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Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate
MR-11822; No of Pages 8 Microelectronics Reliability xxx (2015) xxx–xxx
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Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate Su-Yan Zhao a, Xin Li a,⁎, Yun-Hui Mei a, Guo-Quan Lu a,b a b
School of Materials Science and Engineering, Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, Tianjin, PR China Department of Materials Science and Engineering, Virginia Tech, Blacksburg, VA, USA
a r t i c l e
i n f o
Article history: Received 24 May 2015 Received in revised form 11 October 2015 Accepted 15 October 2015 Available online xxxx Keywords: Nano-silver paste Bare copper plate Interface reaction Bonding strength Thermal aging
a b s t r a c t Nano-silver paste has become an alternative lead-free (Pb-free) die attach material for microelectronic packaging, compared to traditional solders and adhesive films, due to its higher thermal and electrical conductivity, higher temperature operation and higher heat dissipation. In this study, the reliable joints of sintered nanosilver paste on bare copper plate with large-area dummy chips were introduced and thermally aged at 150 °C, 180 °C, or even higher than 250 °C in air or in a coarse vacuum environment. The bonding strength and interfacial reaction were investigated. The results showed, after aging specimens at 150 °C for 960 h in air or at 250 °C for 960 h in a coarse vacuum, the interfaces between the sintered nano-silver and bare copper still consisted of simple inter-diffusion bands and with almost no change in bonding strength. However, the bonding strength sharply decreased to 50% after aging at 180 °C for 72 h in air and it decreased to 20% after aging at 250 °C for 72 h in air. The decrease in bonding strength was mainly attributed to the oxidation of copper at relative elevated temperature in this work. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction In electronic packaging, the die-attach materials play a very important role in providing the packaging interconnection, physical protection, and mechanical support to ensure that the entire system works functionally [1–3]. Nowadays, both solder alloys (leaded or lead-free) and conductive adhesives are commonly used as die-attach materials for use at operating temperatures below 220 °C [4]. However, these solder alloys and conductive adhesives could not meet the stringent requirements for higher temperature applications due to their low melting and operating temperatures, such as in the automotive, aviation, space, and nuclear industries. Nano-silver paste has been reported as a promising die-attach material due to its high melting temperature (960 °C), low sintering temperature (275 °C), high electrical/thermal conductivity (4.1 × 107 S·m− 1 and 240 W·m− 1·K− 1, respectively), and high reliability [5]. Simultaneously, the properties of nano-silver joints have been extensively studied [2,6,7]. Mei et al. [6] studied the effect of sintering condition on the bonding quality of sintered nano-silver on Cu metallization under different temperatures and holding times, showing that the bonding strength mostly increased with increasing applied pressure, pressing temperature, pressing time, sintering temperature, and sintering time. Wang et al. [2] analyzed the bonding strength of sintered nano-silver joints and the microstructure evolution of the ⁎ Corresponding author. E-mail address: [email protected] (X. Li).
nano-silver paste during sintering. They found that the excellent bonding strength was in accordance with the densification behavior of the nanoparticles. In addition, the adhesion strength of the Ag nanoparticles on the bonding substrate surface is important for this bonding strength [7]. Copper as the material of die-attach substrate is commonly metalized by electroplating, physical or chemical vapor deposition or sputtering to prevent oxidation [8]. Consequently, the metallization increases the cost of manufacturing of the substrate, and cracks are likely initiated between the metalized film and bare copper, leading to poor bonding reliability. Nano-silver paste was introduced to bare copper connections without surface metallization to obtain reliable joints [9–14]. Strong joints were obtained by controlling the surface preparation methods that removed copper oxides from the substrate with diluted nitric acid [9] or hydrochloric acid [10,11] prior to application of nano-silver paste. Some researchers demonstrated that sintering the nano-silver paste in air produced lower bonding strength than sintering the nano-silver paste in nitrogen [12,13]. Currently, most studies on nano-silver paste have focused on its feasibility as interconnected materials, as opposed to its long-term behaviors in the field. However, electronic devices are subjected to long-term high-temperature operations due to the increasing demands for miniaturization, integration, and harsh environment applications [15]. Some researchers studied interfacial microstructure development to determine the reliability of solder joints under high-temperature operations. The formation and change of various intermetallic compounds (such as
http://dx.doi.org/10.1016/j.microrel.2015.10.017 0026-2714/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 1. Temperature profile for nano-silver paste sintering.
Cu6Sn5 and Cu3Sn in a SnAgCu/Cu joint) in the joints guide the joint reliability evaluation [16–19]. The microstructure evaluations of nanosilver joints at high temperatures are urgently needed. In this study, a reliable sandwich joint of sintered nano-silver paste on bare copper with dummy chips was introduced. The bonding strength evolutions were investigated with specimens aged at 150 °C, 180 °C, and even higher than 250 °C in air or in a coarse vacuum. The interfacial reaction of the joints as microstructure evaluation was used to probe the bonding strength evolution. 2. Experiment To better simulate the actual nano-silver sintered joints in the electronics industry, we designed a sandwich structure of dummy chip (silver coated Si wafer)/nano-silver/bare Cu plate. The nano-silver paste for interconnection with 82 wt.% of spherical nanoparticles smaller than 50 nm was provided by NBE Technologies LLC (www.nbetech. com). Bare copper plates with dimensions of 15 × 22 × 1.5 mm3 (width × length × thickness) were 99.99% purity. Large area dummy Si chips were 13.5 × 13.5 × 0.2 mm3 (width × length × thickness). For chip connection, first we polished and cleaned the surface of the bare Cu plate. Then a layer of nano-silver paste with a thickness of 50 μm was stencil printed onto the plate and dried at 70 °C for 5 min. Finally, the chip was placed on the dried paste and pressure aided sintering was done on a hot plate inside a tank according to the temperature profile shown in Fig. 1. A pressure of 1.5 MPa was held for 10 min during sintering at 225 °C. During the sintering process, the sintering tank was filled with protective gas (4% H2/96% N2) as shown in Fig. 2 to
