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Rapid pressureless low-temperature sintering of Ag nanoparticles for high-power density electronic packaging
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ScienceDirect Scripta Materialia 69 (2013) 789–792 www.elsevier.com/locate/scriptamat
Rapid pressureless low-temperature sintering of Ag nanoparticles for high-power density electronic packaging Shuai Wang,a Mingyu Li,a,b,⇑ Hongjun Jia and Chunqing Wangb a
Shenzhen Key Laboratory of Advanced Materials, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, People’s Republic of China b State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Received 29 July 2013; revised 30 August 2013; accepted 30 August 2013 Available online 5 September 2013
This paper describes a method to achieve rapid pressureless low-temperature sintering of Ag nanoparticles for bonding. Organic shells adsorbing on the surface of Ag nanoparticles to stabilize them were thinned to create a sparse protecting layer. The numerous coherent twin boundaries formed in sintered Ag nanoparticles with a grain size of 21 nm induce ultrahigh thermal conductivity (229 W m1 K1), which overcomes the intrinsic defect that metals with nanosized grains generally exhibit a significantly reduced thermal conductivity because of the grain boundary scattering effect. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ag nanoparticle; Sintering; Bonding; Thermal conductivity; Twinning
Ag nanoparticle paste is a promising bonding material in electronic devices such as ultrafast computer chips and high-power light-emitting diodes [1]. Sintered joints formed by Ag nanoparticle paste possess a much wider working temperature range than solders because Ag nanoparticles can be sintered easily to form a sintered metal with bulk melting temperature [2,3]. Moreover, of special interest is the advantage offered by these nanoparticles in heat dissipation for high-powerdensity packaging systems, which is crucial for device reliability and lifespan. Recently, various studies on the sintering of Ag nanoparticle paste for bonding have been reported [1–9]. However, the application of Ag nanoparticle paste is still restricted by the long sintering times required, 20–30 min [1,3–7], and the high sintering temperatures, normally >250 °C [1–6]. These two sintering parameters are unsuitable for polymeric substrates and electronic components based on organic groups. Additionally, the sintering pressure applied to form joints [1–6,8,9] also hinders the application of these nanoparti-
⇑ Corresponding author at: Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Advanced Materials, Shenzhen 518055, China. Tel./fax: +86 755 26033463; e-mail: [email protected]
cles in terms of production automation and the integrity of wafers. These adverse effects may be caused by organic shells adsorbing on the surface of Ag nanoparticles which disperse and stabilize them. The removal of organic shells is a prerequisite for sintering to occur, thus requiring a high temperature [3,6,10,11]. In fact, according to the analysis presented herein, the thermal decomposition of organic shells occurs along with the sintering. Residual organics participate in material transport during the sintering process. The morphology of sintered nanoparticles and the sintering mechanism will be changed by the influence of organic shells. Therefore, the sintering of Ag nanoparticles should not be discussed without considering the effect of organic shells, especially for rapid low-temperature sintering, because of their incomplete decomposition. In the present work, we investigate how the organic shells influence the sintered morphology and sintering mechanism of Ag nanoparticles. High-density twins and numerous coherent twin boundaries are formed in sintered Ag nanoparticles, inducing ultrahigh thermal conductivity when the organic shells are thinned. Pressureless bonding process is achieved at 150–200 °C in a very short period with high shear strength. Ag nanoparticles were synthesized according to a modification of Carey Lea’s method [12]. The average
1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.08.031
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S. Wang et al. / Scripta Materialia 69 (2013) 789–792
