Ultrasonics Sonochemistry 21 (2014) 924–929
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Short Communication
Ultrarapid formation of homogeneous Cu6Sn5 and Cu3Sn intermetallic compound joints at room temperature using ultrasonic waves Zhuolin Li a, Mingyu Li a,b,⇑, Yong Xiao a, Chunqing Wang b,⇑ a b
Shenzhen Key Laboratory of Advanced Materials, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 29 September 2013 Accepted 29 September 2013 Available online 17 October 2013 Keywords: Ultrasonic waves Intermetallic compounds Cavitation Erosion
a b s t r a c t Homogeneous intermetallic compound joints are demanded by the semiconductor industry because of their high melting point. In the present work, ultrasonic vibration was applied to Cu/Sn foil/Cu interconnection system at room temperature to form homogeneous Cu6Sn5 and Cu3Sn joints. Compared with other studies based on transient-liquid-phase soldering, the processing time of our method was dramatically reduced from several hours to several seconds. This ultrarapid intermetallic phase formation process resulted from accelerated interdiffusion kinetics, which can be attributed to the sonochemical effects of acoustic cavitation at the interface between the liquid Sn and the solid Cu during the ultrasonic bonding process. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The semiconductor industry has developed remarkably over the past two decades, however, the essential requirements for interconnections among various types of electronic components have remained unchanged. Tin-based solder alloy are most commonly used to form mechanically and electronically reliable joints between electronic components for electronic packaging [1]. However, conventional joining methods and materials often fail to meet the requirements of electronic systems that operate at elevated temperatures (e.g., 400 °C encountered by power electronics used for space exploration [2]). Such high temperatures will remelt most common solder joints. Furthermore, for some advanced integration technologies such as 3D chip-stacking packaging, highmelting-point joints are required to enable repeated multilevel 3D stacking of additional layers without remelting of the joints at lower levels [3]. Traditional efforts to increase the maximum operating temperature of joints employ joining materials with higher melting points. Unfortunately, increasing the joining temperature increases the likelihood of inducing microstructural changes that degrade the material properties [4]. Recently, an alternative transient-liquid-phase (TLP) bonding process was developed that can be performed at relatively low temperatures, resulting in higher melting temperatures of the joints [4–10]. A TLP joint can be formed by sandwiching a thin interlayer, ⇑ Corresponding authors at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China. Tel.: +86 755 26033463; fax: +86 755 26033504 (M. Li). Tel.: +86 451 86418725; fax: +86 451 86416186 (C. Wang). E-mail addresses: [email protected] (M. Li), [email protected] (C. Wang). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.09.020
containing melting point depressants (MPD), between two base metal substrates and heating the entire assembly to melt the interlayer. When the processing time is extended, the MPD diffuses into the surrounding base metal, which results in isothermal solidification until no trace of the melted liquid phase. Ideally the joint becomes homogeneous consisting of intermetallic compounds (IMCs) or solid solution of the interlayer metal in the base metal, imparting a melting point higher than the bonding temperature. However, an inevitable drawback of TLP bonding is that it often requires a long processing time of up to several hours [4,7], which may lead to extra thermal stress on the bond components, which can seriously affect the reliability of the packaging system. Therefore, the development of a new bonding method that can form high-melting-point intermetallic compound joints at low temperature using a short bonding time is highly desirable. The use of ultrasound for the intensification of chemical/physical processing applications has been well established [11]. Through the chemical and physical effects of acoustically-induced microbubble formation and collapse (acoustic cavitation) that can occur simultaneously at millions of locations in a reactor, conditions of very high temperatures and pressures (few thousand atmosphere pressure and few thousand Kelvin temperature) can be created locally, although the overall environment remains at ambient conditions [11,12]. Thus, chemical reactions that requires stringent conditions can be effectively performed using ultrasound at ambient temperature. Moreover, acoustic cavitation can generate numerous spectacular effects (e.g., acoustic streaming, shock waves and micro-jets) that can increase mass transportation to accelerate chemical reaction [13]. In the present work, we used a Cu/Sn foil/Cu interconnection system similar to the ones used in
Z. Li et al. / Ultrasonics Sonochemistry 21 (2014) 924–929
previous TLP bonding studies, while applying ultrasonic waves to rapidly form homogeneous Cu6Sn5 and Cu3Sn intermetallic compound joints at room temperature. The ultrasonic waves enable the formation of intermetallic compound joints by two mechanisms. First, the vibration of the ultrasonic horn induces friction at the interface between the solid solder and the metal substrate, this effect serves as a frictional heating source to melt the solder foil into a liquid phase during the bonding process. Second, the propagation of the ultrasonic waves in the molten foil causes acoustic cavitation that intensifies the interfacial bonding reaction between the liquid solder and the solid metal substrate, which enables rapid formation of intermetallic compound joints. 2. Experiment 2.1. Materials Schematics for the sandwich Cu/Sn foil/Cu interconnection system and ultrasonic bonding are shown in Fig. 1a and b. The interlayer is one piece of pure Sn foil with a thickness of 25 lm. The base metal substrates are two pieces of pure Cu plate with a thickness of 0.3 mm. The Sn foil and the Cu substrates were cut into 3 3 mm2 pieces and were manually flattened using a level press. The Cu/Sn foil/Cu system was chosen as a joining couple because it is widely used for electronic packaging interconnections. 2.2. Experimental process The Sn foil was sandwiched between two Cu substrates and placed in an ultrasonic bonder with a horn-type reactor as shown in Fig. 1b, the bonding process was performed at ambient temperature, using pressure and ultrasonic vibration. The frequency and power of the ultrasonic bonder were 20 kHz and 750 W, respectively. The bonding pressure was optimized at 0.6 MPa in order to guarantee good contact between the ultrasonic horn and the
