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In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction M.L. Huang n, F. Yang, N. Zhao, Z.J. Zhang Electronic Packaging Materials Laboratory, School of Materials Science & Engineering, Dalian University of Technology, Dalian 116024, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 August 2014 Accepted 8 October 2014
Synchrotron radiation real-time imaging technology was used for in situ study of dissolution and growth behavior of interfacial Cu6Sn5 intermetallic compound (IMC) in Sn/Cu solder interconnect during reflow soldering. The pre-formed Cu6Sn5 grains dissolved into the liquid solder with decreasing aspect ratio in the heating stage maintained a thin layer of scallop-type in the dwelling stage, and re-precipitated on the existing Cu6Sn5 grains at a faster growth rate with increasing aspect ratio in the cooling stage. The Cu concentration gradient at the interface is responsible for the aspect ratio variation (corresponding to dissolution and re-precipitation of interfacial Cu6Sn5 grains), which is also supported by the simulation of atomic diffusion in the solder based on Fick's second law. The growth behavior was well explained by a proposed model based on Cu concentration gradient. & 2014 Published by Elsevier B.V.
Keywords: Synchrotron radiation Crystal growth Intermetallic alloys and compounds Precipitation Dissolution Cu6Sn5
1. Introduction The formation of interfacial intermetallic compounds (IMCs) between solders and under bump metallizations (UBMs) is necessary to guarantee reliable interconnections [1]. However, due to the brittle nature of IMCs, thick IMC is detrimental to the reliability of solder joints. Nowadays, due to environmental concerns, Sn-rich lead-free solders with higher soldering temperature and higher Sn content have been widely applied to replace the traditional Sn-Pb solders. As a result, thicker interfacial IMCs form during lead-free soldering, which brings the reliability concerns. The growth mechanism of interfacial IMCs during lead-free soldering has attracted much research interest. Tu et al. [2,3] proposed a ripening theory to describe the growth of Cu6Sn5 grains during soldering, in which the increase of average grain size obeyed a t1/3 law when the liquid solder was saturated with Cu. Dybkov [4] proposed a model considering the growth of IMC with simultaneous dissolution, and Ma et al. [5] used this model to describe the growth of Cu6Sn5 layer at the Sn-Pb/Cu interface during reflow soldering and a good agreement with the experimental data was obtained when the solder served as a large reservoir. Moreover, the fast growth of interfacial Cu6Sn5 in cooling stage was recently investigated by centrifugally separating the liquid solder [6], or blowing off the molten solder [7]. The whole evolution process of interfacial Cu6Sn5 grains during one
n
Corresponding author. Tel./fax: þ 86 411 84706595. E-mail address: [email protected] (M.L. Huang).
reflow soldering is of interest to better understand the reaction mechanism. Synchrotron radiation X-ray real-time imaging technology was used to in situ investigate the evolution of interfacial Cu6Sn5 grains during reflow soldering in the present work, and the growth kinetics and mechanism of interfacial Cu6Sn5 were discussed.
2. Experimental Pure Cu plate (99.95 wt.%, 7 mm 7 mm 1 mm) and 200 g pure Sn (99.99 wt.%) bath were used to prepare the Sn/Cu interconnect. After dip soldering, the Sn/Cu interconnect was prereacted at 260 1C for 2 h to form a thick Cu6Sn5 layer for in situ observation on the dissolution and precipitation behavior of Cu6Sn5. Then the solder interconnect was grounded and polished to a final size of (1 mm Cu þ0.5 mm Sn) 7 mm 100 μm. The synchrotron radiation experiment was carried out at the BL13W1 beam line of Shanghai synchrotron radiation facility. The Sn/Cu interconnect, coated with a thin green oil film to avoid the flowing and oxidation of molten Sn, was placed in an oven and heated up to 280 1C at a heating rate of 15 1C/min. After dwelling at 280 1C for 226 s, the interconnect was cooled down at a cooling rate of 10 1C/min. X-ray beam with an energy of 18 keV transmitted through the solder interconnect was collected by a charged couple device (CCD) camera with a resolution of 0.74 μm/pixel and an exposure time of 3 s. After the experiment, the Sn/Cu interconnect was analyzed using a scanning electron microscope (SEM).
http://dx.doi.org/10.1016/j.matlet.2014.10.041 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Huang ML, et al. In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.041i
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M.L. Huang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Cu 6Sn 5
280 oC Cu 6Sn 5
0s
60 s
120 s
180 s
207 s
240 s
300 s
360 s
420 s
433 s
480 s
600 s
720 s
840 s
1140 s 20 μm
Fig. 1. Synchrotron radiation images of the Sn/Cu interconnect during the reflow soldering: (a)–(e) the heating stage ((a) at the melting point, (e) reached the dwelling temperature of 280 1C), (e)–(j) the dwelling stage ((j) the last image before cooling), and (j)–(o) the cooling stage ((o) after solidification).
