth rate constants ðD ¼ x2 =tÞ, in turn, were then used to calculate the activation energies from Eq. (3) using linear regression analysis of a plot of ln(D) versus (1/T).
Fig. 6. Surface morphologies of Sn3.5Ag0.5Cu composite solder/Cu interface after soldering reactions: (a) 250 °C–30 min, (b) 250 °C–60 min, (c) 325 °C–10 min, and (d) 325 °C–40 min.
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Fig. 9. Arrhenius plot of the growth rate constants (D) of Cu–Sn IMC formed after the Sn3.5Ag0.5Cu composite solder/Cu interfacial reactions.
Table 1 Various diffusion constants in Sn and Cu.
Cu in Sn [1 0 0] Sn in Cu Fig. 7. Schematic diagrams showing the possible paths of the composite solder/Cu substrate interfacial reaction: (a) before soldering and (b) soldering process.
D0 (cm2/s)
Q (kJ/mode)
Range (°C)
Refs.
2.4 103 1.8 104 1.3 10+0 1.13 10+0
33 (S) 17.6 (L) 184 (L) 188 (S)
25 240–480 700–860 200
[19] [20] [21] [22]
S, solid; L, liquid.
of the IMC layers. However, adding to TiO2 nanoparticle was significant improvements in the refinement of Ag3Sn grains. Therefore, the TiO2 nanoparticle and submicro-Ag3Sn phase after soldering between Sn3.5Ag0.5Cu composite solder and Cu is thought to play the major role in raising the activation energy of the growth. So, the composite solder joints also were also effective in retarding the growth of the Cu6Sn5 IMC layer [24].
4. Conclusions
Fig. 8. Thickness (x2) of Cu–Sn IMC as a function if the reaction time (t) after soldering reactions between liquid Sn3.5Ag0.5Cu composite solder and Cu substrate at various temperatures.
An Arrhenius plot of the growth-rate constants versus reaction temperature is shown in Fig. 9. From the slope of the Arrhenius plot, the activation energy (Q) for the growth of overall Cu-Sn IMC (Cu6Sn5 + Cu3Sn) during the SAC composite solder/Cu soldering reactions is estimated to be 67.72 kJ/mol, which is higher than that of the diffusion control of solid Cu in Sn single crystals (33 kJ/mol) [19] and liquid Cu in Sn (17 kJ/mol) [20], but lower than that of liquid Sn in bulk Cu (184–188 kJ/mol) [21,22], as shows Table1. From these data it is obvious to conclude that the mobility of Cu in Sn is much faster than that of Sn in Cu. This implies that the growth of Cu-Sn intermetallic compounds during the SAC composite solder/ Cu soldering reactions was predominantly controlled by the diffusion of Cu into the Sn solders. Previous studies [16,17] indicate that the nanoparticles effect was used to explain the dramatic effect on the growth patterns
A discontinuous layer of scallop-shaped Cu6Sn5 intermetallic compounds appears at the Sn3.5Ag0.5Cu composite/Cu interface after the soldering reactions between liquid Sn3.5Ag0.5Cu composite solder and Cu substrate. Accompanying such a soldering reaction, intermixing of TiO2 nanopowder is found to refine the Ag3Sn phase and b-Sn, which results in the formation of a large number of dot-shaped submicro-Ag3Sn precipitates in the Sn3.5Ag0.5Cu matrix after solidification. The linear growth rate with the square root of reaction time indicates that the interfacial reactions are diffusion controlled. The activation energy, as calculated from the Arrhenius plot of reaction rate constants, is 67.72 kJ/mol, which is higher than that of the diffusion control of solid Cu in Sn single crystals (33 kJ/ mol) and liquid Cu in Sn (17 kJ/mol), but lower than that of liquid Sn in bulk Cu (184–188 kJ/mol). This result implies that the growth of Cu–Sn intermetallic compounds during the Sn3.5Ag0.5Cu composite/Cu soldering reaction is dominated by the diffusion of Cu into Sn solders. Acknowledgments The authors acknowledge the financial support of this work from the National Science Council of the Republic of China under Project No. NSC97-2218-E-020-004.