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bonding component without squeezing the solder out of the joint during the bonding process. Joint formation was examined for bonding times that ranged from 1 to 4 s. 2.3. Characterization To determine the overall temperature profile of the interconnection system during the ultrasonic bonding process, we placed a thermocouple underneath the bottom Cu substrate, and connected it to a thermoelectric-signal collector and a data recording system. Metallographic cross-sections of the bonded samples were prepared for microstructural characterization of the intermetallic phase joints by first mounting the samples in epoxy resin (cured at room temperature for 6 h) and then grinding the samples with 400, 800, 1200, and 2500 grit SiC papers in succession before final polishing in successive diamond slurries of 3, 1 and 0.05 lm (2 min each). A scanning electron microscope (SEM, Hitachi S-4700) using backscattered electron (BSE) signals was used for imaging and analysis of the microstructural features. The intermetallic phases that formed the joints were identified using X-ray diffraction (XRD, Rigaku X-ray diffractometer) analysis. 3. Results and discussion Fig. 1c shows the overall temperature profile of the interconnection system during the ultrasonic bonding process for 3 s and its subsequent cooling procedure. With the onset of ultrasonic bonding, the temperature of the entire assembly increased sharply to 297 °C and then reached a sustained temperature of approximately 277 °C until the bonding time for 3 s ended; the interconnection system was subsequently cooled in air to ambient temperature. Given that the melting point of Sn is 232 °C, a liquid Sn interlayer certainly formed during the bonding process. The initial abrupt increase in temperature can be attributed to frictional heat generated by the interfacial rubbing between the solid solder
Fig. 1. (a and b) Schematics of the Cu/Sn foil/Cu interconnection system and ultrasonic bonding. (c) Overall temperature profile of the interconnection system during the ultrasonic bonding process for 3 s and subsequent cooling.
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and base metal. The thermal effect after the formation of the liquid interlayer can be attributed to the propagation of ultrasonic waves within the liquid interlayer that induces acoustic cavitation, where bubble collapse results in an enormous concentration of energy due to the conversion of the surface energy and the kinetic energy of liquid motion into heat and chemical energy [14–16]. This energy conversion was sufficient to sustain the bonding temperature. The cross-sectional BSE images and XRD analyses of the joints formed by ultrasonic bonding at times that ranged from 1 to 4 s are shown in Fig. 2. After 1 s of the ultrasonic bonding time, the resulted joints contained both residual solder and intermetallic compounds. XRD analysis revealed that the intermetallic compounds formed a Cu6Sn5 (g) phase. After 2 s and 3 s of ultrasonic bonding, the connection layers of the joints were observed to be 20 lm
thick, with the Sn interlayer completely consumed and with the homogeneous Cu6Sn5 (g) phase presented as a connection layer. At the boundary between the intermetallic phase and the base metal, the Cu6Sn5 phase advanced into the Cu base metal, forming pot-shaped damage pits on the once-flat Cu surface. An increase in the ultrasonic bonding time to 4 s, resulted in the joint presented coexisting Cu6Sn5 (g) and Cu3Sn (e) phases. This process produced structures similar to those reported in a previous study on TLP soldering of the Cu/Sn foil/Cu system [10], where a much longer processing time (at least of 90 min) was necessary to form the intermetallic compound joints as those formed in current work within several seconds. This ultrarapid formation of intermetallic phases can be wholly attributed to the sonochemical effects induced by acoustic cavitation at the liquid/solid interface.
Fig. 2. Cross-sectional BSE images and XRD analyses of ultrarapidly formed joints with bonding times that ranged from 1 to 4 s. (a–d) An increase in the ultrasonic bonding time resulted in the formation a series of joints consisting of coexisting Sn solder and Cu6Sn5 (g) phases, a homogeneous Cu6Sn5 (g) phase, and coexisting Cu6Sn5 (g) and Cu3Sn (e) phases. (e) XRD spectra of the intermetallic phases.
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Propagation of ultrasonic waves in the liquid interlayer generated acoustic cavitation, which is a phenomenon involving the formation, growth and rapid implosive collapse of a vapor-filled microbubble. The sonochemical effects in our study were primarily derived from acoustic cavitation. Rapid bubble implosion can induce tiny localized hot spots that reach extreme temperatures and pressures estimated to be on the order of 5000 K and 0.1 GPa, respectively [17–22]. However, the hot spots are so small that heat dissipation occurs within a short period of 2 ls [17]. Therefore, only bubbles that implode adjacent to the interface can generate effects on the solid surface. In our study, acoustic cavitation was confined to a thin liquid solder interlayer, which means that bubble collapse occurred in the vicinity of either base metal surface. When bubble is close to the base metal surface, the liquid motion in its vicinity is hindered, leading to micro-jet phenomenon
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[23]. In addition, bubble collapse leads to the emission of a shock wave with a pressure of several GPa and a starting shock velocity of 4000 m s 1 [24]. The combined effect of liquid-solder microjets, shock waves and localized high temperatures can produce microscale damages (pits) on the base metal surface. This phenomenon is known as cavitation erosion [25]. In our study, the cavitation erosion process released an excessive amount of Cu from the base metal into the molten Sn during bonding. As the released Cu particles entered the acting region of imploding bubbles, they instantly dissolved into the molten Sn due to the high temperature. Although single cavity collapse lasts for only several microseconds, millions of bubbles confined in the thin layer of molten Sn form and implod continuously; therefore, the liquid Sn layer was maintained in a dynamically unequilibrium state with supersaturation of Cu. Upon subsequent cooling, intermetallic compound nuclei rapidly precipitated and grew as a result of reaction crystallization,
Fig. 3. Cross-sectional BSE images and XRD analyses of the joints formed by ultrasonic bonding for 3 s with the interval distances between Cu substrates that ranged from 20 to 12 lm. (a–d) An decrease in the interval distance between Cu substrates resulted in the formation a series of joints consisting of a homogeneous Cu6Sn5 (g) phase, coexisting Cu6Sn5 (g) and Cu3Sn (e) phases and a homogeneous Cu3Sn (e) phase. (e) XRD spectra of the intermetallic phases.