prevent the oxidation of bare copper. The final structure of a sintered sandwich specimen is shown in Fig. 3(a) and the bond-line thickness is about 28 μm, as shown in Fig. 3(b). Thermal aging tests were conducted at constant temperatures of 150 °C, 180 °C and 250 °C in air and in a coarse vacuum. To simulate practical coarse vacuum working condition, the specimens were enclosed into the quartz tube, whose vacuum degree achieves to ~ 0.05 Pa. Samples to be cross sectioned were embedded in an epoxy resin. They were ground with 320, 600, 1200, 1500, 2000 and 3000 grade abrasive sandpapers, and then polished with 2.5 μm, 1 μm, and 0.25 μm diamond suspensions. To reveal details of the interfacial structure, the polished cross-section of a sample was corroded in an etching solution (25% NH4:distilled water:100% H2O2 = 11:10:16). The etching time was 2–3 s. The interfacial microstructure and reaction between the sintered nano-silver and copper were analyzed by scanning electron microscopy (SEM, Hitachi S4800, Japan), energy dispersive spectroscopy (EDS, EDAX Genesis XM2, USA) and electron probe microanalysis (EPMA, Shimadzu EPMA-1600, Japan). The bonding strength of a sintered nano-silver joint was measured by using a bond tester (XTZTEC Condor 150) at a velocity of 100 μm/s. A schematic of the die-shear test is shown in Fig. 4. 3. Results and discussion 3.1. Bonding strength evolution after aging 3.1.1. Bonding strength and fracture surface after aging in air Fig. 5 shows the bonding strengths and corresponding fracture surfaces of sandwich specimens before and after aging at 150 °C, 180 °C and 250 °C in air. At least three specimens are made for each aging level to obtain an average bonding strength. The average bonding strength was found to be 27.7 MPa for 13.5 × 13.5 mm2 chips before aging and did not obviously change after aging at 150 °C for 72 h. With increasing the aging temperature to 180 °C and aging this temperature for 72 h, the bonding strengths are reduced to 50% of the initial bonding strength value. Further increasing the aging temperature to 250 °C, the bonding strengths are reduced to 20%. The variation of bonding strengths with aging temperature is closely related to the fracture morphologies (in the inset Fig. 5). The fractures occur in the sintered silver layer rather than at the Ag/Cu or Ag/chip interface before and after aging at 150 °C for 72 h. Moreover, the sheared-off surfaces of silver die attachment before (Fig. 6(a)) and after (Fig. 6(b)) thermal aging show significant plastic flow, indicating that the bonding strength in this case is high. However, the fractures begin to occur at the Ag/Cu
Fig. 2. Apparatus for pressure-assisted sintering with protective gas surrounding.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 3. Sintered nano-silver joint: (a) texture; (b) cross-section.
interface as the aging temperature increases to higher than 180 °C, though plastic flows still exist, as shown in Fig. 6(c) and (d). The area of the exposed copper, where the sintered nano-silver was peeled off, is dark red and increasing from 37% to 56%, probably due to copper oxidation. This manifests as a decrease in bonding strength in air at aging temperatures higher than 180 °C. The bonding strengths does not change after thermal aging at 150 °C for 72 h, further aging studies for traditional solder joint focuses on 150 °C. A longer aging time of 960 h was used at 150 °C, and the average bonding strength is around 27 MPa which is about the same as the initial bonding strength. Thus, the effect of aging time on the bonding strength is not significant at 150 °C. Moreover, the high bonding strength is approved by the fracture morphology, as shown in the insets of Figs. 5 and 6(e). 3.1.2. Bonding strength and fracture surface after aging in a coarse vacuum To further study the reliability of the sandwich specimens (dummy chip/sintered nano-silver/bare Cu) used in capsulation at elevated temperatures, we conducted thermal aging tests in a coarse vacuum. Fig. 7 shows the bonding strengths and corresponding fracture surface of the specimens after thermal aging at 250 °C in a coarse vacuum. There are no obvious changes from the initial bonding strength after the specimens were aged for 72 h or even 960 h. The fracture surfaces (in the insets of Fig. 7) show that less dummy chip is peeled off as the aging time increases. The uniform plastic flows presented in the micrographs of the fracture surfaces (shown in Fig. 8) demonstrate the relatively high bonding strengths of the sintered joints.
3.2. Interfacial reaction of Cu–Ag bond line after aging 3.2.1. Interfacial reaction of Cu–Ag bond line after aging in air The variations in the presented bonding strength values for each of the multiple die attachment samples may be attributed to the different interfacial reaction between sintered Ag and Cu after sintering. Fig. 9 shows the interfacial element distributions before and after aging at 150 °C for 72 h or even 960 h, and at 180 °C for 72 h in air. As shown in Fig. 9(a), a reliable metallurgical bonding is formed between the sintered porous silver and bare Cu after sintering. The bond line is a simple inter-diffusion band of Cu–Ag. The thickness of initial inter-diffusion band, which was determined by an element line scan, is around 1 μm (the thickness is considered as the distance between 10 at.% Cu or Ag on opposite sides). After aging at the conventional temperature of 150 °C for 72 h or 960 h, there are no visible changes of the morphologies at the interface, as shown in Fig. 9(b) and (c). The bond lines are still simple inter-diffusion bands because barely any oxygen element was obtained according to the interfacial element distribution. After thermal aging at 180 °C for 72 h in air, there is no obvious change of the interfacial morphology, as shown in Fig. 9(d). But there is around an oxygen peak of about 1 μm on the bond line; therefore, the bonding strength decreases by 50%.. Fig. 10(a) shows the morphology of an aged joint interface after thermal aging at 250 °C for 72 h in air. Unlike the initial bond line, it is not a simple inter-diffusion band anymore. A complicated diffusion band with two typical morphologies, as indicated by region A and region B, is formed after thermal aging at 250 °C for 72 h. Fig. 10(b) and
Fig. 4. Schematic of die-shear test.
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Fig. 5. Effect of thermal aging condition on bonding strength in air. Insets: photo images of fracture surfaces.
Fig. 7. Effect of thermal aging condition on bonding strength in a coarse vacuum. Inset: photo images of fracture surfaces.
Fig. 6. SEM micrographs of fracture surfaces of sintered nano-silver joints (a) before thermal aging and (b) after thermal aging at 150 °C for 72 h, (c) at 180 °C for 72 h, (d) at 250 °C for 72 h, and (e) at 150 °C for 960 h in air.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 8. SEM micrographs of fracture surfaces of sintered nano-silver joints after thermal aging at (a) 250 °C for 72 h and (b) 250 °C for 960 h in a coarse vacuum.
