Cu system via a micro-resistance spot welding process

Journal of Alloys and Compounds 687 (2016) 667e673

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Extremely fast formation of CueSn intermetallic compounds in Cu/Sn/ Cu system via a micro-resistance spot welding process Baolei Liu a, Yanhong Tian a, *, Chenxi Wang a, Rong An a, b, Yang Liu a a b

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China Key Laboratory of Micro-Systems and Micro-Structures Manufacturing, Ministry of Education, Harbin Institute of Technology, Harbin, 150080, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 March 2016 Received in revised form 22 May 2016 Accepted 20 June 2016 Available online 22 June 2016

High-temperature-stable circuit interconnects are highly desirable for the wide band-gap semiconductor devices to operate in harsh environment (>200
C). In this study, a full Cu3Sn interconnect with the melting point of 679
C was fabricated by using a micro-resistance spot welding process in an extremely short time of 200 ms and under a low pressure of 0.08 MPa at ambient temperature in Cu/Sn (30 mm)/Cu interconnection system. The microstructure evolution of CueSn intermetallic compounds indicated that the joule heat-induced temperature coupled with the passage of electric current significantly enhanced the interfacial reaction at the liquid Sn/solid Cu metallization interface. Accompanying with the speedy electromigration of Cu atoms in the molten Sn solder, the columnar dendritic Cu6Sn5 compounds were formed due to constitutional supercooling and then were totally transformed into Cu3Sn compounds. The resulted Cu3Sn interconnect exhibited a higher mechanical strength than Sn-based interconnects, offering this type of interconnect a promising application in high-temperature power electronics. © 2016 Elsevier B.V. All rights reserved.

Keywords: Intermetallics Liquid-solid reactions Microstructure Liquid-solid electromigration High-temperature interconnect

1. Introduction With the trend toward multi-functionalization and miniaturization, the power density and high temperature capabilities of microelectronic devices are undergoing unprecedented growth. Especially for the high-temperature power electronics used in aircrafts, automotive, space exploration, and deep oil/gas extraction, the required operating temperature can be above 200
C [1]. Thus, high-temperature-stable circuit interconnection materials are highly desirable. Although high-melting-point solders such as high Pb-, Bi-, Zn-, Au-based alloys can be used in the high-temperature applications, they all have their own drawbacks such as toxicity, low electric/thermal conductivity, poor oxidation resistance, and high cost [2]. Recently, the high-melting-point interconnects fully consisting of intermetallic compounds (IMCs) like Cu6Sn5 and Cu3Sn have been expected to solve those technological challenges [3e5]. Compared with Cu6Sn5 IMC, Cu3Sn IMC with excellent thermal stability up to 600
C is a more promising candidate due to its higher elastic modulus, fracture toughness, and lower electrical resistivity [6e8]. Transient liquid phase (TLP) bonding and eutectic

* Corresponding author. E-mail address: [email protected] (Y. Tian). http://dx.doi.org/10.1016/j.jallcom.2016.06.184 0925-8388/© 2016 Elsevier B.V. All rights reserved.

bonding processes are the common used technologies for fabricating full IMCs interconnects [8e11]. However, those bonding processes are highly time-consuming (>100 min) for completely consuming Sn solder interlayer and transforming into thermodynamically stable IMC interconnects. Multiple hours of overall annealing at high bonding temperature (260e300
C) would induce extra thermal stress and then reduce the reliability of microelectronic devices. Therefore, the development of fast forming process for IMC interconnects at low temperature is imperative. As a member of micro- or small-scale resistance welding processes, micro-resistance spot welding (MRSW) has been widely used for fabricating high-temperature-stable interconnects in electronic packaging due to the advantages of high speed and low cost [12]. With sufficient joule heat generated at the faying interfaces, bonding with a short holding time is conventionally achieved by melting two target metals such as Cu/Cu, Al/Al and Ni/Ni [13,14]. If a low-melting-point solder interlayer is used for MRSW of base metals (like Cu, Ag, Au and Ni), rapid bonding can be achieved at a temperature lower than the melting point of the targeted base metals. The joule heat-induced temperature coupled with the imposed current will strongly influence the interfacial interaction at the liquid solder/solid metallization interface. Recent investigations of current stressing on Cu/molten Sn/Cu reaction systems were conducted to understand the liquid-solid