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driven by the supersaturation of Cu in the liquid solder interlayer. Eventually joint was bonded by the formation of intermetallic phases. These sonochemical effects result in the ultrarapid formation of intermetallic compound joints. The amount of Cu released into the liquid Sn interlayer during the bonding process is in direct proportional to the bonding time. An increase in the bonding time resulted in more mass transportation of Cu metal and increased the concentration ratio of Cu in the Cu/Sn reaction system. Therefore, with the bonding time increased from 2 to 4 s, we observed that the intermetallic phase of the joint transformed from a homogeneous Cu6Sn5 (g) phase to coexisting Cu6Sn5 (g) and Cu3Sn (e) phases. However, bonding time is not the only factor that can influence the formation of intermetallic phases in the joint during the bonding process. By adjusting the interval distance between the two base metal substrates during bonding (which was controlled by the working position of ultrasonic horn), we could also produce joints that consisted of various intermetallic phases (as shown in Fig. 3). When the ultrasonic bonding time was fixed at 3 s, with reducing the interval distance between the two base metal substrates during bonding, the thickness of connection layer hc in the joint decreased from 20 to 12 lm (the Sn foil melted into liquid phase during the bonding process, hence the thickness of the connection layer in the joint was determined by the interval distance between base metal plates). The intermetallic phase of the joint transformed from a homogeneous Cu6Sn5 (g) phase to coexisting Cu6Sn5 (g) and Cu3Sn (e) phases and finally to a homogeneous Cu3Sn (e) phase. This evolution of intermetallic phase can be attributed to the amplification of sonochemical effects induced by changes in the localized cavitation condition. Cavitation erosion on solid surfaces is generally attributed to two principle effects [25]: micro-jets and shock waves. The relative importance of each effect depends on the localized cavitation conditions, such as the bubble radius and the distance between the bubble center and the solid surface [23–25]. The source of impact pressure at the solid surface can be classified into three categories based on L/Rmax [26], where L is the distance between the bubble center and the solid surface and Rmax is the radius of the bubble at its maximum size: (1) When L/Rmax < 0.3 and >1.5, the shock wave is dominant. (2) When 0.6 < L/Rmax < 0.8, the liquid jet is dominant. (3) When 0.3 < L/Rmax < 0.6 and 0.8 < L/Rmax < 1.5, the shock wave and the liquid are equal. The actual radius of the bubble at its maximum size (Rmax) is difficult to measure in the liquid Sn interlayer during the bonding process. However, according to previous studies on the approximate size of cavitation bubbles, the bubble size can usually be inferred from the ultrasonic frequency [27]. For the ultrasonic frequency of 20 kHz employed in our work, the bubble should have an estimated radius of 5 lm [28,29]. For the distance L between the bubble center and the solid surface, we can assume that the bubble center is localized at the midpoint between the inner surfaces of the two base metals. The interval distance between the two Cu substrates can be approximated to the eventual thickness of connection layer in the bonded joint, which ranged from 20 to 12 lm, therefore, L is between 5 and 1 lm. This means that L/Rmax is between 1 and 0.2 in our study, which indicates that, for the joints fabricated in our study, the mechanism of cavitation erosion at the liquid Sn/solid Cu interface gradually transformed from being dominated by the impact of micro-jets to being dominated by the impact of shock waves. For the acoustic cavitation with a frequency of 20 kHz in water, the impact pressure exerted by the micro-jet at the impact zone would be 0.225 GPa with an average velocity of the micro-jet hitting the solid surface of