(c) show the magnified images of regions A and B, respectively. It appears that the loose structure in region A consists of powder and particles, but the compact region B contains some “isolated islands” separated by sintered nano-silver. The cross-section of an aged sample was corroded to reveal the morphology of the complicated diffusion band. Representative SEM morphologies of the complicated diffusion band are shown in Fig. 10(d). The thickness of the complicated diffusion band is not uniform, ranging from 2 μm to 9 μm. A compact stripe structure is dominant in the thinner section of the band. But the powder/particle structure loosens the complicated diffusion band into a thicker section. Some of the powder/particle structures are adjacent to the bare Cu side, but others are far away from the bare Cu side separated by compact strip structure. Mapping element analyses were performed on complicated diffusion band of a sample aged at 250 °C for 72 h due to irregular
morphology at the interface. The results (Fig. 11) show that the loosened structure along the interface consists of only copper oxides, but not silver oxide, because the silver oxide has decomposed at around 215 °C [20]. To further confirm the representative morphologies compositions, EDS and EPMA element analyses were performed. Fig. 12 shows the analysis points of the complicated diffusion band of a sample aged after aging at 250 °C for 72 h. Tables 1 and 2 show the results of EDS and EPMA element analyses, respectively. No element O detected at the clump-like point A far away from the bare copper plate. Point A is determined to have the dual phase structure of Ag (Cu), whose atom content (6.14 at.% Cu in Ag) is higher than the traditional one (0.8–0.3 at.% Cu in Ag at 400–500 °C [21]) because with the nonequilibrium vacancy concentration in sinter silver, additional Cu atoms diffuse into and dissolve in the medium. In addition, the compact stripe structure at points B and C
Fig. 9. Interfacial morphologies and element line scanning results (a) before thermal aging (b) and after thermal aging at 150 °C for 72 h, (c) at 150 °C for 960 h, and (d) at 180 °C for 72 h in air.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 10. Interfacial morphologies of complicated diffusion band after thermal aging at 250 °C for 72 h in air: (a) overall of the complicated diffusion band; (b) amplified image of region A; (c) amplified image of region B; (d) complicated diffusion band after etching.
mainly consists of CuO because the ratios of element Cu to element O are approximately 1:1. In the same way, the powder and particle structures at point D and E showed are mainly consists of Cu2O. The EPMA element analyses results are consistent with the EDS results. The compact strip
structure at F and powder and particle structures at G are consist of CuO or Cu2O, respectively. The dual-layer structure (Cu2O/CuO) and the single-layer structure (CuO) at the interface, as shown in Fig. 10(d), resulted in a sharp decrease of bonding strength after thermal aging.
Fig. 11. Results of mapping element analyses of loosened structure at the interface of sample aged at 250 °C for 72 h.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 12. Analysis points of complicated diffusion band after thermal aging at 250 °C for 72 h.
3.2.2. Interfacial reaction of Cu–Ag bond line after aging in a coarse vacuum Fig. 13(a) and (b) shows the interfacial element distribution after aging at 250 °C for 72 h and 960 in a coarse vacuum, respectively. There are no visible changes in the morphology at the interface. Both of the bond lines are still simple inter-diffusion bands. Meanwhile, the bonding quality of Cu/porous sintered silver is improved by the metallurgical bonding effect between Cu and Ag resulting from further Cu– Ag inter-diffusion after thermal aging. 3.3. Evolution mechanism of Cu–Ag bond line after aging It is necessary to explain the development of different Cu–Ag bond line interface reactions, which may determined the bonding strength evolution, after aged nano-silver joints in air and in a coarse vacuum at relatively high temperatures. The model schematically shown in Fig. 14 is used to probe the bond line evolution mechanism during thermal aging. As aging specimens in a coarse vacuum, the bond line is still simple Cu–Ag inter-diffusion band due to the poor-oxygen atmosphere. The interfacial morphologies are not obviously changed, because the diffusion of Ag and Cu atoms is very slow at 250 °C and there is no new phase forming of Cu–Ag. If aging specimens in air, more oxygen permeated into the sintered nano-silver layer and contacted with Cu at the interface. However, only Cu–Ag inter-diffusion occurred in bond line at low temperature (such as 150 °C) though the aging time is long, because the oxygen potential is not sufficient for copper oxidation. The evolution mechanism in air at 150 °C is actually similar to the evolution behavior of bond line after thermal aging in a coarse vacuum. With increasing the aging temperature, the Cu and O atoms are more active. They easily detour around the compact sintered nano-silver and diffuse into the porous structure at relative high temperatures, as shown in Fig. 14, because sintered nanostructure silver has high energy boundaries and lattice defects, where have relatively high energies. When the oxygen potential could be sufficient for copper oxidation, interface reaction of copper reacting with oxygen in bond line may occur as follows: 2CuðsÞ þ O2 ðgÞ ¼ 2CuOðsÞ
ð1Þ
Table 1 Results of EDS element analyses. Position
Ag at.%
Cu at.%
O at.%
Composition
A B C D E
93.9 1.1 0 2.3 0
6.1 53.4 56.1 61.4 63.9
0 45.5 43.9 36.3 36.1
Ag (Cu) CuO CuO Cu2O Cu2O
According to reaction (1), the copper reacts with oxygen to form a thin CuO film. Many O and Cu atoms continuously diffuse into the lattice defects and formed uniformly 1–2 μm CuO layer (see compact strip in Fig. 10(d)), what is far away from the bare Cu plate like a “isolated islands” as shown in Fig. 10(c). As the thickness of the CuO layer increases, the dense CuO layer gradually hinders the meeting of oxygen and bare Cu. Thus, the oxygen potential at the CuO/Cu interface decreases. When the oxygen potential decreases to the pressure that Cu2O is stable [22] at some locations, the nuclei of Cu2O will occur and the oxide will grow according to the reaction (2) in the positions where enough Cu atoms have. CuOðsÞ þ CuðsÞ ¼ Cu2 OðsÞ
ð2Þ
Once small Cu2O nuclei are formed, they will grow in volume and aggregate to form powder-like Cu2O clusters in the coalescence of oxide stage [22] as shown in Fig. 10(b). Further growth of these Cu2O clusters will lead to a continuous Cu2O layer adjacent to CuO. In other words, a complicated diffusion band is formed and grown when CuO is gradually forming and converting into stable Cu2O based on the proper oxygen potential and enough Cu atoms adjacent to CuO at relatively elevated temperature. So basically, the distribution of Cu2O is not uniform, as shown in Fig. 10(d), because the Cu atoms diffusion trail is untraceable. 4. Conclusion The joints of sintered nano-silver paste on bare copper with large area dummy chip are not degraded at 150 °C in air or 250 °C in a coarse vacuum in thermal aging. After thermal aging, the bond lines of sintered nano-silver and copper are still simple inter-diffusion bands. The bonding strength of sandwich joints is gradually reduced after aging at 180 °C or 250 °C for 72 h in air because oxygen permeated into the sintered silver and reacted with bare copper. The oxidation process resulted in a sharp decrease of bonding strength. A complicated diffusion band with a compact stripe structure (CuO) and loose powder/ particle structure (Cu2O) between the sintered nano-silver and bare copper were formed at the interface after aging at 250 °C for 72 h. We expect that the nano-silver paste for bonding bare copper has high reliability under severe operating conditions in air/in a coarse Table 2 Results of EPMA analyses. Position
Ag at.%
Cu at.%
O at.%
Composition
F G
0.5 0
47.9 64.1
51.6 35.9
CuO Cu2O
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 13. Interfacial morphologies and element line scanning after thermal aging (a) at 250 °C for 72 h and (b) at 250 °C for 960 h in a coarse vacuum.