diameter was about 13 nm according to transmission electron microscopy (TEM, Tecnai G2 F20, FEI) measurements. The organic shells comprised of dense layers of citrates adsorbing on the surface of Ag nanoparticles. To thin the organic shells, Ag nanoparticles were redispersed in water and subsequently precipitated with NaNO3 solution (1 M). Ag nanoparticle paste was obtained by condensing Ag nanoparticle solution with a centrifuge at 3500 rpm for 5 min. The substrates for bonding were Ag-plated Cu sheets with dimensions of 1.5 1.5 0.5 mm3. The prepared Cu–Cu joint specimens with Ag nanoparticle paste were heated by a hot-air reflow system (PACE ST325). The surface states of the sintered Ag nanoparticles were detected by Raman spectrometry (inVia, Renishaw). The porosity and density of the sintered Ag samples were measured by the Archimedes method. Thermal conductivity (k) data was obtained from the equation, k = aqc, where a is thermal diffusivity measured by a laser flash apparatus (LFA477, Netzsch), q is density, and c is the heat capacity obtained by differential scanning calorimetry (STA449, Netzsch) using a sapphire sample as a reference. The shear strength of the joint was measured by a bond tester (4000PXY, Dage) with a shear speed of 100 lm s1. The grain size was determined via X-ray diffraction using Scherrer’s formula (X’ Pert, Philips). Fig. 1 shows Raman spectra collected from sintered Ag nanoparticle paste. Citrates have decomposed during the sintering process. The decomposition product of citrate, acetone dicarboxylic acid, remains in the sintered Ag sample because the sintering temperature is below the final decomposition temperature of citrate (230 °C). For thick organic shells, the intensities of characteristic peaks which correspond to functional groups dramatically decrease as the sintering temperature increases but the relative intensities of the characteristic peaks at 1542, 1345 and 688 cm1, corresponding to acetone dicarboxylic acid, and 1055 cm1, corresponding to citrate [13], are still high when the sample is sintered at 200 °C. For thin organic shells, Raman curves show very weak characteristic peaks, and the relative intensities of these characteristic peaks are much lower than those of thick organic shells. Therefore, the thinning process effectively reduces the amount of organic shells. The weight loss of dried Ag nanoparticle paste that occurs during the thermal process is attributed to the vaporization and decomposition of the organic content. Therefore, the amount of organic content can be deter-
Figure 1. Raman spectroscopy of Ag nanoparticles sintered for 20 min at 150 and 200 °C.
mined by thermogravimetric analysis (STA499, Netzsch). The weight ratios of organic content to metallic Ag for thin and thick organic shells were about 2.5:97.5 and 7.6:92.4, respectively, as shown in Supplementary Fig. S1. These organics have a strong impact on sintered morphology because they hinder the contact of Ag nanoparticles. For thick organic shells, Ag nanoparticles have fewer contact chances. For one Ag nanoparticle, there may be only one or two parts of the surface of the Ag nanoparticle contacting with another. In this case, the sintered morphology formed is chain-like, as sketched in Fig. 2a, i.e. as the sintering proceeds, the approximately spherical nanoparticles (Fig. 2g) are elongated. When there are sufficient Ag nanoparticles, the sintered morphology is pinecone-like, i.e. is composed of many chains of sintered Ag nanoparticles. These morphologies are shown in Fig. 2c,e. The average width of chains is about 9.5 nm, which is slightly less than the average Ag nanoparticle diameter of 13 nm because the sintered Ag nanoparticles are elongated during the formation of chains. Organic residues remain between these chains. After the thinning process, fewer organics hinder the sintering process, so the sintered morphology is formed to be net-like as sketched in Fig. 2b. According to the TEM image shown in Fig. 2d, the recrystallization mechanism causes the original approximately spherical Ag nanoparticles to disappear, and the net-like sintered morphology is shown in Fig. 2f. From the above analysis, organic shells significantly affect the sintering process and sintered morphology, and consequently the properties of sintered Ag metal will dramatically change. The thermal conductivity and porosity of sintered Ag nanoparticle paste are shown in Fig. 3a. When the paste
Figure 2. Schematic diagrams of the sintering process of Ag nanoparticles with (a) thick organic shells and (b) thin organic shells. TEM images of Ag nanoparticles sintered at 200 °C for 20 min with (c) thick organic shells and (d) thin organic shells. (e) and (f) are the lowmagnification TEM images of (c) and (d), respectively. (g) TEM image of initial Ag nanoparticles.