150/m s 1 [24]. However, shock wave pressure estimated from single bubble sonoluminescence was in the range of 4–6 GPa with an impacting velocity of 4000 m s 1 [30]. Obviously, greater damage effects would result from the impact of shock waves than from the impact of micro-jets. This explains the reason for the gradual expansion of cavitation erosion pits on the Cu substrate surfaces with decreasing of the interval distance between base metals (as shown in Fig. 3a–d). As more amplified damage effects generated on the Cu substrate surfaces caused by changes in localized cavitation condition, larger amounts of Cu were released into the liquid melt interlayer during the bonding process, which increased the concentration ratio of Cu in the Cu/Sn reacting system and promoted the transformation of the intermetallic phase from a homogeneous Cu6Sn5 (g) phase to coexisting Cu6Sn5 (g) and Cu3Sn (e) phases and finally to a homogeneous Cu3Sn (e) phase. 4. Conclusion In summary, we demonstrated ultrarapid bonding in a Cu/Sn foil/Cu system at room temperature using ultrasonic waves. Our process yielded homogeneous Cu6Sn5 and Cu3Sn joints with high melting points, which are especially suitable for electronic systems operated at elevated temperatures. Ultrasonic vibration served as a heating source to rapidly melt the solder interlayer and subsequent sonochemical effects induced by acoustic cavitation at the interface between the liquid Sn and the solid Cu dramatically accelerate the melting diffusion kinetics. This efficient joining method is a promising technique for improving electronic packaging. Acknowledgement This work was supported by National Nature Science Foundation of China under Grant No. 51175116. References [1] Y. Li, C.P. Wong, Recent advances of conductive adhesives as a lead-free alternative in electronic packaging: materials, processing, reliability and applications, Mater. Sci. Eng. R 51 (2006) 1–35. [2] C. Buttay, D. Planson, B. Allard, D. Bergogne, P. Bevilacqua, C. Joubert, M. Lazar, C. Martin, H. Morel, D. Tournier, C. Raynaud, State of the art of high temperature power electronics, Mater. Sci. Eng. B 176 (2011) 283–288. [3] W. Zhang, W. Ruythooren, Study of the Au/In reaction for transient liquidphase bonding and 3D chip stacking, J. Electron. Mater. 37 (2008) 1095–1101. [4] S.M. Hong, C.C. Bartlow, T.B. Reynolda, J.T. McKeown, A.M. Glaeser, Ultrarapid transient-liquid-phase bonding of Al2O3 ceramics, Adv. Mater. 20 (2008) 4799–4803. [5] W.D. MacDonald, T.W. Eagar, Transient liquid phase bonding, Annu. Rev. Mater. Sci. 22 (1992) 23–46. [6] Y. Zhou, W.F. Gale, T.H. North, Modelling of transient liquid phase bonding, Int. Mater. Rev. 40 (1995) 181–196. [7] G.O. Cook, C.D. Sorensen, Overview of transient liquid phase and partial transient liquid phase bonding, J. Mater. Sci. 46 (2011) 5305–5323. [8] N.S. Bosco, F.W. Zok, Critical interlayer thickness for transient liquid phase bonding in the Cu–Sn system, Acta Mater. 52 (2004) 2965–2972. [9] J.F. Li, P.A. Agyakwa, C.M. Johnson, Kinetics of Ag3Sn growth in Ag–Sn–Ag system during transient liquid phase soldering process, Acta Mater. 58 (2010) 3429–3443. [10] J.F. Li, P.A. Agyakwa, C.M. Johnson, Interfacial reaction in Cu/Sn/Cu system during the transient liquid phase soldering process, Acta Mater. 59 (2011) 1198–1211. [11] P.R. Gogate, R.K. Tayal, A.B. Pandit, Cavitation: a technology on the horizon, Curr. Sci. INDIA 91 (2006) 35–46. [12] P.R. Gogate, V.S. Sutkar, A.B. Pandit, Sonochemical reactor: important design and scale up considerations with a special emphasis on hetergeneous systems, Chem. Eng. J. 166 (2011) 1066–1082. [13] R.K. Chinnam, C. Fauteux, J. Neutenschwander, J. Janczak-Rusch, Evolution of the microstructure of Sn–Ag–Cu solder joints exposed to ultrasonic waves during solidifacation, Acta Mater. 59 (2011) 1474–1481. [14] D.G. Shchukin, S. Ekaterina, V. Belova, H. Möhwald, Ultrasonic cavitation at solid surfaces, Adv. Mater. 23 (2011) 1922–1934. [15] K.S. Suslick, D.J. Flannigan, Inside a collapsing bubble: sonoluminescence and the conditions during cavitation, Annu. Rev. Phys. Chem. 59 (2008) 659–683.
Z. Li et al. / Ultrasonics Sonochemistry 21 (2014) 924–929 [16] N.P. Vichare, P.S. Sentchilkumar, V.S. Moholkar, P.R. Gogate, A.B. Pandit, Energy analysis in acoustic cavitation, Ind. Eng. Chem. Res. 39 (2000) 1480–1486. [17] K.S. Suslick, D.A. Hammerton, R.E. Cline, The sonochemical hot spot, J. Am. Chem. Soc. 108 (1986) 5641–5642. [18] K.S. Suslick, Y. Didenko, M.M. Fang, T. Hyeon, K.J. Kolbeck, W.B. Mdleleni, M.M. McNamara, M. Wong, Acoustic cavitation and its chemical consequences, Philos. Trans. R. Soc. A 357 (1999) 335–353. [19] E.B. Flint, K.S. Suslick, The temperature of cavitation, Science 253 (1991) 1397– 1399. [20] K.S. Suslick, G.J. Price, Application of ultrasound to materials chemistry, Annu. Rev. Mater. Sci. 29 (1999) 295–326. [21] N.P. Vichare, P.R. Gogate, V.Y. Dindore, A.B. Pandit, Mixing time analysis of a sonochemical reactor, Ultrason. Sonochem. 8 (2001) 23–33. [22] P.R. Gogate, A.B. Pandit, Engineering design method for cavitational reactors: 1. Sonochemical reactors, AIChE J. 46 (2000) 372–379. [23] X.G. Chen, J.C. Yan, F. Gao, J.H. Wei, Z.W. Xu, G.H. Fan, Interaction behaviors at the interface between liquid Al–Si and solid Ti-6Al-4V in ultrasonic-assisted brazing in air, Ultrason. Sonochem. 20 (2013) 144–154.