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[12] Z.D. Xia, Z.G. Chen, Y.W. Shi, N. Mu, N. Sun, J. Electron. Mater. 31 (2002) 564– 567. [13] Q.J. Zhai, S.K. Guan, Q.Y. Shang, Metallurgy Industry Press, Beijing, 1999, pp. 156–160 [14] S. Ahat, M. Shang, L. Luo, J. Mater. Res. 6 (2001) 2914–2921. [15] H.T. Lee, M.H. Chen, Mater. Sci. Eng. A 333 (2002) 24–34. [16] S.M.L. Nai, J. Wei, M. Gupta, J. Alloy. Comp. 473 (2009) 100–106. [17] Y. Wu, J.A. Sees, C. Pouraghabagher, L.A. Foster, J.L. Marshall, E.G. Jacobs, R.F. Pinizzotto, J. Electron. Mater. 22 (1993) 769–777. [18] R.J.K. Wassink, Electrochemical Publications Ltd., Ayr, Scotland, 1989, pp. 149– 159. [19] B.F. Dyson, T.R. Anthony, D. Turnbull, J. Appl. Phys. 38 (1967) 3408. [20] C.H. Ma, R.A. Swalin, Acta Metall. 8 (1960) 388–395. [21] S.K. Sen, M.B. Dutt, A.K. Barua, Phys. Stat. Sol. A 32 (1975) 345–350. [22] H.C. Bhedwar, K.K. Ray, S.D. Kulkarni, V. Balasubramanisn, Scripta Metall. 6 (1972) 919–922. [24] L.C. Tsao, J. Alloy Comp. 509 (2011) 2326–2333.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Study of interfacial reactions between Sn3.5Ag0.5Cu composite alloys and Cu substrate L.C. Tsao ⇑, C.P. Chu, S.F. Peng Department of Materials Engineering, National Pingtung University of Science & Technology, Neipu, Pingtung 91201, Taiwan
a r t i c l e
i n f o
Article history: Received 12 June 2010 Accepted 9 April 2011 Available online 28 April 2011 Keywords: TiO2 nanopowder Sn3.5Ag0.5Cu composite solder Kinetics analysis
a b s t r a c t For development of a lead-free composite solder for advance electrical components, lead-free Sn3.5Ag0.5Cu solder was produced by mechanically mixing 0.5 wt.% TiO2 nanopowder with Sn3.5Ag0.5Cu solder. The morphology and growth kinetics of the intermetallic compounds that formed during the soldering reactions between Sn3.5Ag0.5Cu solder with intermixed TiO2 nanopowder and Cu substrates at various temperatures ranging from 250 to 325 °C were investigated. A scanning electron microscope (SEM) was used to quantify the interfacial microstructure at each processing condition. The thickness of interfacial intermetallic layers was quantitatively evaluated from SEM micrographs using imaging software. Experimental results show that a discontinuous layer of scallop-shaped Cu–Sn intermetallic compounds formed during the soldering. Kinetics analysis shows that the growth of such interfacial Cu–Sn intermetallic compounds is diffusion controlled with an activation energy of 67.72 kJ/mol. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The solder is an important part of the microelectronic interconnection for the transfer of both electrons and thermal energy. Due to health and legislative pressures, the electronics industry is moving to adoption of lead-free solders. Many lead free solders have been studied as replacements for Sn–Pb solders. Among the various lead-free solder alloys, such Sn–Ag, Sn–Cu, Sn–Zn and Sn– Ag–Cu eutectic system, Sn–Ag–Cu solder (SAC) has been proposed as the most promising substitute for lead-containing solders because of its relatively low melting temperature compared with Sn–Ag system, for it provides better mechanical properties than those of eutectic Sn37Pb solder [1–3]. During the soldering process, the metallurgical reaction between liquid solder and copper or Ni/Au metallization result in a intermetallic compound (IMC) layer at the solder/metallization interface. The interfacial reactions of Sn-base solders with Cu pads have been investigated by a number of researchers [4–6]. They all showed that the intermetallic compounds formed during these interfacial reactions below 350 °C were of the Cu3Sn (e) and Cu6Sn5 (g) phases [7]. Because these intermetallics are quite brittle, excessive thickness may decrease solder joint ductility and strength [8]. Recently, the composite solders have been identified as potential materials that may provide higher strength as compared to the conventional solders. ⇑ Corresponding author. Address: Department of Materials Engineering, National Pingtung University of Science & Technology, 1, Hseuhfu Road, Neipu, Pingtung 91201, Taiwan. Tel.: +886 8 7703202x7560, fax: +886 8 7740552. E-mail address: [email protected] (L.C. Tsao). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.04.034