Fig. 14. Schematic of interfacial reaction between sintered nano-silver and bare copper plate in air at relatively high temperatures.
vacuum and is a promising of Pb-free die-attach material for high temperature power devices. Acknowledgments The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (No. 51401145), the Natural Science Fund of Tianjin, China (No. 13JCQNJC02400) and the Ph. D. Programs Foundation of the Ministry of Education of China (No. 20130032120002). References [1] H.S. Chin, K.Y. Cheong, A.B. Ismail, A review on die attach materials for sic-based high-temperature power devices [J], Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 41 (4) (2010) 824–832. [2] T. Wang, X. Chen, G.Q. Lu, G.Y. Lei, Low-temperature sintering with nano-silver paste in die-attached interconnection [J], J. Electron. Mater. 36 (10) (2007) 1333–1340.
[3] V.R. Manikam, K.Y. Cheong, Die attach materials for high temperature applications: a review [J], IEEE Trans. Compon. Packag. Technol. 1 (4) (2011) 457–478. [4] Y. Yamada, Y. Takaku, Y. Yagi, K. Ishida, et al., Reliability of wire-bonding and solder joint for high temperature operation of power semiconductor device [J], Microelectron. Reliab. 47 (12) (2007) 2147–2151. [5] Z.Y. Zhang, G.Q. Lu, Pressure-assisted low-temperature sintering of silver paste as an alternative die-attach solution to solder reflow [J], IEEE Trans. Compon. Packag. Technol. 25 (4) (2002) 279–283. [6] Y.H. Mei, G. Chen, Y.J. Cao, G.Q. Lu, et al., Simplification of low-temperature sintering nano-silver for power electronics packaging [J], J. Electron. Mater. 42 (6) (2013) 1209–1218. [7] K.S. Siow, Mechanical properties of nano-silver joints as die attach materials [J], J. Alloys Compd. 514 (2012) 6–19. [8] J. Schulz-Harder, Advantages and new development of direct bonded copper substrates [J], Microelectron. Reliab. 43 (3) (2003) 359–365. [9] D. Wakuda, K.S. Kim, K. Suganuma, Ag nanoparticle paste synthesis for room temperature bonding [J], IEEE Trans. Compon. Packag. Technol. 33 (2) (2010) 437–442. [10] E. Ide, S. Angata, A. Hirose, K.F. Kobayashi, Metal–metal bonding process using Ag metallo-organic nanoparticles [J], Acta Mater. 53 (8) (2005) 2385–2393. [11] Y. Morisada, T. Nagaoka, M. Fukusumi, M. Nakamoto, et al., A low-temperature bonding process using mixed Cu–Ag nanoparticles [J], J. Electron. Mater. 39 (8) (2010) 1283–1288. [12] H. Ogura, M. Maruyama, R. Matsubayashi, S. Isoda, et al., Carboxylate-passivated silver nanoparticles and their application to sintered interconnection: a replacement for high temperature lead-rich solders [J], J. Electron. Mater. 39 (8) (2010) 1233–1240. [13] H.G. Zheng, D. Berry, J.N. Calata, G.Q. Lu, et al., Low-pressure joining of large-area devices on copper using nano-silver paste [J], IEEE Trans. Compon. Hybrids Manuf. Technol. 3 (6) (2013) 915–922. [14] J.W. Elmer, E.D. Specht, Metall. Mater. Trans. A 43 (5) (2012) 1528–1537. [15] W.Q. Peng, M.E. Marques, Effect of thermal aging on drop performance of chip scale packages with SnAgCu solder joints on Cu pads [J], J. Electron. Mater. 36 (12) (2007) 1679–1690. [16] X.Y. Li, F.Q. Zu, Z.Y. Huang, W.J. Zhang, et al., Correlation of intermetallic compound growth behavior and melt state of Sn–3.5 Ag–3.5Bi/Cu joint during soldering and isothermal aging [J], J. Mater. Sci. Mater. Electron. 24 (4) (2013) 1231–1237. [17] X. Yu, X.W. Hu, Y.L., Z.X. Min, et al., Tensile properties of Cu/Sn–58Bi/Cu soldered joints subjected to isothermal aging [J], J. Mater. Sci. Mater. Electron. 25 (6) (2014) 2416–2425. [18] Y. Akada, H. Tatsumi, T. Yamaguchi, A. Hirose, Interfacial bonding mechanism using silver metallo-organic nanoparticles to bulk metals and observation of sintering behavior [J], Mater. Trans. 49 (7) (2008) 1537–1545. [19] L. Zhang, X.Y. Fan, C.W. He, Y.H. Guo, Intermetallic compound layer growth between SnAgCu solder and Cu substrate in electronic packaging [J], J. Mater. Sci. Mater. Electron. 24 (9) (2013) 3249–3254. [20] R.W. Zhang, W. Lin, K.S. Moon, C.P. Wong, Fast preparation of printable highly conductive polymer nanocomposites by thermal decomposition of silver carboxylate and sintering of silver nanoparticles, J. Am. Chem. Soc. 2 (9) (2010) 2637–2645. [21] L. Zhang, L. Meng, S.P. Zhou, F.T. Yang, Behaviors of the interface and matrix for the Ag/Cu bimetallic laminates prepared by roll bonding and diffusion annealing [J], Mater. Sci. Eng. A 371 (1–2) (2004) 65–71. [22] Y.F. Zhu, K.J. Mimura, M. Isshiki, A study of the initial oxidation of copper in 0.1 MPa oxygen and the effect of purity by metallographic methods. Corrosion, Science 46 (10) (2004) 2445–2454.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate Su-Yan Zhao a, Xin Li a,⁎, Yun-Hui Mei a, Guo-Quan Lu a,b a b
School of Materials Science and Engineering, Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, Tianjin, PR China Department of Materials Science and Engineering, Virginia Tech, Blacksburg, VA, USA
a r t i c l e
i n f o
Article history: Received 24 May 2015 Received in revised form 11 October 2015 Accepted 15 October 2015 Available online xxxx Keywords: Nano-silver paste Bare copper plate Interface reaction Bonding strength Thermal aging
a b s t r a c t Nano-silver paste has become an alternative lead-free (Pb-free) die attach material for microelectronic packaging, compared to traditional solders and adhesive films, due to its higher thermal and electrical conductivity, higher temperature operation and higher heat dissipation. In this study, the reliable joints of sintered nanosilver paste on bare copper plate with large-area dummy chips were introduced and thermally aged at 150 °C, 180 °C, or even higher than 250 °C in air or in a coarse vacuum environment. The bonding strength and interfacial reaction were investigated. The results showed, after aging specimens at 150 °C for 960 h in air or at 250 °C for 960 h in a coarse vacuum, the interfaces between the sintered nano-silver and bare copper still consisted of simple inter-diffusion bands and with almost no change in bonding strength. However, the bonding strength sharply decreased to 50% after aging at 180 °C for 72 h in air and it decreased to 20% after aging at 250 °C for 72 h in air. The decrease in bonding strength was mainly attributed to the oxidation of copper at relative elevated temperature in this work. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction In electronic packaging, the die-attach materials play a very important role in providing the packaging interconnection, physical protection, and mechanical support to ensure that the entire system works functionally [1–3]. Nowadays, both solder alloys (leaded or lead-free) and conductive adhesives are commonly used as die-attach materials for use at operating temperatures below 220 °C [4]. However, these solder alloys and conductive adhesives could not meet the stringent requirements for higher temperature applications due to their low melting and operating temperatures, such as in the automotive, aviation, space, and nuclear industries. Nano-silver paste has been reported as a promising die-attach material due to its high melting temperature (960 °C), low sintering temperature (275 °C), high electrical/thermal conductivity (4.1 × 107 S·m− 1 and 240 W·m− 1·K− 1, respectively), and high reliability [5]. Simultaneously, the properties of nano-silver joints have been extensively studied [2,6,7]. Mei et al. [6] studied the effect of sintering condition on the bonding quality of sintered nano-silver on Cu metallization under different temperatures and holding times, showing that the bonding strength mostly increased with increasing applied pressure, pressing temperature, pressing time, sintering temperature, and sintering time. Wang et al. [2] analyzed the bonding strength of sintered nano-silver joints and the microstructure evolution of the ⁎ Corresponding author. E-mail address: [email protected] (X. Li).
nano-silver paste during sintering. They found that the excellent bonding strength was in accordance with the densification behavior of the nanoparticles. In addition, the adhesion strength of the Ag nanoparticles on the bonding substrate surface is important for this bonding strength [7]. Copper as the material of die-attach substrate is commonly metalized by electroplating, physical or chemical vapor deposition or sputtering to prevent oxidation [8]. Consequently, the metallization increases the cost of manufacturing of the substrate, and cracks are likely initiated between the metalized film and bare copper, leading to poor bonding reliability. Nano-silver paste was introduced to bare copper connections without surface metallization to obtain reliable joints [9–14]. Strong joints were obtained by controlling the surface preparation methods that removed copper oxides from the substrate with diluted nitric acid [9] or hydrochloric acid [10,11] prior to application of nano-silver paste. Some researchers demonstrated that sintering the nano-silver paste in air produced lower bonding strength than sintering the nano-silver paste in nitrogen [12,13]. Currently, most studies on nano-silver paste have focused on its feasibility as interconnected materials, as opposed to its long-term behaviors in the field. However, electronic devices are subjected to long-term high-temperature operations due to the increasing demands for miniaturization, integration, and harsh environment applications [15]. Some researchers studied interfacial microstructure development to determine the reliability of solder joints under high-temperature operations. The formation and change of various intermetallic compounds (such as
http://dx.doi.org/10.1016/j.microrel.2015.10.017 0026-2714/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 1. Temperature profile for nano-silver paste sintering.
Cu6Sn5 and Cu3Sn in a SnAgCu/Cu joint) in the joints guide the joint reliability evaluation [16–19]. The microstructure evaluations of nanosilver joints at high temperatures are urgently needed. In this study, a reliable sandwich joint of sintered nano-silver paste on bare copper with dummy chips was introduced. The bonding strength evolutions were investigated with specimens aged at 150 °C, 180 °C, and even higher than 250 °C in air or in a coarse vacuum. The interfacial reaction of the joints as microstructure evaluation was used to probe the bonding strength evolution. 2. Experiment To better simulate the actual nano-silver sintered joints in the electronics industry, we designed a sandwich structure of dummy chip (silver coated Si wafer)/nano-silver/bare Cu plate. The nano-silver paste for interconnection with 82 wt.% of spherical nanoparticles smaller than 50 nm was provided by NBE Technologies LLC (www.nbetech. com). Bare copper plates with dimensions of 15 × 22 × 1.5 mm3 (width × length × thickness) were 99.99% purity. Large area dummy Si chips were 13.5 × 13.5 × 0.2 mm3 (width × length × thickness). For chip connection, first we polished and cleaned the surface of the bare Cu plate. Then a layer of nano-silver paste with a thickness of 50 μm was stencil printed onto the plate and dried at 70 °C for 5 min. Finally, the chip was placed on the dried paste and pressure aided sintering was done on a hot plate inside a tank according to the temperature profile shown in Fig. 1. A pressure of 1.5 MPa was held for 10 min during sintering at 225 °C. During the sintering process, the sintering tank was filled with protective gas (4% H2/96% N2) as shown in Fig. 2 to