S. Wang et al. / Scripta Materialia 69 (2013) 789–792
was sintered at 200 °C for 20 min, after the thinning process, the thermal conductivity shows a remarkable increase of 3 times compared to that of thick organic shells (74 W m1 K1) in previous work [7], and is as high as 229 W m1 K1 with a porosity of 27%. This value is higher than half the thermal conductivity of bulk Ag (410 W m1 K1) and 4.5 times higher than that of Sn–Pb solders (51 W m1 K1). Details of this phenomenon are described below. Fig. 3b shows the shear strength of sintered joints as a function of sintering time. Joints formed in 30 s at 200 °C with a shear strength of 24 MPa, which is similar to that of Sn–Pb solders (19– 24 MPa) [3]. Extending the sintering time to 120 s gave a shear strength of 34 MPa, which is higher than that in previous works in which the material was sintered at 300 °C for 30 min [3–6,8]. When the sintering time increases to 20 min, the shear strength reaches 60 MPa, which is the highest value so far reported for bonding by sintering Ag nanoparticles [2–8]. However, the joint strength is also significantly affected by the size of the bonding area using Ag nanoparticle paste. For the smaller bonding area, the pathways for vaporization and decomposition of the organics are short, which results in a fast and efficient sintering process. On the other hand, it is difficult to decompose organics from large bonding areas because of the lack of contact with air, which results in insufficient sintering and a lower bonding strength. Moreover, the shrinkage of the paste due to vaporization from large bonding areas is more obvious than that from small bonding areas. Therefore, the size effect should be taken into consideration when the joint strength is evaluated. Moreover, the size of the specimens used in this work is much smaller than those in previous works [2–4,6], but still larger than most chips
Figure 3. (a) Thermal conductivity and porosity of Ag nanoparticles with thin organic shells sintered for 20 min as a function of sintering temperature. (b) Shear strength of joints as a function of sintering time using the Ag nanoparticles with thin organic shells sintered at 150 and 200 °C. Inset: Scanning electron microscopy image of sintered Cu–Cu joint.
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used in high-power light-emitting diodes. In fact, the growth of sintering necks and coalescence are both very fast [8]. For Ag nanoparticles with thick organic shells, after sintered chains are formed, no more Ag nanoparticles participate in the three-dimensional sintering process due to the hindering effect of the organics, which can be proved by the chain-like morphology in Fig. 2c,e. Therefore, the bonding strength will not increase significantly with the extended sintering times. However, for Ag nanoparticles with thin organic shells, during the formation of sintering necks and the subsequent heat process, there are more Ag nanoparticles participating in the three-dimensional sintering process to form net-like sintered morphology because of the weaker hindering effect from thin organic shells as shown in Fig. 2d,f. Therefore, the bonding strength dramatically improves as the sintering time increases. It is worth noting that the considerably high shear strength, thermal conductivity and short sintering time are obtained without applying pressure. Considering the porous sintered Ag nanoparticles as a composite material comprised of Ag matrix with voids, the thermal conductivity is expressed as [14]: k ¼ k 0 ð1 nÞ
3=2
;
ð1Þ
where k0 is the thermal conductivity of solid material, k is the conductivity of the system and n is the porosity. The calculated ideal thermal conductivity of Ag nanoparticles is 256 W m1 K1 using the porosity value of 27% and sintering at 200 °C (Fig. 3a). It is amazing that the calculated value is very close to the corresponding experimental value of 229 W m1 K1. Commonly, thermal conductivity will be dramatically degraded, even to two orders of magnitude lower than that of corresponding bulk material, when the grain size is reduced to the nanoscale because of the grain boundary scattering effect [15–20]. Fig. 4a shows the grain sizes of Ag nanoparticles sintered at different temperatures. When sintered at 200 °C, the grain size is 21 nm. The thermal conductivity can be written as k = nkbulkCm/3NA, where n is particles per unit volume, is mean particle speed, kbulk is bulk mean free length, Cm is molar heat capacity and NA is Avogrado’s number [15]. Because the grain size of sintered Ag nanoparticles is much smaller than the bulk mean free length of Ag (51 nm) [21], the mean free length in sintered Ag nanoparticles (ksintered) can be assumed to be equal to the grain size and as a first approximation, substituting the values in ksintered/kbulk = ksintered/kbulk, where kbulk is the calculated value of 256 W m1 K1, ksintered is 21 nm, kbulk is 51 nm, gives the value of ksintered as 105 W m1 K1, which is much lower than the corresponding experimental value of 229 W m1 K1. This abnormal result originates from the high density twins formed in the sintered microstructure as shown in Figs. 2d,f and 4b. The scattering effect of a coherent twin boundary is about one order of magnitude lower than that of a conventional high-angle grain boundary [22]. Therefore, the coherent twin boundaries possess an extremely low thermal resistivity. The high density of twins can be obtained in the metals with low stacking fault energy, which is 29 mJ cm2 in Ag [23]. From a thermodynamic point
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.scriptamat.2013.08.031.