Especially, the addition of dioxide nanopowder enhances the strength of the conventional solder. Tsao and Chang added TiO2 nanoparticles to Sn3.5Ag0.25Cu lead-free solder alloy [9]. The mechanical properties (microhardness, 0.2%YS and UTS) increase with the increasing presence of reinforcement. Tsao et al. added nanopowder of Al2O3 to a conventional Sn3.5Ag0.5Cu solder [10]. Microhardness improved with the addition of Al2O3 nanopowders. This improved mechanical property was due to the composite microstructure, which is close to the theoretical prediction from dispersion strengthening theory. Shen et al. [11] investigated the Sn–3.5Ag–ZrO2 composite solders and found that the size of primary b-Sn dendrites and Ag3Sn phases was refined. Since information on the growth kinetics of such intermetallic compounds in the liquid SAC composite solder/solid Cu interface is scarce, this study focuses on that area. The present work investigates the effect of TiO2 nanopowder addition to the Sn3.5Ag0.5Cu solder on the interfacial reaction between solder and Cu substrate. The morphologies of Cu–Sn IMC formed at various temperatures were observed and their thicknesses measured as a function of reaction time. Through analysis of the kinetics, the activation energy for the growth of these intermetallic compounds can be obtained. 2. Experimental Pure Sn, Ag and Cu were melted in a vacuum furnace at 650 °C for 2.5 h to produce solder alloys of eutectic Sn3.5Ag0.5Cu (wt.%). The purity of all the materials was 99.99 wt.%. The lead-free
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Fig. 1. (a) FEG-SEM image of the TiO2 nanopowders and (b) X-ray diffraction spectra of the TiO2 nanopowders.
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composite solders were prepared by mechanically dispersing 0.5 wt.% of TiO2 nanopowder into the eutectic Sn3.5Ag0.5Cu (wt.%) solder with subsequent remelting in vacuum furnace at 650 °C for 2.5 h and casting in a mold. The solder was melted in a crucible and chill cast in a water–cooled copper mold to form square ingots of 8 10 20 mm. The average size of the nominally spherical TiO2 nanopowder was 20 nm diameter (Nanostructured & Amorphous Materials, USA). Fig. 1a shows a field-emission gun scanning electron microscope (FEG-SEM) image of the TiO2 nanopowders taken by a LEO 1530. X-ray diffraction (XRD, D/max 2500 V/PC) analysis was performed to determine the phase composition of the TiO2 nanopowders (Fig. 1b). The ingot was rolled into 1 mm thick foil. Cu substrates with a dimension of 8 mm 12 mm were cut from a 1 mm-thick Cu plate (3N5), ground with SiC paper, and polished with 1 lm and 0.3 lm Al2O3 powders. Before the soldering reaction, substrates were dipped in a rosin mildly activated (RAM) type flux. For the study of interfacial reactions, the SAC solder foil was placed on the Cu substrate and heated in a furnace under a vacuum of 103 torr. Through a water cooling system installed within the furnace, the specimens were quickly cooled to room temperature. Soldering reactions were conducted at temperatures between 250 and 325 °C for various heating times. For the observations of the morphology of intermetallic compounds formed at the Sn3.5Ag0.5Cu composite solder/Cu interface after soldering reactions, scanning electron microscopy (SEM) was used. The specimens were cross-sectioned, ground with SiC paper, polished with 1 lm and 0.3 lm Al2O3 powders, and etched with 5% HCl and 95% H2O solution. The chemical compositions of intermetallic compounds were analyzed by energy-dispersion spectroscopy (EDS). For growth kinetics analysis, the thickness of intermetallic compounds that formed with various temperatures and time periods were calculated through dividing the total area
Fig. 2. The as-cast Sn3.5Ag0.5Cu composite solder: (a) microstructure; (b) DSC; (c) EDS analysis to show the eutectic area in (a).