Fig. 2. Cross-sectional SEM image of interfacial Cu6Sn5 grains.
The thicknesses of the interfacial IMC layers in the synchrotron radiation images were measured using image processing software.
3. Results and discussion Fig. 1(a) shows the synchrotron radiation images of the preformed elongated Cu6Sn5 grains with large aspect ratio (height/ width) at the Sn/Cu interface. As shown in Fig. 1(b)-(e), all the interfacial Cu6Sn5 grains gradually dissolved into the molten solder at a dissolution rate of 0.11 μm/s and transformed into scallop-type grains with small aspect ratio in the heating stage. As shown in Fig. 1(e)-(j), no obvious growth of the interfacial Cu6Sn5 grains was observed, indicating a relatively slow growth rate in the dwelling stage. The morphology of the Cu6Sn5 grains remained scallop-type. As shown in Fig. 1(j)-(p), the Cu6Sn5 grains grew fast and extended into the molten solder at a rate of 0.04 μm/s in the cooling stage. The morphology of Cu6Sn5 grains gradually transformed from scallop-type into elongated rod-like with large aspect ratio, which is similar to the morphology of the pre-formed Cu6Sn5 grains. The morphological evolutions of two Cu6Sn5 grains during the whole reflow soldering were clearly indicated by the two black arrows in Fig. 1. The top parts of the two elongated grains dissolved into the molten solder in the heating stage while the re-precipitation on the remaining two scallop Cu6Sn5 grains occurred in the cooling stage. However, the widths of the Cu6Sn5 grains seemed stable. Fig. 2 shows the cross-sectional SEM image of the Sn/Cu interconnect after synchrotron radiation experiment. Elongated Cu6Sn5 grains with large aspect ratio were observed at the interface, which is in good agreement with the synchrotron radiation observations. The dissolution and precipitation of interfacial Cu6Sn5 grains during reflow soldering are strongly influenced by the Cu concentration gradient normal to the Sn/Cu interface, which provides
the driving force for Cu diffusion flux. Fig. 3(a)-(d) shows the simulated Cu concentration distributions after reflow soldering for 60 s (heating stage), 420 s (dwelling stage), 600 s and 800 s (cooling stages), respectively, using the parameters of the present reflow condition. The initial Cu concentration in the molten solder was set as 0. The solubility of Cu in Sn-Cu solder varied from 0.94 wt.% at 230 1C to 1.67 wt.% at 280 1C, which is considered to follow a linear relationship with temperature [9]. The diffusivity D can be expressed as D¼ D0exp(-Q/RT), where D0 is 1.8 10-7 m2/s, Q is the diffusion activation energy that equals 17.58 kJ/mol [10], R is the gas constant and T is the temperature. The results show that there is a Cu concentration gradient form the molten solder toward the interface due to the continuously increasing Cu solution saturation limit in the heating stage, resulting in a Cu diffusion flux toward molten solder and consequently the dissolution of interfacial Cu6Sn5; there is no Cu concentration gradient in the dwelling stage (saturation of Cu), resulting in a relatively slow growth rate of interfacial Cu6Sn5; and there is a Cu concentration gradient from the interface toward the molten solder due to the continuously decreasing Cu solution saturation limit in the cooling stage, resulting in a Cu diffusion flux toward the interface and consequently the fast precipitation of Cu6Sn5 on the existing interfacial Cu6Sn5 grains. The growth rate of interfacial Cu6Sn5 IMC can be described by the following equation [8]: pffiffiffi 2 dl Dρsolder ½ðC b C e Þ=ð 3l =2δ þ lÞ dC=dx ¼ ð1Þ dt wρIMC where l is the average thickness of interfacial Cu6Sn5 layer, t is reflow time, w is the mass percent of Cu in Cu6Sn5, Ce is the equilibrium concentration of Cu (in wt.%) in solder at the planar interface between Cu6Sn5 and molten solder, Cb is the equilibrium concentration of Cu (in wt.