prevent the oxidation of bare copper. The final structure of a sintered sandwich specimen is shown in Fig. 3(a) and the bond-line thickness is about 28 μm, as shown in Fig. 3(b). Thermal aging tests were conducted at constant temperatures of 150 °C, 180 °C and 250 °C in air and in a coarse vacuum. To simulate practical coarse vacuum working condition, the specimens were enclosed into the quartz tube, whose vacuum degree achieves to ~ 0.05 Pa. Samples to be cross sectioned were embedded in an epoxy resin. They were ground with 320, 600, 1200, 1500, 2000 and 3000 grade abrasive sandpapers, and then polished with 2.5 μm, 1 μm, and 0.25 μm diamond suspensions. To reveal details of the interfacial structure, the polished cross-section of a sample was corroded in an etching solution (25% NH4:distilled water:100% H2O2 = 11:10:16). The etching time was 2–3 s. The interfacial microstructure and reaction between the sintered nano-silver and copper were analyzed by scanning electron microscopy (SEM, Hitachi S4800, Japan), energy dispersive spectroscopy (EDS, EDAX Genesis XM2, USA) and electron probe microanalysis (EPMA, Shimadzu EPMA-1600, Japan). The bonding strength of a sintered nano-silver joint was measured by using a bond tester (XTZTEC Condor 150) at a velocity of 100 μm/s. A schematic of the die-shear test is shown in Fig. 4. 3. Results and discussion 3.1. Bonding strength evolution after aging 3.1.1. Bonding strength and fracture surface after aging in air Fig. 5 shows the bonding strengths and corresponding fracture surfaces of sandwich specimens before and after aging at 150 °C, 180 °C and 250 °C in air. At least three specimens are made for each aging level to obtain an average bonding strength. The average bonding strength was found to be 27.7 MPa for 13.5 × 13.5 mm2 chips before aging and did not obviously change after aging at 150 °C for 72 h. With increasing the aging temperature to 180 °C and aging this temperature for 72 h, the bonding strengths are reduced to 50% of the initial bonding strength value. Further increasing the aging temperature to 250 °C, the bonding strengths are reduced to 20%. The variation of bonding strengths with aging temperature is closely related to the fracture morphologies (in the inset Fig. 5). The fractures occur in the sintered silver layer rather than at the Ag/Cu or Ag/chip interface before and after aging at 150 °C for 72 h. Moreover, the sheared-off surfaces of silver die attachment before (Fig. 6(a)) and after (Fig. 6(b)) thermal aging show significant plastic flow, indicating that the bonding strength in this case is high. However, the fractures begin to occur at the Ag/Cu
Fig. 2. Apparatus for pressure-assisted sintering with protective gas surrounding.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 3. Sintered nano-silver joint: (a) texture; (b) cross-section.
interface as the aging temperature increases to higher than 180 °C, though plastic flows still exist, as shown in Fig. 6(c) and (d). The area of the exposed copper, where the sintered nano-silver was peeled off, is dark red and increasing from 37% to 56%, probably due to copper oxidation. This manifests as a decrease in bonding strength in air at aging temperatures higher than 180 °C. The bonding strengths does not change after thermal aging at 150 °C for 72 h, further aging studies for traditional solder joint focuses on 150 °C. A longer aging time of 960 h was used at 150 °C, and the average bonding strength is around 27 MPa which is about the same as the initial bonding strength. Thus, the effect of aging time on the bonding strength is not significant at 150 °C. Moreover, the high bonding strength is approved by the fracture morphology, as shown in the insets of Figs. 5 and 6(e). 3.1.2. Bonding strength and fracture surface after aging in a coarse vacuum To further study the reliability of the sandwich specimens (dummy chip/sintered nano-silver/bare Cu) used in capsulation at elevated temperatures, we conducted thermal aging tests in a coarse vacuum. Fig. 7 shows the bonding strengths and corresponding fracture surface of the specimens after thermal aging at 250 °C in a coarse vacuum. There are no obvious changes from the initial bonding strength after the specimens were aged for 72 h or even 960 h. The fracture surfaces (in the insets of Fig. 7) show that less dummy chip is peeled off as the aging time increases. The uniform plastic flows presented in the micrographs of the fracture surfaces (shown in Fig. 8) demonstrate the relatively high bonding strengths of the sintered joints.
3.2. Interfacial reaction of Cu–Ag bond line after aging 3.2.1. Interfacial reaction of Cu–Ag bond line after aging in air The variations in the presented bonding strength values for each of the multiple die attachment samples may be attributed to the different interfacial reaction between sintered Ag and Cu after sintering. Fig. 9 shows the interfacial element distributions before and after aging at 150 °C for 72 h or even 960 h, and at 180 °C for 72 h in air. As shown in Fig. 9(a), a reliable metallurgical bonding is formed between the sintered porous silver and bare Cu after sintering. The bond line is a simple inter-diffusion band of Cu–Ag. The thickness of initial inter-diffusion band, which was determined by an element line scan, is around 1 μm (the thickness is considered as the distance between 10 at.% Cu or Ag on opposite sides). After aging at the conventional temperature of 150 °C for 72 h or 960 h, there are no visible changes of the morphologies at the interface, as shown in Fig. 9(b) and (c). The bond lines are still simple inter-diffusion bands because barely any oxygen element was obtained according to the interfacial element distribution. After thermal aging at 180 °C for 72 h in air, there is no obvious change of the interfacial morphology, as shown in Fig. 9(d). But there is around an oxygen peak of about 1 μm on the bond line; therefore, the bonding strength decreases by 50%.. Fig. 10(a) shows the morphology of an aged joint interface after thermal aging at 250 °C for 72 h in air. Unlike the initial bond line, it is not a simple inter-diffusion band anymore. A complicated diffusion band with two typical morphologies, as indicated by region A and region B, is formed after thermal aging at 250 °C for 72 h. Fig. 10(b) and
Fig. 4. Schematic of die-shear test.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 5. Effect of thermal aging condition on bonding strength in air. Insets: photo images of fracture surfaces.
Fig. 7. Effect of thermal aging condition on bonding strength in a coarse vacuum. Inset: photo images of fracture surfaces.
Fig. 6. SEM micrographs of fracture surfaces of sintered nano-silver joints (a) before thermal aging and (b) after thermal aging at 150 °C for 72 h, (c) at 180 °C for 72 h, (d) at 250 °C for 72 h, and (e) at 150 °C for 960 h in air.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 8. SEM micrographs of fracture surfaces of sintered nano-silver joints after thermal aging at (a) 250 °C for 72 h and (b) 250 °C for 960 h in a coarse vacuum.