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Fig. 3. X-ray diffraction scan of the Sn3.5Ag0.5Cu composite solder.
of intermetallic cells spread out on the micrograph by the width of these intermetallics. 3. Results and discussion The microstructure of the as-cast Sn3.5Ag0.5Cu composite solder is shown in Fig. 2a. The melting temperature of the solder as
Fig. 4. Typical morphology of intermetallic compounds formed at the SAC composite solder/Cu interface after soldering reactions at 250 °C for various times of (a) 30 min, (b) 45 min, and (c) 60 min.
measured by DSC was 224.1 °C, as shown in Fig. 2b, which is slightly higher than the eutectic point of Sn3.5Ag0.5Cu alloy (223.6 °C). The microstructure of the lead-free Sn3.5Ag0.5Cu solder was composed of dendritic b-Sn with a size of 15–36 lm, small Ag3Sn particles, and an eutectic area where the needle-like Ag3Sn phase is finely dispersed. The needle-like Ag3Sn grains are in average 12.1 lm long and 1.21 lm wide. However, large particles of Ag3Sn and Cu6Sn5 were not observed in the rapidly-cooled leadfree Sn3.5Ag0.5Cu composite solder. According to the EDS analysis results shown in Fig. 2c, the eutectic areas were found to contain Ti, O, Cu, Sn and Ag. Since the solders used were SAC solder intermixing TiO2 nanopowder and Ag3Sn was the only precipitate phase containing Ag element in the solder, it can be concluded that these network eutectic areas are submicro-Ag3Sn and TiO2 nanopowders. Cu element is solution in the matrix. Fig. 3 presents the X-ray diffraction scan of SAC composite solder, indicating that the TiO2 nanopowders were successfully blended with the SAC solder. As observed from the microstructural observation, both the reinforcing TiO2 nanopowder and the refined IMC of submicro-Ag3Sn can be considered the main reasons for the improved microhardness of the SAC composite solders. As reported in Refs. [12,11], this results in greatly-refined Ag3Sn IMC in the microstructure of the solder. Furthermore, surface absorption theory also can be used to explain the refinement of Ag3Sn particles in the prepared Sn3.5Ag0.5Cu composite solder [13].
Fig. 5. Typical morphology of intermetallic compounds formed at the Sn3.5Ag0.5Cu composite solder/Cu interface after soldering reactions at various temperatures for 40 min: (a) 275 °C, (b) 300 °C, and (c) 325 °C.