%) in solder at the interface between substrate and solder at the bottom of grain boundary. ρIMC is the density of Cu6Sn5, ρsolder is the density of solder, δ is the width of Cu6Sn5 grain boundaries (width of liquid channels between Cu6Sn5 grains), and dC/dx is the Cu concentration gradient normal to the solder/Cu6Sn5 interface (positive with a direction from the molten solder toward the interface). Fig. 4(a) also shows the simulated dC/dx as a function of reflow time and the aspect ratio variations of the two Cu6Sn5 grains indicated by the black arrows in Fig. 1. The aspect ratio variations agree well with the Cu concentration gradient. The positive Cu concentration gradient in the heating stage caused the dissolution flux from the interface toward the molten solder, resulting in the decreasing IMC grain height and thus the decreasing aspect ratio; while the negative Cu concentration gradient in the cooling stage caused the precipitation flux from the molten solder toward the
Please cite this article as: Huang ML, et al. In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.041i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
M.L. Huang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 3. Simulated Cu concentration distribution at (a) 60 s, (b) 420 s, (c) 600 s and (d) 840 s (arrows indicate the Sn/Cu interfaces).
b
a -1
Average Cu C concentratio on gradient (m )
24
100 80
Grain 1
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20 0 point
0 Heating
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0 0
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200
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Fig. 4. (a) Simulated Cu concentration gradient at the interface and the aspect ratio of grains, (b) calculated and experimental Cu6Sn5 thicknesses as a function of reflow time.
interface, resulting in the increasing IMC grain height and thus the increasing aspect ratio. Assuming δ¼0.05 μm [3], Cb-Ce E 0.001 [3], ρCu ¼ 8.96 g/cm3, ρSolder ¼7.4 g/cm3, ρIMC ¼8.28 g/cm3, and w¼ 0.39, the average thickness of the Cu6Sn5 IMC layer was calculated by Eq. (1) using a fourth/fifth-order Ronge-Kutta-Fehlberg algorithm. The calculated results agree well with the experimental results, as shown in Fig. 4(b), indicating a reasonable understanding on the growth and dissolution mechanism of interfacial Cu6Sn5 IMC.
4. Conclusions The interfacial Cu6Sn5 grains dissolved into the liquid solder with decreasing aspect ratio in the heating stage, maintained a thin layer of scallop-type in the dwelling stage, and re-precipitated on the existing Cu6Sn5 grains with increasing aspect ratio in the cooling stage. The Cu concentration gradient at the interface is responsible for the aspect ratio variation of interfacial Cu6Sn5 grains. The increasing Cu solubility in the molten solder in the
Please cite this article as: Huang ML, et al. In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.041i
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M.L. Huang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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heating stage resulted in a Cu concentration gradient toward the interface and thus the dissolution of Cu6Sn5 IMC, while the decreasing solubility of Cu in the molten solder in the cooling stage resulted in a Cu concentration gradient toward the molten solder and thus the faster precipitation of Cu6Sn5 IMC. The proposed IMC growth model based on Cu diffusion flux, in fact induced by the concentration gradient, well explained the dissolution and precipitation mechanism of interfacial Cu6Sn5 IMC. Acknowledgments This work is supported by the National Natural Science Foundation of China under Grant Nos. 51475072 and 51171036 and the BL13W1 beam line of Shanghai Synchrotron Radiation Facility.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Abtew M, Selvaduray G. Mater Sci Eng R 2000;27:95–141. Kim HK, Tu KN. Phys Rev B 1996;53:16027–34. Gusak AM, Tu KN. Phys Rev B 2002;66(115403):1–14. Dybkov VI. Growth kinetics of chemical compound layers. Cambridge: Cambridge International Science Publishing; 1998. Ma D, Wang WD, Lahiri SK. J Appl Phys 2002;91:3312–7. Gong JC, Liu CQ, Conway PP, Silberschmidt VV. Acta Mater 2008;56:4291–7. Yang M, Cao Y, Joo SM, Chen HT, Ma X, Li MY. J Alloys Compd 2014;582:688–95. Huang ML, Yang F, Zhao N, Liu XY. ECS T 2013;52:753–8. Moon KW, Boettinger WJ, Kattner UR, Biancaniello FS, Handwerker CA. J Electron Mater 2000;29:1122–36. Shewmon PG. Transformations in metals. Tulsa: J. Williams Book Company; 1983.