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[24] M. Virot, T. Chave, S.I. Nikitenko, D.G. Shchukin, T. Zemb, H. Möhwald, Acoustic cavitation at the water–glass interface, J. Phys. Chem. C 114 (2010) 13083– 13091. [25] A. Karimi, J.L. Martin, Cavitation erosion of materials, Int. Met. Rev. 31 (1986) 1–26. [26] A. Shima, K. Takayama, Y. Tomita, N. Miura, An experimental study on effects of a solid wall on the motion of bubbles and shock waves in bubble collapse, Acoustica 48 (1981) 293–301. [27] A. Brotchie, F. Grieser, M. Ashokkumar, Effect of power and frequency on bubble-size distributions in acoustic cavitation, Phys. Rev. Lett. 102 (2009) 084302. [28] P. Kanthale, M. Ashokkumar, F. Grieser, Sonoluminescence, sonochemistry (H2O2 yield) and bubble dynamics: frequency and power effects, Ultrason. Sonochem. 15 (2008) 143–150. [29] M. Ashokkumar, The characterization of acoustic cavitation bubbles–an overview, Ultrason. Sonochem. 18 (2011) 864–872. [30] R. Pecha, B. Gompf, Microimplosions: cavitation collapse and shock wave emission on a nanosecond time scale, Phys. Rev. Lett. 84 (2000) 1328–1330.
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Short Communication
Ultrarapid formation of homogeneous Cu6Sn5 and Cu3Sn intermetallic compound joints at room temperature using ultrasonic waves Zhuolin Li a, Mingyu Li a,b,⇑, Yong Xiao a, Chunqing Wang b,⇑ a b
Shenzhen Key Laboratory of Advanced Materials, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 29 September 2013 Accepted 29 September 2013 Available online 17 October 2013 Keywords: Ultrasonic waves Intermetallic compounds Cavitation Erosion
a b s t r a c t Homogeneous intermetallic compound joints are demanded by the semiconductor industry because of their high melting point. In the present work, ultrasonic vibration was applied to Cu/Sn foil/Cu interconnection system at room temperature to form homogeneous Cu6Sn5 and Cu3Sn joints. Compared with other studies based on transient-liquid-phase soldering, the processing time of our method was dramatically reduced from several hours to several seconds. This ultrarapid intermetallic phase formation process resulted from accelerated interdiffusion kinetics, which can be attributed to the sonochemical effects of acoustic cavitation at the interface between the liquid Sn and the solid Cu during the ultrasonic bonding process. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The semiconductor industry has developed remarkably over the past two decades, however, the essential requirements for interconnections among various types of electronic components have remained unchanged. Tin-based solder alloy are most commonly used to form mechanically and electronically reliable joints between electronic components for electronic packaging [1]. However, conventional joining methods and materials often fail to meet the requirements of electronic systems that operate at elevated temperatures (e.g., 400 °C encountered by power electronics used for space exploration [2]). Such high temperatures will remelt most common solder joints. Furthermore, for some advanced integration technologies such as 3D chip-stacking packaging, highmelting-point joints are required to enable repeated multilevel 3D stacking of additional layers without remelting of the joints at lower levels [3]. Traditional efforts to increase the maximum operating temperature of joints employ joining materials with higher melting points. Unfortunately, increasing the joining temperature increases the likelihood of inducing microstructural changes that degrade the material properties [4]. Recently, an alternative transient-liquid-phase (TLP) bonding process was developed that can be performed at relatively low temperatures, resulting in higher melting temperatures of the joints [4–10]. A TLP joint can be formed by sandwiching a thin interlayer, ⇑ Corresponding authors at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China. Tel.: +86 755 26033463; fax: +86 755 26033504 (M. Li). Tel.: +86 451 86418725; fax: +86 451 86416186 (C. Wang). E-mail addresses: [email protected] (M. Li), [email protected] (C. Wang). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.09.020
containing melting point depressants (MPD), between two base metal substrates and heating the entire assembly to melt the interlayer. When the processing time is extended, the MPD diffuses into the surrounding base metal, which results in isothermal solidification until no trace of the melted liquid phase. Ideally the joint becomes homogeneous consisting of intermetallic compounds (IMCs) or solid solution of the interlayer metal in the base metal, imparting a melting point higher than the bonding temperature. However, an inevitable drawback of TLP bonding is that it often requires a long processing time of up to several hours [4,7], which may lead to extra thermal stress on the bond components, which can seriously affect the reliability of the packaging system. Therefore, the development of a new bonding method that can form high-melting-point intermetallic compound joints at low temperature using a short bonding time is highly desirable. The use of ultrasound for the intensification of chemical/physical processing applications has been well established [11]. Through the chemical and physical effects of acoustically-induced microbubble formation and collapse (acoustic cavitation) that can occur simultaneously at millions of locations in a reactor, conditions of very high temperatures and pressures (few thousand atmosphere pressure and few thousand Kelvin temperature) can be created locally, although the overall environment remains at ambient conditions [11,12]. Thus, chemical reactions that requires stringent conditions can be effectively performed using ultrasound at ambient temperature. Moreover, acoustic cavitation can generate numerous spectacular effects (e.g., acoustic streaming, shock waves and micro-jets) that can increase mass transportation to accelerate chemical reaction [13]. In the present work, we used a Cu/Sn foil/Cu interconnection system similar to the ones used in