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Fig. 4 shows the microstructural evolutions of the Sn3.5Ag0.5Cu composite solder/Cu interface at lower temperatures such as 250 °C for different reaction times. It can be seen from Fig. 4a, that a discontinuous scallop-shaped Cu6Sn5 IMC layer was formed at the Sn3.5Ag0.5Cu composite solder/Cu interface after soldering reaction. EDS analysis showed that the intermetallic compounds comprised only one layer of Cu6Sn5 IMC. However, large particles of Ag3Sn and Cu6Sn5 were not observed in the Sn3.5Ag0.5Cu composite matrix. In addition, a large number of dot-shaped submicroAg3Sn precipitates can be observed around the eutectic network structure in Fig. 4c (Mark a). Fig. 5 shows the morphology of intermetallic compounds formed after interfacial reaction between liquid Sn3.5Ag0.5Cu composite solder and Cu substrate at various temperatures. The EDS analysis indicated that the inner layer adjacent to the Cu substrate the inner layer adjacent to the Cu substrate is Cu3Sn and the outer layer is Cu6Sn5, whereas only Cu6Sn5 is observed in Fig. 5c. Another, the dot scallop-shaped Ag3Sn IMC became widely distributed over the Sn3.5Ag0.5Cu composite solder matrix (Fig. 5c). It can therefore be expected that the mechanical properties of soldered joints are improved. Previous studies [14,15] declared that the formation of Cu3Sn was inhibited because of the prevention of Sn diffusion through the Cu6Sn5 layer, which forms at soldering. Notably, the growth of the dale-IMC site (Mark b) of the Sn3.5Ag0.5Cu composite solder/Cu specimens was not significantly influenced by temperature (Fig. 5) after soldering for 40 min. Compare to the low temperature for 30 min specimen (Fig. 4a), the dale-IMC site is more similar after high temperature solder. Fig. 6 shows the top surface morphology of the IMC formed at the interfacial between liquid Sn3.5Ag0.5Cu composite solder/Cu interface soldered. The scallops-type grains appear rounded, and there are deep channels between them (Fig. 6a). The surface of the Cu6Sn5 grains at the Sn3.5Ag0.5Cu composite solder/Cu interface reaction is rough, and light-colored particles are visible on the surface, as shown in Fig. 6a. With increased soldering time, the size of the nano-Ag3Sn particles increased (Fig. 6b–d). According to the EDS analysis, the scallop-type grains contained Cu, Sn, Ti,
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O and Ag. It is clear that the lighted-colored nano-particles were to confirmed to be Ag3Sn and miner TiO2 nanoparticles, which is adsorbed and filled on the surface of Cu6Sn5 IMC layer soldered. These results that the presence of these TiO2 nanoparticles and submicro-Ag3Sn IMC are a good barrier for preventing the interdiffusion between Cu and Sn (Fig. 7). The growth of interfacial IMC layer was thus delayed [16,17]. The relationship between IMC thickness and soldering time of SAC composite solder/Cu is show in Fig. 8. The intermetallic growth model by diffusion mechanism used to evaluate the diffusion constants of the Cu–Sn phases is the so-called parabolic growth law [18]: 1
Xðt; TÞ ¼ X 0 þ ðDtÞ2
ð1Þ
where D is the interdiffusion coefficient Arrhenius expression, X is the thickness of the IMC layer after soldering, t is the interface reaction time, and T is the absolute temperature. In the case of isothermal liquid-state reaction, the following boundary condition must be satisfied:
Xð0; TÞ ¼ X 0 ¼ 0
ð2Þ
The thickness variation of the Cu–Sn IMC layer of SAC composite solder/Cu at different reaction temperatures (Fig. 8) shows a linear relationship between the thickness (x2) of the IMC and the reaction time (t). The result indicates that the growth of Cu–Sn IMC during the interfacial reactions between liquid SAC composite solder and Cu substrates is diffusion-controlled. The temperature variations of D for the IMC layer growth can be expressed by the following Arrhenius equation in terms of the interdiffusion coefficient [18]:
D ¼ D0 exp ðQ =RTÞ
ð3Þ
where D0 is the interdiffusion constant, Q is activation energy for growth of the interfacial IMC layer, R is the gas constant, and T is the absolute temperature. The growth rate constants ðD ¼ x2 =tÞ, in turn, were then used to calculate the activation energies from Eq. (3) using linear regression analysis of a plot of ln(D) versus (1/T).