Please cite this article as: Huang ML, et al. In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.041i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction M.L. Huang n, F. Yang, N. Zhao, Z.J. Zhang Electronic Packaging Materials Laboratory, School of Materials Science & Engineering, Dalian University of Technology, Dalian 116024, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 August 2014 Accepted 8 October 2014
Synchrotron radiation real-time imaging technology was used for in situ study of dissolution and growth behavior of interfacial Cu6Sn5 intermetallic compound (IMC) in Sn/Cu solder interconnect during reflow soldering. The pre-formed Cu6Sn5 grains dissolved into the liquid solder with decreasing aspect ratio in the heating stage maintained a thin layer of scallop-type in the dwelling stage, and re-precipitated on the existing Cu6Sn5 grains at a faster growth rate with increasing aspect ratio in the cooling stage. The Cu concentration gradient at the interface is responsible for the aspect ratio variation (corresponding to dissolution and re-precipitation of interfacial Cu6Sn5 grains), which is also supported by the simulation of atomic diffusion in the solder based on Fick's second law. The growth behavior was well explained by a proposed model based on Cu concentration gradient. & 2014 Published by Elsevier B.V.
Keywords: Synchrotron radiation Crystal growth Intermetallic alloys and compounds Precipitation Dissolution Cu6Sn5
1. Introduction The formation of interfacial intermetallic compounds (IMCs) between solders and under bump metallizations (UBMs) is necessary to guarantee reliable interconnections [1]. However, due to the brittle nature of IMCs, thick IMC is detrimental to the reliability of solder joints. Nowadays, due to environmental concerns, Sn-rich lead-free solders with higher soldering temperature and higher Sn content have been widely applied to replace the traditional Sn-Pb solders. As a result, thicker interfacial IMCs form during lead-free soldering, which brings the reliability concerns. The growth mechanism of interfacial IMCs during lead-free soldering has attracted much research interest. Tu et al. [2,3] proposed a ripening theory to describe the growth of Cu6Sn5 grains during soldering, in which the increase of average grain size obeyed a t1/3 law when the liquid solder was saturated with Cu. Dybkov [4] proposed a model considering the growth of IMC with simultaneous dissolution, and Ma et al. [5] used this model to describe the growth of Cu6Sn5 layer at the Sn-Pb/Cu interface during reflow soldering and a good agreement with the experimental data was obtained when the solder served as a large reservoir. Moreover, the fast growth of interfacial Cu6Sn5 in cooling stage was recently investigated by centrifugally separating the liquid solder [6], or blowing off the molten solder [7]. The whole evolution process of interfacial Cu6Sn5 grains during one
n
Corresponding author. Tel./fax: þ 86 411 84706595. E-mail address: [email protected] (M.L. Huang).
reflow soldering is of interest to better understand the reaction mechanism. Synchrotron radiation X-ray real-time imaging technology was used to in situ investigate the evolution of interfacial Cu6Sn5 grains during reflow soldering in the present work, and the growth kinetics and mechanism of interfacial Cu6Sn5 were discussed.
2. Experimental Pure Cu plate (99.95 wt.%, 7 mm 7 mm 1 mm) and 200 g pure Sn (99.99 wt.%) bath were used to prepare the Sn/Cu interconnect. After dip soldering, the Sn/Cu interconnect was prereacted at 260 1C for 2 h to form a thick Cu6Sn5 layer for in situ observation on the dissolution and precipitation behavior of Cu6Sn5. Then the solder interconnect was grounded and polished to a final size of (1 mm Cu þ0.5 mm Sn) 7 mm 100 μm. The synchrotron radiation experiment was carried out at the BL13W1 beam line of Shanghai synchrotron radiation facility. The Sn/Cu interconnect, coated with a thin green oil film to avoid the flowing and oxidation of molten Sn, was placed in an oven and heated up to 280 1C at a heating rate of 15 1C/min. After dwelling at 280 1C for 226 s, the interconnect was cooled down at a cooling rate of 10 1C/min. X-ray beam with an energy of 18 keV transmitted through the solder interconnect was collected by a charged couple device (CCD) camera with a resolution of 0.74 μm/pixel and an exposure time of 3 s. After the experiment, the Sn/Cu interconnect was analyzed using a scanning electron microscope (SEM).