Figure 4. (a) Grain size of sintered Ag nanoparticles with thin organic shells sintered for 20 min as a function of sintering temperature. (b) TEM image of twins formed in Ag nanoparticles with thin organic shells sintered at 200 °C for 20 min. Inset: High-resolution TEM image of twins.
of view, the formation of twins decreases the total interfacial energy, because the excess energy of coherent twin boundaries is much smaller than that for conventional high-angle grain boundaries [22]. Therefore, it is reasonable that sintered Ag nanoparticles with numerous coherent twin boundaries possess ultrahigh thermal conductivity even though the grain size is small. In conclusion, rapid pressureless low-temperature sintering of Ag nanoparticle paste for bonding is achieved. By thinning the organic shells, the sintering time is dramatically shortened, and the sintered morphology changes from pinecone-like to net-like. High-density twins are formed in the net-like sintered microstructure and numerous twin boundaries effectively reduce the grain boundary scattering effect, thus inducing ultrahigh thermal conductivity. This work is supported by Shenzhen Science and Technology Plan Project under Grant No. XCL201110007.
[1] H. Yu, L.L. Li, Y.J. Zhang, Scr. Mater. 66 (2012) 931. [2] E. Ide, S. Angatas, A. Hirose, K.F. Kobayashi, Acta Mater. 53 (2005) 2385. [3] M. Maruyama, R. Matsubayashi, H. Iwakuro, S. Isoda, T. Komatsu, Appl. Phys. A 93 (2008) 467. [4] H. Ogura, M. Maruyama, R. Matsubayashi, T. Ogawa, S. Nakamura, T. Komatsu, H. Nagasawa, A. Ichimura, S. Isoda, J. Electron. Mater. 39 (2010) 1233. [5] H. Alarifi, A. Hu, M. Yavuz, Y.N. Zhou, J. Electron. Mater. 40 (2011) 1394. [6] J.F. Yan, G.S. Zou, A.P. Wu, J.L. Ren, J.C. Yan, A.M. Hu, Y. Zhou, Scr. Mater. 66 (2012) 582. [7] S. Wang, H.J. Ji, M.Y. Li, C.Q. Wang, Mater. Lett. 85 (2012) 61. [8] A. Hu, J.Y. Guo, H. Alarifi, G. Patane, Y. Zhou, G. Compagnini, C.X. Xu, Appl. Phys. Lett. 97 (2010) 153117. [9] S.J. Kim, E.A. Stach, C.A. Handwerker, Appl. Phys. Lett. 96 (2010) 144101. [10] R.W. Zhang, K.S. Moon, W. Lin, C.P. Wong, J. Mater. Chem. 20 (2010) 2018. [11] B.T. Anto, S. Sivaramakrishnan, L.L. Chua, P.K.H. Ho, Adv. Funct. Mater. 20 (2010) 296. [12] G. Frens, J.T.G. Overbeek, Colloid Polym. Sci. 233 (1969) 922. [13] C.H. Munro, W.E. Smith, M. Garner, J. Clarkson, P.C. White, Langmuir 11 (1995) 3712. [14] M.I. Aivazov, I.A. Domashnev, Powder Metall. Met. Ceram. 7 (1968) 708. [15] T.G. Desai, Appl. Phys. Lett. 98 (2011) 193107. [16] G. Soyez, J.A. Eastman, L.J. Thompson, G.R. Bai, P.M. Baldo, A.W. Mccormick, Appl. Phys. Lett. 77 (2000) 1155. [17] X.Y. Qin, B.M. Wu, Y.L. Du, L.D. Zhang, H.X. Tang, Nanostruct. Mater. 7 (1996) 383. [18] A. Zhang, H.Q. Xie, M. Fujii, H. Ago, Appl. Phys. Lett. 86 (2005) 171912. [19] A.K. Collins, M.A. Pickering, R.L. Taylor, J. Appl. Phys. 68 (1990) 6510. [20] M.F. Ashby, P.J. Ferreira, D.L. Schodek, Nanomaterials, Nanotechnologies and Design, Elsevier, Oxford, 2009, p. 214. [21] X.Y. Qin, W. Zhang, L.D. Zhang, L.D. Jiang, Phys. Rev. B 56 (1997) 10596. [22] L. Lu, Y.F. Shen, X.H. Chen, L.H. Qian, K. Lu, Science 304 (2004) 422. ¨ m, B.I. Lundqvist, [23] J. Hartford, B.V. Sydow, G. WahnstrO Phys. Rev. B 58 (1998) 2487.