(c) show the magnified images of regions A and B, respectively. It appears that the loose structure in region A consists of powder and particles, but the compact region B contains some “isolated islands” separated by sintered nano-silver. The cross-section of an aged sample was corroded to reveal the morphology of the complicated diffusion band. Representative SEM morphologies of the complicated diffusion band are shown in Fig. 10(d). The thickness of the complicated diffusion band is not uniform, ranging from 2 μm to 9 μm. A compact stripe structure is dominant in the thinner section of the band. But the powder/particle structure loosens the complicated diffusion band into a thicker section. Some of the powder/particle structures are adjacent to the bare Cu side, but others are far away from the bare Cu side separated by compact strip structure. Mapping element analyses were performed on complicated diffusion band of a sample aged at 250 °C for 72 h due to irregular
morphology at the interface. The results (Fig. 11) show that the loosened structure along the interface consists of only copper oxides, but not silver oxide, because the silver oxide has decomposed at around 215 °C [20]. To further confirm the representative morphologies compositions, EDS and EPMA element analyses were performed. Fig. 12 shows the analysis points of the complicated diffusion band of a sample aged after aging at 250 °C for 72 h. Tables 1 and 2 show the results of EDS and EPMA element analyses, respectively. No element O detected at the clump-like point A far away from the bare copper plate. Point A is determined to have the dual phase structure of Ag (Cu), whose atom content (6.14 at.% Cu in Ag) is higher than the traditional one (0.8–0.3 at.% Cu in Ag at 400–500 °C [21]) because with the nonequilibrium vacancy concentration in sinter silver, additional Cu atoms diffuse into and dissolve in the medium. In addition, the compact stripe structure at points B and C
Fig. 9. Interfacial morphologies and element line scanning results (a) before thermal aging (b) and after thermal aging at 150 °C for 72 h, (c) at 150 °C for 960 h, and (d) at 180 °C for 72 h in air.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 10. Interfacial morphologies of complicated diffusion band after thermal aging at 250 °C for 72 h in air: (a) overall of the complicated diffusion band; (b) amplified image of region A; (c) amplified image of region B; (d) complicated diffusion band after etching.
mainly consists of CuO because the ratios of element Cu to element O are approximately 1:1. In the same way, the powder and particle structures at point D and E showed are mainly consists of Cu2O. The EPMA element analyses results are consistent with the EDS results. The compact strip
structure at F and powder and particle structures at G are consist of CuO or Cu2O, respectively. The dual-layer structure (Cu2O/CuO) and the single-layer structure (CuO) at the interface, as shown in Fig. 10(d), resulted in a sharp decrease of bonding strength after thermal aging.
Fig. 11. Results of mapping element analyses of loosened structure at the interface of sample aged at 250 °C for 72 h.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 12. Analysis points of complicated diffusion band after thermal aging at 250 °C for 72 h.
3.2.2. Interfacial reaction of Cu–Ag bond line after aging in a coarse vacuum Fig. 13(a) and (b) shows the interfacial element distribution after aging at 250 °C for 72 h and 960 in a coarse vacuum, respectively. There are no visible changes in the morphology at the interface. Both of the bond lines are still simple inter-diffusion bands. Meanwhile, the bonding quality of Cu/porous sintered silver is improved by the metallurgical bonding effect between Cu and Ag resulting from further Cu– Ag inter-diffusion after thermal aging. 3.3. Evolution mechanism of Cu–Ag bond line after aging It is necessary to explain the development of different Cu–Ag bond line interface reactions, which may determined the bonding strength evolution, after aged nano-silver joints in air and in a coarse vacuum at relatively high temperatures. The model schematically shown in Fig. 14 is used to probe the bond line evolution mechanism during thermal aging. As aging specimens in a coarse vacuum, the bond line is still simple Cu–Ag inter-diffusion band due to the poor-oxygen atmosphere. The interfacial morphologies are not obviously changed, because the diffusion of Ag and Cu atoms is very slow at 250 °C and there is no new phase forming of Cu–Ag. If aging specimens in air, more oxygen permeated into the sintered nano-silver layer and contacted with Cu at the interface. However, only Cu–Ag inter-diffusion occurred in bond line at low temperature (such as 150 °C) though the aging time is long, because the oxygen potential is not sufficient for copper oxidation. The evolution mechanism in air at 150 °C is actually similar to the evolution behavior of bond line after thermal aging in a coarse vacuum. With increasing the aging temperature, the Cu and O atoms are more active. They easily detour around the compact sintered nano-silver and diffuse into the porous structure at relative high temperatures, as shown in Fig. 14, because sintered nanostructure silver has high energy boundaries and lattice defects, where have relatively high energies. When the oxygen potential could be sufficient for copper oxidation, interface reaction of copper reacting with oxygen in bond line may occur as follows: 2CuðsÞ þ O2 ðgÞ ¼ 2CuOðsÞ
ð1Þ
Table 1 Results of EDS element analyses. Position
Ag at.%
Cu at.%
O at.%
Composition
A B C D E
93.9 1.1 0 2.3 0
6.1 53.4 56.1 61.4 63.9
0 45.5 43.9 36.3 36.1
Ag (Cu) CuO CuO Cu2O Cu2O
According to reaction (1), the copper reacts with oxygen to form a thin CuO film. Many O and Cu atoms continuously diffuse into the lattice defects and formed uniformly 1–2 μm CuO layer (see compact strip in Fig. 10(d)), what is far away from the bare Cu plate like a “isolated islands” as shown in Fig. 10(c). As the thickness of the CuO layer increases, the dense CuO layer gradually hinders the meeting of oxygen and bare Cu. Thus, the oxygen potential at the CuO/Cu interface decreases. When the oxygen potential decreases to the pressure that Cu2O is stable [22] at some locations, the nuclei of Cu2O will occur and the oxide will grow according to the reaction (2) in the positions where enough Cu atoms have. CuOðsÞ þ CuðsÞ ¼ Cu2 OðsÞ
ð2Þ
Once small Cu2O nuclei are formed, they will grow in volume and aggregate to form powder-like Cu2O clusters in the coalescence of oxide stage [22] as shown in Fig. 10(b). Further growth of these Cu2O clusters will lead to a continuous Cu2O layer adjacent to CuO. In other words, a complicated diffusion band is formed and grown when CuO is gradually forming and converting into stable Cu2O based on the proper oxygen potential and enough Cu atoms adjacent to CuO at relatively elevated temperature. So basically, the distribution of Cu2O is not uniform, as shown in Fig. 10(d), because the Cu atoms diffusion trail is untraceable. 4. Conclusion The joints of sintered nano-silver paste on bare copper with large area dummy chip are not degraded at 150 °C in air or 250 °C in a coarse vacuum in thermal aging. After thermal aging, the bond lines of sintered nano-silver and copper are still simple inter-diffusion bands. The bonding strength of sandwich joints is gradually reduced after aging at 180 °C or 250 °C for 72 h in air because oxygen permeated into the sintered silver and reacted with bare copper. The oxidation process resulted in a sharp decrease of bonding strength. A complicated diffusion band with a compact stripe structure (CuO) and loose powder/ particle structure (Cu2O) between the sintered nano-silver and bare copper were formed at the interface after aging at 250 °C for 72 h. We expect that the nano-silver paste for bonding bare copper has high reliability under severe operating conditions in air/in a coarse Table 2 Results of EPMA analyses. Position
Ag at.%
Cu at.%
O at.%
Composition
F G
0.5 0
47.9 64.1
51.6 35.9
CuO Cu2O
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017
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Fig. 13. Interfacial morphologies and element line scanning after thermal aging (a) at 250 °C for 72 h and (b) at 250 °C for 960 h in a coarse vacuum.