http://dx.doi.org/10.1016/j.matlet.2014.10.041 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Huang ML, et al. In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.041i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
M.L. Huang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Cu 6Sn 5
280 oC Cu 6Sn 5
0s
60 s
120 s
180 s
207 s
240 s
300 s
360 s
420 s
433 s
480 s
600 s
720 s
840 s
1140 s 20 μm
Fig. 1. Synchrotron radiation images of the Sn/Cu interconnect during the reflow soldering: (a)–(e) the heating stage ((a) at the melting point, (e) reached the dwelling temperature of 280 1C), (e)–(j) the dwelling stage ((j) the last image before cooling), and (j)–(o) the cooling stage ((o) after solidification).
Fig. 2. Cross-sectional SEM image of interfacial Cu6Sn5 grains.
The thicknesses of the interfacial IMC layers in the synchrotron radiation images were measured using image processing software.
3. Results and discussion Fig. 1(a) shows the synchrotron radiation images of the preformed elongated Cu6Sn5 grains with large aspect ratio (height/ width) at the Sn/Cu interface. As shown in Fig. 1(b)-(e), all the interfacial Cu6Sn5 grains gradually dissolved into the molten solder at a dissolution rate of 0.11 μm/s and transformed into scallop-type grains with small aspect ratio in the heating stage. As shown in Fig. 1(e)-(j), no obvious growth of the interfacial Cu6Sn5 grains was observed, indicating a relatively slow growth rate in the dwelling stage. The morphology of the Cu6Sn5 grains remained scallop-type. As shown in Fig. 1(j)-(p), the Cu6Sn5 grains grew fast and extended into the molten solder at a rate of 0.04 μm/s in the cooling stage. The morphology of Cu6Sn5 grains gradually transformed from scallop-type into elongated rod-like with large aspect ratio, which is similar to the morphology of the pre-formed Cu6Sn5 grains. The morphological evolutions of two Cu6Sn5 grains during the whole reflow soldering were clearly indicated by the two black arrows in Fig. 1. The top parts of the two elongated grains dissolved into the molten solder in the heating stage while the re-precipitation on the remaining two scallop Cu6Sn5 grains occurred in the cooling stage. However, the widths of the Cu6Sn5 grains seemed stable. Fig. 2 shows the cross-sectional SEM image of the Sn/Cu interconnect after synchrotron radiation experiment. Elongated Cu6Sn5 grains with large aspect ratio were observed at the interface, which is in good agreement with the synchrotron radiation observations. The dissolution and precipitation of interfacial Cu6Sn5 grains during reflow soldering are strongly influenced by the Cu concentration gradient normal to the Sn/Cu interface, which provides
the driving force for Cu diffusion flux. Fig. 3(a)-(d) shows the simulated Cu concentration distributions after reflow soldering for 60 s (heating stage), 420 s (dwelling stage), 600 s and 800 s (cooling stages), respectively, using the parameters of the present reflow condition. The initial Cu concentration in the molten solder was set as 0. The solubility of Cu in Sn-Cu solder varied from 0.94 wt.% at 230 1C to 1.67 wt.% at 280 1C, which is considered to follow a linear relationship with temperature [9]. The diffusivity D can be expressed as D¼ D0exp(-Q/RT), where D0 is 1.8 10-7 m2/s, Q is the diffusion activation energy that equals 17.58 kJ/mol [10], R is the gas constant and T is the temperature. The results show that there is a Cu concentration gradient form the molten solder toward the interface due to the continuously increasing Cu solution saturation limit in the heating stage, resulting in a Cu diffusion flux toward molten solder and consequently the dissolution of interfacial Cu6Sn5; there is no Cu concentration gradient in the dwelling stage (saturation of Cu), resulting in a relatively slow growth rate of interfacial Cu6Sn5; and there is a Cu concentration gradient from the interface toward the molten solder due to the continuously decreasing Cu solution saturation limit in the cooling stage, resulting in a Cu diffusion flux toward the interface and consequently the fast precipitation of Cu6Sn5 on the existing interfacial Cu6Sn5 grains. The growth rate of interfacial Cu6Sn5 IMC can be described by the following equation [8]: pffiffiffi 2 dl Dρsolder ½ðC b C e Þ=ð 3l =2δ þ lÞ dC=dx ¼ ð1Þ dt wρIMC where l is the average thickness of interfacial Cu6Sn5 layer, t is reflow time, w is the mass percent of Cu in Cu6Sn5, Ce is the equilibrium concentration of Cu (in wt.