ScienceDirect Scripta Materialia 69 (2013) 789–792 www.elsevier.com/locate/scriptamat
Rapid pressureless low-temperature sintering of Ag nanoparticles for high-power density electronic packaging Shuai Wang,a Mingyu Li,a,b,⇑ Hongjun Jia and Chunqing Wangb a
Shenzhen Key Laboratory of Advanced Materials, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, People’s Republic of China b State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Received 29 July 2013; revised 30 August 2013; accepted 30 August 2013 Available online 5 September 2013
This paper describes a method to achieve rapid pressureless low-temperature sintering of Ag nanoparticles for bonding. Organic shells adsorbing on the surface of Ag nanoparticles to stabilize them were thinned to create a sparse protecting layer. The numerous coherent twin boundaries formed in sintered Ag nanoparticles with a grain size of 21 nm induce ultrahigh thermal conductivity (229 W m1 K1), which overcomes the intrinsic defect that metals with nanosized grains generally exhibit a significantly reduced thermal conductivity because of the grain boundary scattering effect. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ag nanoparticle; Sintering; Bonding; Thermal conductivity; Twinning
Ag nanoparticle paste is a promising bonding material in electronic devices such as ultrafast computer chips and high-power light-emitting diodes [1]. Sintered joints formed by Ag nanoparticle paste possess a much wider working temperature range than solders because Ag nanoparticles can be sintered easily to form a sintered metal with bulk melting temperature [2,3]. Moreover, of special interest is the advantage offered by these nanoparticles in heat dissipation for high-powerdensity packaging systems, which is crucial for device reliability and lifespan. Recently, various studies on the sintering of Ag nanoparticle paste for bonding have been reported [1–9]. However, the application of Ag nanoparticle paste is still restricted by the long sintering times required, 20–30 min [1,3–7], and the high sintering temperatures, normally >250 °C [1–6]. These two sintering parameters are unsuitable for polymeric substrates and electronic components based on organic groups. Additionally, the sintering pressure applied to form joints [1–6,8,9] also hinders the application of these nanoparti-
⇑ Corresponding author at: Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Advanced Materials, Shenzhen 518055, China. Tel./fax: +86 755 26033463; e-mail: [email protected]
cles in terms of production automation and the integrity of wafers. These adverse effects may be caused by organic shells adsorbing on the surface of Ag nanoparticles which disperse and stabilize them. The removal of organic shells is a prerequisite for sintering to occur, thus requiring a high temperature [3,6,10,11]. In fact, according to the analysis presented herein, the thermal decomposition of organic shells occurs along with the sintering. Residual organics participate in material transport during the sintering process. The morphology of sintered nanoparticles and the sintering mechanism will be changed by the influence of organic shells. Therefore, the sintering of Ag nanoparticles should not be discussed without considering the effect of organic shells, especially for rapid low-temperature sintering, because of their incomplete decomposition. In the present work, we investigate how the organic shells influence the sintered morphology and sintering mechanism of Ag nanoparticles. High-density twins and numerous coherent twin boundaries are formed in sintered Ag nanoparticles, inducing ultrahigh thermal conductivity when the organic shells are thinned. Pressureless bonding process is achieved at 150–200 °C in a very short period with high shear strength. Ag nanoparticles were synthesized according to a modification of Carey Lea’s method [12]. The average
1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.08.031
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S. Wang et al. / Scripta Materialia 69 (2013) 789–792