Z. Li et al. / Ultrasonics Sonochemistry 21 (2014) 924–929
previous TLP bonding studies, while applying ultrasonic waves to rapidly form homogeneous Cu6Sn5 and Cu3Sn intermetallic compound joints at room temperature. The ultrasonic waves enable the formation of intermetallic compound joints by two mechanisms. First, the vibration of the ultrasonic horn induces friction at the interface between the solid solder and the metal substrate, this effect serves as a frictional heating source to melt the solder foil into a liquid phase during the bonding process. Second, the propagation of the ultrasonic waves in the molten foil causes acoustic cavitation that intensifies the interfacial bonding reaction between the liquid solder and the solid metal substrate, which enables rapid formation of intermetallic compound joints. 2. Experiment 2.1. Materials Schematics for the sandwich Cu/Sn foil/Cu interconnection system and ultrasonic bonding are shown in Fig. 1a and b. The interlayer is one piece of pure Sn foil with a thickness of 25 lm. The base metal substrates are two pieces of pure Cu plate with a thickness of 0.3 mm. The Sn foil and the Cu substrates were cut into 3 3 mm2 pieces and were manually flattened using a level press. The Cu/Sn foil/Cu system was chosen as a joining couple because it is widely used for electronic packaging interconnections. 2.2. Experimental process The Sn foil was sandwiched between two Cu substrates and placed in an ultrasonic bonder with a horn-type reactor as shown in Fig. 1b, the bonding process was performed at ambient temperature, using pressure and ultrasonic vibration. The frequency and power of the ultrasonic bonder were 20 kHz and 750 W, respectively. The bonding pressure was optimized at 0.6 MPa in order to guarantee good contact between the ultrasonic horn and the
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bonding component without squeezing the solder out of the joint during the bonding process. Joint formation was examined for bonding times that ranged from 1 to 4 s. 2.3. Characterization To determine the overall temperature profile of the interconnection system during the ultrasonic bonding process, we placed a thermocouple underneath the bottom Cu substrate, and connected it to a thermoelectric-signal collector and a data recording system. Metallographic cross-sections of the bonded samples were prepared for microstructural characterization of the intermetallic phase joints by first mounting the samples in epoxy resin (cured at room temperature for 6 h) and then grinding the samples with 400, 800, 1200, and 2500 grit SiC papers in succession before final polishing in successive diamond slurries of 3, 1 and 0.05 lm (2 min each). A scanning electron microscope (SEM, Hitachi S-4700) using backscattered electron (BSE) signals was used for imaging and analysis of the microstructural features. The intermetallic phases that formed the joints were identified using X-ray diffraction (XRD, Rigaku X-ray diffractometer) analysis. 3. Results and discussion Fig. 1c shows the overall temperature profile of the interconnection system during the ultrasonic bonding process for 3 s and its subsequent cooling procedure. With the onset of ultrasonic bonding, the temperature of the entire assembly increased sharply to 297 °C and then reached a sustained temperature of approximately 277 °C until the bonding time for 3 s ended; the interconnection system was subsequently cooled in air to ambient temperature. Given that the melting point of Sn is 232 °C, a liquid Sn interlayer certainly formed during the bonding process. The initial abrupt increase in temperature can be attributed to frictional heat generated by the interfacial rubbing between the solid solder
Fig. 1. (a and b) Schematics of the Cu/Sn foil/Cu interconnection system and ultrasonic bonding. (c) Overall temperature profile of the interconnection system during the ultrasonic bonding process for 3 s and subsequent cooling.
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and base metal. The thermal effect after the formation of the liquid interlayer can be attributed to the propagation of ultrasonic waves within the liquid interlayer that induces acoustic cavitation, where bubble collapse results in an enormous concentration of energy due to the conversion of the surface energy and the kinetic energy of liquid motion into heat and chemical energy [14–16]. This energy conversion was sufficient to sustain the bonding temperature. The cross-sectional BSE images and XRD analyses of the joints formed by ultrasonic bonding at times that ranged from 1 to 4 s are shown in Fig. 2. After 1 s of the ultrasonic bonding time, the resulted joints contained both residual solder and intermetallic compounds. XRD analysis revealed that the intermetallic compounds formed a Cu6Sn5 (g) phase. After 2 s and 3 s of ultrasonic bonding, the connection layers of the joints were observed to be 20 lm
thick, with the Sn interlayer completely consumed and with the homogeneous Cu6Sn5 (g) phase presented as a connection layer. At the boundary between the intermetallic phase and the base metal, the Cu6Sn5 phase advanced into the Cu base metal, forming pot-shaped damage pits on the once-flat Cu surface. An increase in the ultrasonic bonding time to 4 s, resulted in the joint presented coexisting Cu6Sn5 (g) and Cu3Sn (e) phases. This process produced structures similar to those reported in a previous study on TLP soldering of the Cu/Sn foil/Cu system [10], where a much longer processing time (at least of 90 min) was necessary to form the intermetallic compound joints as those formed in current work within several seconds. This ultrarapid formation of intermetallic phases can be wholly attributed to the sonochemical effects induced by acoustic cavitation at the liquid/solid interface.