Fig. 14. Schematic of interfacial reaction between sintered nano-silver and bare copper plate in air at relatively high temperatures.
vacuum and is a promising of Pb-free die-attach material for high temperature power devices. Acknowledgments The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (No. 51401145), the Natural Science Fund of Tianjin, China (No. 13JCQNJC02400) and the Ph. D. Programs Foundation of the Ministry of Education of China (No. 20130032120002). References [1] H.S. Chin, K.Y. Cheong, A.B. Ismail, A review on die attach materials for sic-based high-temperature power devices [J], Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 41 (4) (2010) 824–832. [2] T. Wang, X. Chen, G.Q. Lu, G.Y. Lei, Low-temperature sintering with nano-silver paste in die-attached interconnection [J], J. Electron. Mater. 36 (10) (2007) 1333–1340.
[3] V.R. Manikam, K.Y. Cheong, Die attach materials for high temperature applications: a review [J], IEEE Trans. Compon. Packag. Technol. 1 (4) (2011) 457–478. [4] Y. Yamada, Y. Takaku, Y. Yagi, K. Ishida, et al., Reliability of wire-bonding and solder joint for high temperature operation of power semiconductor device [J], Microelectron. Reliab. 47 (12) (2007) 2147–2151. [5] Z.Y. Zhang, G.Q. Lu, Pressure-assisted low-temperature sintering of silver paste as an alternative die-attach solution to solder reflow [J], IEEE Trans. Compon. Packag. Technol. 25 (4) (2002) 279–283. [6] Y.H. Mei, G. Chen, Y.J. Cao, G.Q. Lu, et al., Simplification of low-temperature sintering nano-silver for power electronics packaging [J], J. Electron. Mater. 42 (6) (2013) 1209–1218. [7] K.S. Siow, Mechanical properties of nano-silver joints as die attach materials [J], J. Alloys Compd. 514 (2012) 6–19. [8] J. Schulz-Harder, Advantages and new development of direct bonded copper substrates [J], Microelectron. Reliab. 43 (3) (2003) 359–365. [9] D. Wakuda, K.S. Kim, K. Suganuma, Ag nanoparticle paste synthesis for room temperature bonding [J], IEEE Trans. Compon. Packag. Technol. 33 (2) (2010) 437–442. [10] E. Ide, S. Angata, A. Hirose, K.F. Kobayashi, Metal–metal bonding process using Ag metallo-organic nanoparticles [J], Acta Mater. 53 (8) (2005) 2385–2393. [11] Y. Morisada, T. Nagaoka, M. Fukusumi, M. Nakamoto, et al., A low-temperature bonding process using mixed Cu–Ag nanoparticles [J], J. Electron. Mater. 39 (8) (2010) 1283–1288. [12] H. Ogura, M. Maruyama, R. Matsubayashi, S. Isoda, et al., Carboxylate-passivated silver nanoparticles and their application to sintered interconnection: a replacement for high temperature lead-rich solders [J], J. Electron. Mater. 39 (8) (2010) 1233–1240. [13] H.G. Zheng, D. Berry, J.N. Calata, G.Q. Lu, et al., Low-pressure joining of large-area devices on copper using nano-silver paste [J], IEEE Trans. Compon. Hybrids Manuf. Technol. 3 (6) (2013) 915–922. [14] J.W. Elmer, E.D. Specht, Metall. Mater. Trans. A 43 (5) (2012) 1528–1537. [15] W.Q. Peng, M.E. Marques, Effect of thermal aging on drop performance of chip scale packages with SnAgCu solder joints on Cu pads [J], J. Electron. Mater. 36 (12) (2007) 1679–1690. [16] X.Y. Li, F.Q. Zu, Z.Y. Huang, W.J. Zhang, et al., Correlation of intermetallic compound growth behavior and melt state of Sn–3.5 Ag–3.5Bi/Cu joint during soldering and isothermal aging [J], J. Mater. Sci. Mater. Electron. 24 (4) (2013) 1231–1237. [17] X. Yu, X.W. Hu, Y.L., Z.X. Min, et al., Tensile properties of Cu/Sn–58Bi/Cu soldered joints subjected to isothermal aging [J], J. Mater. Sci. Mater. Electron. 25 (6) (2014) 2416–2425. [18] Y. Akada, H. Tatsumi, T. Yamaguchi, A. Hirose, Interfacial bonding mechanism using silver metallo-organic nanoparticles to bulk metals and observation of sintering behavior [J], Mater. Trans. 49 (7) (2008) 1537–1545. [19] L. Zhang, X.Y. Fan, C.W. He, Y.H. Guo, Intermetallic compound layer growth between SnAgCu solder and Cu substrate in electronic packaging [J], J. Mater. Sci. Mater. Electron. 24 (9) (2013) 3249–3254. [20] R.W. Zhang, W. Lin, K.S. Moon, C.P. Wong, Fast preparation of printable highly conductive polymer nanocomposites by thermal decomposition of silver carboxylate and sintering of silver nanoparticles, J. Am. Chem. Soc. 2 (9) (2010) 2637–2645. [21] L. Zhang, L. Meng, S.P. Zhou, F.T. Yang, Behaviors of the interface and matrix for the Ag/Cu bimetallic laminates prepared by roll bonding and diffusion annealing [J], Mater. Sci. Eng. A 371 (1–2) (2004) 65–71. [22] Y.F. Zhu, K.J. Mimura, M. Isshiki, A study of the initial oxidation of copper in 0.1 MPa oxygen and the effect of purity by metallographic methods. Corrosion, Science 46 (10) (2004) 2445–2454.
Please cite this article as: S.-Y. Zhao, et al., Study on high temperature bonding reliability of sintered nano-silver joint on bare copper plate, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.10.017