diameter was about 13 nm according to transmission electron microscopy (TEM, Tecnai G2 F20, FEI) measurements. The organic shells comprised of dense layers of citrates adsorbing on the surface of Ag nanoparticles. To thin the organic shells, Ag nanoparticles were redispersed in water and subsequently precipitated with NaNO3 solution (1 M). Ag nanoparticle paste was obtained by condensing Ag nanoparticle solution with a centrifuge at 3500 rpm for 5 min. The substrates for bonding were Ag-plated Cu sheets with dimensions of 1.5 1.5 0.5 mm3. The prepared Cu–Cu joint specimens with Ag nanoparticle paste were heated by a hot-air reflow system (PACE ST325). The surface states of the sintered Ag nanoparticles were detected by Raman spectrometry (inVia, Renishaw). The porosity and density of the sintered Ag samples were measured by the Archimedes method. Thermal conductivity (k) data was obtained from the equation, k = aqc, where a is thermal diffusivity measured by a laser flash apparatus (LFA477, Netzsch), q is density, and c is the heat capacity obtained by differential scanning calorimetry (STA449, Netzsch) using a sapphire sample as a reference. The shear strength of the joint was measured by a bond tester (4000PXY, Dage) with a shear speed of 100 lm s1. The grain size was determined via X-ray diffraction using Scherrer’s formula (X’ Pert, Philips). Fig. 1 shows Raman spectra collected from sintered Ag nanoparticle paste. Citrates have decomposed during the sintering process. The decomposition product of citrate, acetone dicarboxylic acid, remains in the sintered Ag sample because the sintering temperature is below the final decomposition temperature of citrate (230 °C). For thick organic shells, the intensities of characteristic peaks which correspond to functional groups dramatically decrease as the sintering temperature increases but the relative intensities of the characteristic peaks at 1542, 1345 and 688 cm1, corresponding to acetone dicarboxylic acid, and 1055 cm1, corresponding to citrate [13], are still high when the sample is sintered at 200 °C. For thin organic shells, Raman curves show very weak characteristic peaks, and the relative intensities of these characteristic peaks are much lower than those of thick organic shells. Therefore, the thinning process effectively reduces the amount of organic shells. The weight loss of dried Ag nanoparticle paste that occurs during the thermal process is attributed to the vaporization and decomposition of the organic content. Therefore, the amount of organic content can be deter-
Figure 1. Raman spectroscopy of Ag nanoparticles sintered for 20 min at 150 and 200 °C.
mined by thermogravimetric analysis (STA499, Netzsch). The weight ratios of organic content to metallic Ag for thin and thick organic shells were about 2.5:97.5 and 7.6:92.4, respectively, as shown in Supplementary Fig. S1. These organics have a strong impact on sintered morphology because they hinder the contact of Ag nanoparticles. For thick organic shells, Ag nanoparticles have fewer contact chances. For one Ag nanoparticle, there may be only one or two parts of the surface of the Ag nanoparticle contacting with another. In this case, the sintered morphology formed is chain-like, as sketched in Fig. 2a, i.e. as the sintering proceeds, the approximately spherical nanoparticles (Fig. 2g) are elongated. When there are sufficient Ag nanoparticles, the sintered morphology is pinecone-like, i.e. is composed of many chains of sintered Ag nanoparticles. These morphologies are shown in Fig. 2c,e. The average width of chains is about 9.5 nm, which is slightly less than the average Ag nanoparticle diameter of 13 nm because the sintered Ag nanoparticles are elongated during the formation of chains. Organic residues remain between these chains. After the thinning process, fewer organics hinder the sintering process, so the sintered morphology is formed to be net-like as sketched in Fig. 2b. According to the TEM image shown in Fig. 2d, the recrystallization mechanism causes the original approximately spherical Ag nanoparticles to disappear, and the net-like sintered morphology is shown in Fig. 2f. From the above analysis, organic shells significantly affect the sintering process and sintered morphology, and consequently the properties of sintered Ag metal will dramatically change. The thermal conductivity and porosity of sintered Ag nanoparticle paste are shown in Fig. 3a. When the paste
Figure 2. Schematic diagrams of the sintering process of Ag nanoparticles with (a) thick organic shells and (b) thin organic shells. TEM images of Ag nanoparticles sintered at 200 °C for 20 min with (c) thick organic shells and (d) thin organic shells. (e) and (f) are the lowmagnification TEM images of (c) and (d), respectively. (g) TEM image of initial Ag nanoparticles.