Fig. 2. Cross-sectional BSE images and XRD analyses of ultrarapidly formed joints with bonding times that ranged from 1 to 4 s. (a–d) An increase in the ultrasonic bonding time resulted in the formation a series of joints consisting of coexisting Sn solder and Cu6Sn5 (g) phases, a homogeneous Cu6Sn5 (g) phase, and coexisting Cu6Sn5 (g) and Cu3Sn (e) phases. (e) XRD spectra of the intermetallic phases.
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Propagation of ultrasonic waves in the liquid interlayer generated acoustic cavitation, which is a phenomenon involving the formation, growth and rapid implosive collapse of a vapor-filled microbubble. The sonochemical effects in our study were primarily derived from acoustic cavitation. Rapid bubble implosion can induce tiny localized hot spots that reach extreme temperatures and pressures estimated to be on the order of 5000 K and 0.1 GPa, respectively [17–22]. However, the hot spots are so small that heat dissipation occurs within a short period of 2 ls [17]. Therefore, only bubbles that implode adjacent to the interface can generate effects on the solid surface. In our study, acoustic cavitation was confined to a thin liquid solder interlayer, which means that bubble collapse occurred in the vicinity of either base metal surface. When bubble is close to the base metal surface, the liquid motion in its vicinity is hindered, leading to micro-jet phenomenon
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[23]. In addition, bubble collapse leads to the emission of a shock wave with a pressure of several GPa and a starting shock velocity of 4000 m s 1 [24]. The combined effect of liquid-solder microjets, shock waves and localized high temperatures can produce microscale damages (pits) on the base metal surface. This phenomenon is known as cavitation erosion [25]. In our study, the cavitation erosion process released an excessive amount of Cu from the base metal into the molten Sn during bonding. As the released Cu particles entered the acting region of imploding bubbles, they instantly dissolved into the molten Sn due to the high temperature. Although single cavity collapse lasts for only several microseconds, millions of bubbles confined in the thin layer of molten Sn form and implod continuously; therefore, the liquid Sn layer was maintained in a dynamically unequilibrium state with supersaturation of Cu. Upon subsequent cooling, intermetallic compound nuclei rapidly precipitated and grew as a result of reaction crystallization,
Fig. 3. Cross-sectional BSE images and XRD analyses of the joints formed by ultrasonic bonding for 3 s with the interval distances between Cu substrates that ranged from 20 to 12 lm. (a–d) An decrease in the interval distance between Cu substrates resulted in the formation a series of joints consisting of a homogeneous Cu6Sn5 (g) phase, coexisting Cu6Sn5 (g) and Cu3Sn (e) phases and a homogeneous Cu3Sn (e) phase. (e) XRD spectra of the intermetallic phases.
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driven by the supersaturation of Cu in the liquid solder interlayer. Eventually joint was bonded by the formation of intermetallic phases. These sonochemical effects result in the ultrarapid formation of intermetallic compound joints. The amount of Cu released into the liquid Sn interlayer during the bonding process is in direct proportional to the bonding time. An increase in the bonding time resulted in more mass transportation of Cu metal and increased the concentration ratio of Cu in the Cu/Sn reaction system. Therefore, with the bonding time increased from 2 to 4 s, we observed that the intermetallic phase of the joint transformed from a homogeneous Cu6Sn5 (g) phase to coexisting Cu6Sn5 (g) and Cu3Sn (e) phases. However, bonding time is not the only factor that can influence the formation of intermetallic phases in the joint during the bonding process. By adjusting the interval distance between the two base metal substrates during bonding (which was controlled by the working position of ultrasonic horn), we could also produce joints that consisted of various intermetallic phases (as shown in Fig. 3). When the ultrasonic bonding time was fixed at 3 s, with reducing the interval distance between the two base metal substrates during bonding, the thickness of connection layer hc in the joint decreased from 20 to 12 lm (the Sn foil melted into liquid phase during the bonding process, hence the thickness of the connection layer in the joint was determined by the interval distance between base metal plates). The intermetallic phase of the joint transformed from a homogeneous Cu6Sn5 (g) phase to coexisting Cu6Sn5 (g) and Cu3Sn (e) phases and finally to a homogeneous Cu3Sn (e) phase. This evolution of intermetallic phase can be attributed to the amplification of sonochemical effects induced by changes in the localized cavitation condition. Cavitation erosion on solid surfaces is generally attributed to two principle effects [25]: micro-jets and shock waves. The relative importance of each effect depends on the localized cavitation conditions, such as the bubble radius and the distance between the bubble center and the solid surface [23–25]. The source of impact pressure at the solid surface can be classified into three categories based on L/Rmax [26], where L is the distance between the bubble center and the solid surface and Rmax is the radius of the bubble at its maximum size: (1) When L/Rmax < 0.3 and >1.5, the shock wave is dominant. (2) When 0.6 < L/Rmax < 0.8, the liquid jet is dominant. (3) When 0.3 < L/Rmax < 0.6 