668

B. Liu et al. / Journal of Alloys and Compounds 687 (2016) 667e673

electromigration (EM) behaviors by Ma et al. [15] and Liu et al. [16]. The mass transport of solid solute would be enhanced by the electric current-induced EM, creating the possibilities of interconnection materials with diverse chemical structures and properties. The objective of the present study was to fabricate full Cu3Sn IMC interconnects in very short durations (~ms) using a MRSW process at ambient temperature. This study focused on the IMCs microstructure evolution and thermal-electric interaction at the molten solder/solid metallization interface, which was crucial for the transient formation of IMC interconnects. 2. Materials and methods Commercially available 30 mm thick 99.9% pure Sn foil and 500 mm thick 99.99% pure Cu foil were used in this experiment. Both Sn and Cu foils were cut into 2.5  2.5 mm2 pieces. After cleaning in an ultrasonic bath, one piece of Sn foil was sandwiched between two pieces of pure Cu foils as solder interlayer, as shown in Fig. 1. After being aligned by a specially designed clamp, the sandwiched structure was fixed on the bottom anode electrode (3 mm in diameter), and then bonded by a micro-resistance welding system (UNITEK, HF25 inverter system) at ambient temperature. The peak value of DC pulsed current was selected as 875 A. Thus, the average current density flowed through the 2.5  2.5 mm2 Sn interlayer was 1.4  104 A/cm2. The welding current waveform increased to the peak value in 10 ms, held constant for 20e180 ms, and then decreased to zero in 10 ms. The pressure supplied by the upper electrode (2 mm in diameter) was 0.08 MPa. After bonded for 40e200 ms, the samples were aircooled to room temperature. The dotted arrow marked by “e” represented the direction of electron flow. Moreover, the real-time temperatures of the interconnection system were measured by an infrared thermal imaging camera (FLIR, A615). The camera emissivity was set as 0.65, and the temperature data were acquired at a rate of 200 frames per second. To observe the microstructure evolution of CueSn IMCs, the cross-sectional samples was deeply etched with 5 g FeCl3 þ 15 mL vol% HCl þ 85 mL deionized water for 5 s. Then, the microstructures were characterized by scanning electron microscopy (SEM, FEI Quanta 200 FEG) equipped with energy dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD, Bruker D8 Advance), and transmission electron microscopy (TEM, FEI Tecnai G2-F30, 300 KV). TEM specimens were prepared by focused ion beam (FIB, FEI Helios Nanolab 600i). An image processing software was utilized to measure the area of the consumed Cu substrate, and the average thickness was obtained by dividing the area by the line length of the interface. Additionally, the shear strength was measured by multifunctional bond tester (Condor Sigma Lite, XYZTEC) with a loading rate of 200 mm/s.

3. Results 3.1. Rapidly increased bonding temperature induced by joule heating effect Fig. 2 shows the overall temperature profile of the Cu/Sn/Cu interconnection system during the MRSW process for 200 ms. Under the bonding current density of 1.4  104 A/cm2, the joule heat-induced temperature increased to the Sn melting point (232
C) within 40 ms, and then to the peak value of 426.5
C for 150 ms. The sufficient joule heat generation was attributed to the larger contact resistances at Cu/Sn and electrode/Cu interfaces. Note that the measured peak temperature, 426.5
C, was between the melting points of Cu6Sn5 (415
C) and Cu3Sn (679
C) [16]. Thus, the resulted interconnects with different microstructures were anticipated to be obtained after different durations of metallurgical reaction between solid Cu and molten Sn. Moreover, no flux was used in this experiment. This was because the strong electric current can not only generate sparking plasma at Cu/Sn interfaces to remove surface oxide layers [17] but also improved the wetting behavior of molten Sn on Cu substrates [18]. 3.2. Microstructures of CueSn IMCs under different bonding times Fig. 3 shows the cross-sectional SEM images of the deep-etched Cu/Sn/Cu interconnect after 40 ms. Combined with XRD analysis in Fig. 3d, Sn and h-Cu6Sn5 phases were both detected at the cathode and anode Cu/Sn interfaces. As shown in Fig. 3a, the long rod-like

Fig. 2. Overall temperature profile of the Cu/Sn/Cu interconnection system during the MRSW process.