S. Wang et al. / Scripta Materialia 69 (2013) 789–792
was sintered at 200 °C for 20 min, after the thinning process, the thermal conductivity shows a remarkable increase of 3 times compared to that of thick organic shells (74 W m1 K1) in previous work [7], and is as high as 229 W m1 K1 with a porosity of 27%. This value is higher than half the thermal conductivity of bulk Ag (410 W m1 K1) and 4.5 times higher than that of Sn–Pb solders (51 W m1 K1). Details of this phenomenon are described below. Fig. 3b shows the shear strength of sintered joints as a function of sintering time. Joints formed in 30 s at 200 °C with a shear strength of 24 MPa, which is similar to that of Sn–Pb solders (19– 24 MPa) [3]. Extending the sintering time to 120 s gave a shear strength of 34 MPa, which is higher than that in previous works in which the material was sintered at 300 °C for 30 min [3–6,8]. When the sintering time increases to 20 min, the shear strength reaches 60 MPa, which is the highest value so far reported for bonding by sintering Ag nanoparticles [2–8]. However, the joint strength is also significantly affected by the size of the bonding area using Ag nanoparticle paste. For the smaller bonding area, the pathways for vaporization and decomposition of the organics are short, which results in a fast and efficient sintering process. On the other hand, it is difficult to decompose organics from large bonding areas because of the lack of contact with air, which results in insufficient sintering and a lower bonding strength. Moreover, the shrinkage of the paste due to vaporization from large bonding areas is more obvious than that from small bonding areas. Therefore, the size effect should be taken into consideration when the joint strength is evaluated. Moreover, the size of the specimens used in this work is much smaller than those in previous works [2–4,6], but still larger than most chips
Figure 3. (a) Thermal conductivity and porosity of Ag nanoparticles with thin organic shells sintered for 20 min as a function of sintering temperature. (b) Shear strength of joints as a function of sintering time using the Ag nanoparticles with thin organic shells sintered at 150 and 200 °C. Inset: Scanning electron microscopy image of sintered Cu–Cu joint.
791
used in high-power light-emitting diodes. In fact, the growth of sintering necks and coalescence are both very fast [8]. For Ag nanoparticles with thick organic shells, after sintered chains are formed, no more Ag nanoparticles participate in the three-dimensional sintering process due to the hindering effect of the organics, which can be proved by the chain-like morphology in Fig. 2c,e. Therefore, the bonding strength will not increase significantly with the extended sintering times. However, for Ag nanoparticles with thin organic shells, during the formation of sintering necks and the subsequent heat process, there are more Ag nanoparticles participating in the three-dimensional sintering process to form net-like sintered morphology because of the weaker hindering effect from thin organic shells as shown in Fig. 2d,f. Therefore, the bonding strength dramatically improves as the sintering time increases. It is worth noting that the considerably high shear strength, thermal conductivity and short sintering time are obtained without applying pressure. Considering the porous sintered Ag nanoparticles as a composite material comprised of Ag matrix with voids, the thermal conductivity is expressed as [14]: k ¼ k 0 ð1 nÞ
3=2
;
ð1Þ
where k0 is the thermal conductivity of solid material, k is the conductivity of the system and n is the porosity. The calculated ideal thermal conductivity of Ag nanoparticles is 256 W m1 K1 using the porosity value of 27% and sintering at 200 °C (Fig. 3a). It is amazing that the calculated value is very close to the corresponding experimental value of 229 W m1 K1. Commonly, thermal conductivity will be dramatically degraded, even to two orders of magnitude lower than that of corresponding bulk material, when the grain size is reduced to the nanoscale because of the grain boundary scattering effect [15–20]. Fig. 4a shows the grain sizes of Ag nanoparticles sintered at different temperatures. When sintered at 200 °C, the grain size is 21 nm. The thermal conductivity can be written as k = n
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S. Wang et al. / Scripta Materialia 69 (2013) 789–792
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.scriptamat.2013.08.031.
Figure 4. (a) Grain size of sintered Ag nanoparticles with thin organic shells sintered for 20 min as a function of sintering temperature. (b) TEM image of twins formed in Ag nanoparticles with thin organic shells sintered at 200 °C for 20 min. Inset: High-resolution TEM image of twins.
of view, the formation of twins decreases the total interfacial energy, because the excess energy of coherent twin boundaries is much smaller than that for conventional high-angle grain boundaries [22]. Therefore, it is reasonable that sintered Ag nanoparticles with numerous coherent twin boundaries possess ultrahigh thermal conductivity even though the grain size is small. In conclusion, rapid pressureless low-temperature sintering of Ag nanoparticle paste for bonding is achieved. By thinning the organic shells, the sintering time is dramatically shortened, and the sintered morphology changes from pinecone-like to net-like. High-density twins are formed in the net-like sintered microstructure and numerous twin boundaries effectively reduce the grain boundary scattering effect, thus inducing ultrahigh thermal conductivity. This work is supported by Shenzhen Science and Technology Plan Project under Grant No. XCL201110007.
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