and 0.8 < L/Rmax < 1.5, the shock wave and the liquid are equal. The actual radius of the bubble at its maximum size (Rmax) is difficult to measure in the liquid Sn interlayer during the bonding process. However, according to previous studies on the approximate size of cavitation bubbles, the bubble size can usually be inferred from the ultrasonic frequency [27]. For the ultrasonic frequency of 20 kHz employed in our work, the bubble should have an estimated radius of 5 lm [28,29]. For the distance L between the bubble center and the solid surface, we can assume that the bubble center is localized at the midpoint between the inner surfaces of the two base metals. The interval distance between the two Cu substrates can be approximated to the eventual thickness of connection layer in the bonded joint, which ranged from 20 to 12 lm, therefore, L is between 5 and 1 lm. This means that L/Rmax is between 1 and 0.2 in our study, which indicates that, for the joints fabricated in our study, the mechanism of cavitation erosion at the liquid Sn/solid Cu interface gradually transformed from being dominated by the impact of micro-jets to being dominated by the impact of shock waves. For the acoustic cavitation with a frequency of 20 kHz in water, the impact pressure exerted by the micro-jet at the impact zone would be 0.225 GPa with an average velocity of the micro-jet hitting the solid surface of
150/m s 1 [24]. However, shock wave pressure estimated from single bubble sonoluminescence was in the range of 4–6 GPa with an impacting velocity of 4000 m s 1 [30]. Obviously, greater damage effects would result from the impact of shock waves than from the impact of micro-jets. This explains the reason for the gradual expansion of cavitation erosion pits on the Cu substrate surfaces with decreasing of the interval distance between base metals (as shown in Fig. 3a–d). As more amplified damage effects generated on the Cu substrate surfaces caused by changes in localized cavitation condition, larger amounts of Cu were released into the liquid melt interlayer during the bonding process, which increased the concentration ratio of Cu in the Cu/Sn reacting system and promoted the transformation of the intermetallic phase from a homogeneous Cu6Sn5 (g) phase to coexisting Cu6Sn5 (g) and Cu3Sn (e) phases and finally to a homogeneous Cu3Sn (e) phase. 4. Conclusion In summary, we demonstrated ultrarapid bonding in a Cu/Sn foil/Cu system at room temperature using ultrasonic waves. Our process yielded homogeneous Cu6Sn5 and Cu3Sn joints with high melting points, which are especially suitable for electronic systems operated at elevated temperatures. Ultrasonic vibration served as a heating source to rapidly melt the solder interlayer and subsequent sonochemical effects induced by acoustic cavitation at the interface between the liquid Sn and the solid Cu dramatically accelerate the melting diffusion kinetics. This efficient joining method is a promising technique for improving electronic packaging. Acknowledgement This work was supported by National Nature Science Foundation of China under Grant No. 51175116. References [1] Y. Li, C.P. Wong, Recent advances of conductive adhesives as a lead-free alternative in electronic packaging: materials, processing, reliability and applications, Mater. Sci. Eng. R 51 (2006) 1–35. [2] C. Buttay, D. Planson, B. Allard, D. Bergogne, P. Bevilacqua, C. Joubert, M. Lazar, C. Martin, H. Morel, D. Tournier, C. Raynaud, State of the art of high temperature power electronics, Mater. Sci. Eng. B 176 (2011) 283–288. [3] W. Zhang, W. Ruythooren, Study of the Au/In reaction for transient liquidphase bonding and 3D chip stacking, J. Electron. Mater. 37 (2008) 1095–1101. [4] S.M. Hong, C.C. Bartlow, T.B. Reynolda, J.T. McKeown, A.M. Glaeser, Ultrarapid transient-liquid-phase bonding of Al2O3 ceramics, Adv. Mater. 20 (2008) 4799–4803. [5] W.D. MacDonald, T.W. Eagar, Transient liquid phase bonding, Annu. Rev. Mater. Sci. 22 (1992) 23–46. [6] Y. Zhou, W.F. Gale, T.H. North, Modelling of transient liquid phase bonding, Int. Mater. Rev. 40 (1995) 181–196. [7] G.O. Cook, C.D. Sorensen, Overview of transient liquid phase and partial transient liquid phase bonding, J. Mater. Sci. 46 (2011) 5305–5323. [8] N.S. Bosco, F.W. Zok, Critical interlayer thickness for transient liquid phase bonding in the Cu–Sn system, Acta Mater. 52 (2004) 2965–2972. [9] J.F. Li, P.A. Agyakwa, C.M. Johnson, Kinetics of Ag3Sn growth in Ag–Sn–Ag system during transient liquid phase soldering process, Acta Mater. 58 (2010) 3429–3443. [10] J.F. Li, P.A. Agyakwa, C.M. Johnson, Interfacial reaction in Cu/Sn/Cu system during the transient liquid phase soldering process, Acta Mater. 59 (2011) 1198–1211. [11] P.R. Gogate, R.K. Tayal, A.B. Pandit, Cavitation: a technology on the horizon, Curr. Sci. INDIA 91 (2006) 35–46. [12] P.R. Gogate, V.S. Sutkar, A.B. Pandit, Sonochemical reactor: important design and scale up considerations with a special emphasis on hetergeneous systems, Chem. Eng. J. 166 (2011) 1066–1082. [13] R.K. Chinnam, C. Fauteux, J. Neutenschwander, J. Janczak-Rusch, Evolution of the microstructure of Sn–Ag–Cu solder joints exposed to ultrasonic waves during solidifacation, Acta Mater. 59 (2011) 1474–1481. [14] D.G. Shchukin, S. Ekaterina, V. Belova, H. Möhwald, Ultrasonic cavitation at solid surfaces, Adv. Mater. 23 (2011) 1922–1934. [15] K.S. Suslick, D.J. Flannigan, Inside a collapsing bubble: sonoluminescence and the conditions during cavitation, Annu. Rev. Phys. Chem. 59 (2008) 659–683.
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