%) in solder at the planar interface between Cu6Sn5 and molten solder, Cb is the equilibrium concentration of Cu (in wt.%) in solder at the interface between substrate and solder at the bottom of grain boundary. ρIMC is the density of Cu6Sn5, ρsolder is the density of solder, δ is the width of Cu6Sn5 grain boundaries (width of liquid channels between Cu6Sn5 grains), and dC/dx is the Cu concentration gradient normal to the solder/Cu6Sn5 interface (positive with a direction from the molten solder toward the interface). Fig. 4(a) also shows the simulated dC/dx as a function of reflow time and the aspect ratio variations of the two Cu6Sn5 grains indicated by the black arrows in Fig. 1. The aspect ratio variations agree well with the Cu concentration gradient. The positive Cu concentration gradient in the heating stage caused the dissolution flux from the interface toward the molten solder, resulting in the decreasing IMC grain height and thus the decreasing aspect ratio; while the negative Cu concentration gradient in the cooling stage caused the precipitation flux from the molten solder toward the
Please cite this article as: Huang ML, et al. In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.041i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
M.L. Huang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 3. Simulated Cu concentration distribution at (a) 60 s, (b) 420 s, (c) 600 s and (d) 840 s (arrows indicate the Sn/Cu interfaces).
b
a -1
Average Cu C concentratio on gradient (m )
24
100 80
Grain 1
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60 Grain 2 40 0.6
20 0 point
0 Heating
Holding
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
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Experimental results Calculated data
0 0
Reaction time (s)
200
400
600
800
1000
Reflow time (s)
Fig. 4. (a) Simulated Cu concentration gradient at the interface and the aspect ratio of grains, (b) calculated and experimental Cu6Sn5 thicknesses as a function of reflow time.
interface, resulting in the increasing IMC grain height and thus the increasing aspect ratio. Assuming δ¼0.05 μm [3], Cb-Ce E 0.001 [3], ρCu ¼ 8.96 g/cm3, ρSolder ¼7.4 g/cm3, ρIMC ¼8.28 g/cm3, and w¼ 0.39, the average thickness of the Cu6Sn5 IMC layer was calculated by Eq. (1) using a fourth/fifth-order Ronge-Kutta-Fehlberg algorithm. The calculated results agree well with the experimental results, as shown in Fig. 4(b), indicating a reasonable understanding on the growth and dissolution mechanism of interfacial Cu6Sn5 IMC.
4. Conclusions The interfacial Cu6Sn5 grains dissolved into the liquid solder with decreasing aspect ratio in the heating stage, maintained a thin layer of scallop-type in the dwelling stage, and re-precipitated on the existing Cu6Sn5 grains with increasing aspect ratio in the cooling stage. The Cu concentration gradient at the interface is responsible for the aspect ratio variation of interfacial Cu6Sn5 grains. The increasing Cu solubility in the molten solder in the
Please cite this article as: Huang ML, et al. In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.041i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
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heating stage resulted in a Cu concentration gradient toward the interface and thus the dissolution of Cu6Sn5 IMC, while the decreasing solubility of Cu in the molten solder in the cooling stage resulted in a Cu concentration gradient toward the molten solder and thus the faster precipitation of Cu6Sn5 IMC. The proposed IMC growth model based on Cu diffusion flux, in fact induced by the concentration gradient, well explained the dissolution and precipitation mechanism of interfacial Cu6Sn5 IMC. Acknowledgments This work is supported by the National Natural Science Foundation of China under Grant Nos. 51475072 and 51171036 and the BL13W1 beam line of Shanghai Synchrotron Radiation Facility.
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Please cite this article as: Huang ML, et al. In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.041i
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