Fig. 6. Surface morphologies of Sn3.5Ag0.5Cu composite solder/Cu interface after soldering reactions: (a) 250 °C–30 min, (b) 250 °C–60 min, (c) 325 °C–10 min, and (d) 325 °C–40 min.
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Fig. 9. Arrhenius plot of the growth rate constants (D) of Cu–Sn IMC formed after the Sn3.5Ag0.5Cu composite solder/Cu interfacial reactions.
Table 1 Various diffusion constants in Sn and Cu.
Cu in Sn [1 0 0] Sn in Cu Fig. 7. Schematic diagrams showing the possible paths of the composite solder/Cu substrate interfacial reaction: (a) before soldering and (b) soldering process.
D0 (cm2/s)
Q (kJ/mode)
Range (°C)
Refs.
2.4 103 1.8 104 1.3 10+0 1.13 10+0
33 (S) 17.6 (L) 184 (L) 188 (S)
25 240–480 700–860 200
[19] [20] [21] [22]
S, solid; L, liquid.
of the IMC layers. However, adding to TiO2 nanoparticle was significant improvements in the refinement of Ag3Sn grains. Therefore, the TiO2 nanoparticle and submicro-Ag3Sn phase after soldering between Sn3.5Ag0.5Cu composite solder and Cu is thought to play the major role in raising the activation energy of the growth. So, the composite solder joints also were also effective in retarding the growth of the Cu6Sn5 IMC layer [24].
4. Conclusions
Fig. 8. Thickness (x2) of Cu–Sn IMC as a function if the reaction time (t) after soldering reactions between liquid Sn3.5Ag0.5Cu composite solder and Cu substrate at various temperatures.
An Arrhenius plot of the growth-rate constants versus reaction temperature is shown in Fig. 9. From the slope of the Arrhenius plot, the activation energy (Q) for the growth of overall Cu-Sn IMC (Cu6Sn5 + Cu3Sn) during the SAC composite solder/Cu soldering reactions is estimated to be 67.72 kJ/mol, which is higher than that of the diffusion control of solid Cu in Sn single crystals (33 kJ/mol) [19] and liquid Cu in Sn (17 kJ/mol) [20], but lower than that of liquid Sn in bulk Cu (184–188 kJ/mol) [21,22], as shows Table1. From these data it is obvious to conclude that the mobility of Cu in Sn is much faster than that of Sn in Cu. This implies that the growth of Cu-Sn intermetallic compounds during the SAC composite solder/ Cu soldering reactions was predominantly controlled by the diffusion of Cu into the Sn solders. Previous studies [16,17] indicate that the nanoparticles effect was used to explain the dramatic effect on the growth patterns
A discontinuous layer of scallop-shaped Cu6Sn5 intermetallic compounds appears at the Sn3.5Ag0.5Cu composite/Cu interface after the soldering reactions between liquid Sn3.5Ag0.5Cu composite solder and Cu substrate. Accompanying such a soldering reaction, intermixing of TiO2 nanopowder is found to refine the Ag3Sn phase and b-Sn, which results in the formation of a large number of dot-shaped submicro-Ag3Sn precipitates in the Sn3.5Ag0.5Cu matrix after solidification. The linear growth rate with the square root of reaction time indicates that the interfacial reactions are diffusion controlled. The activation energy, as calculated from the Arrhenius plot of reaction rate constants, is 67.72 kJ/mol, which is higher than that of the diffusion control of solid Cu in Sn single crystals (33 kJ/ mol) and liquid Cu in Sn (17 kJ/mol), but lower than that of liquid Sn in bulk Cu (184–188 kJ/mol). This result implies that the growth of Cu–Sn intermetallic compounds during the Sn3.5Ag0.5Cu composite/Cu soldering reaction is dominated by the diffusion of Cu into Sn solders. Acknowledgments The authors acknowledge the financial support of this work from the National Science Council of the Republic of China under Project No. NSC97-2218-E-020-004.
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