Fig. 1. Schematic illustration of the micro-resistance spot welding (MRSW) process and the Cu/Sn/Cu interconnection system.

B. Liu et al. / Journal of Alloys and Compounds 687 (2016) 667e673

669

Fig. 3. Cross-sectional SEM images of the deep-etched Cu/Sn/Cu interconnect under the MRSW process for 40 ms. (a) cross-sectional SEM image; (b) and (c) are the magnified morphologies at the cathode and anode interfaces in (a), respectively; (d) XRD analysis of consisting phases.

Cu6Sn5 layer with the average length of 5.5 mm formed at the cathode interface, whereas a less than 2.0 mm longitudinal Cu6Sn5 layer with short rod-like type formed at the anode interface. Besides, as shown in Fig. 3b and c, the cathode Cu substrate was more serrated than the anode. That indicated more Cu atoms at cathode side were dissolved into the molten Sn solder under the upward electron flow. With the passage of electron flow, the enhanced dissolution of Cu cathode in a Cu/molten Sn-based solder/Cu interconnection system has also been reported by Huang et al. [15,19]. Since the Cu was the dominant diffusion species in Sn or Snbased solder [20], the dissolved Cu atoms form the cathode were then diffused into the molten Sn solder under the electric currentinduced electromigration (EM). When the Cu solute concentration around the cathode interface reached a certain level within 40 ms, masses of long rod-like Cu6Sn5 compounds were precipitated out. With the bonding time increasing to 80 ms, joule heat-induced temperature accordingly rose up to 300
C. As shown in Fig. 4a, a certain number of columnar dendritic Cu6Sn5 compounds formed and grew even throughout the Sn interlayer. After 120 ms, the number and the size of dendritic h-Cu6Sn5 grains both increased with the bonding time extending (Fig. 4b). In previous literature, the Cu6Sn5 compound with kinds of morphologies such as layertype [10], scallop-like, long needle-like [21], hollow hexagonal tubular [22], and prism-type [23] have been reported. However, there were a few literature reported the fascinating Cu6Sn5 grains with columnar dendritic morphology [24]. For further increasing the bonding time to 160 ms, the joule heat-induced temperature increased to the peak value of 425
C. Coupled with continuous dissolution of Cu, the composition and morphology of CueSn IMCs were both changed, as shown in Fig. 4c. Based on the CueSn phase diagram and the EDS analysis in Fig. 4d, the darker CueSn IMCs in Fig. 4c were confirmed as Cu3Sn compounds. The dendritic Cu3Sn compounds with denser secondary dendrite arms were almost filled the areas of Sn solder interlayer. The formation mechanism of columnar dendritic CueSn IMCs needs further investigations.

Fig. 5 shows a nearly full Cu3Sn interconnect with few voids after bonded for 200 ms. The Sn interlayer as well as the h-Cu6Sn5 compounds were completely consumed (Fig. 5a). It was also noted that two ultrathin Cu-rich IMC layers formed and adhered well on the Cu substrates, as shown in the magnified image of Fig. 5b and the TEM image of Fig. 5c. Based on the diffraction patterns in Fig. 5d and e, the super-lattice ε-Cu3Sn phase with orthorhombic structure and d-Cu41Sn11 phase with cubic structure were confirmed respectively. The Cu41Sn11 compound was also a high-temperature stable phase up to 590
C, which formed by reacting with Cu3Sn phase (9Cu þ 11Cu3Sn / Cu41Sn11) [25,26]. In addition, under the TLP processes or reflowing conditions, the formation of Cu3Sn interconnects was usually accompanied by the formation of numerous microvoids due to the unbalanced diffusion flux of Cu and Sn across Cu3Sn [8,27,28]. However, there were few microvoids appeared within the Cu3Sn layer under the MRSW process. This was probably because that the Cu3Sn formation mechanism under the MRSW process was different form that under TLP bonding process, as discussed below.

3.3. Shearing properties of the as-fabricated interconnects The dramatic microstructure evolution under different bonding times influenced the mechanical properties of the resulted interconnects. The following shear test results in Fig. 6a shows that shear forces increase with the bonding times. Inversely, the shear displacements decrease with the bonding times. This implied that the increase in the volume fraction of CueSn IMCs in the solder interlayer caused the interconnects to embrittle, resulting in higher shear strength and lower fracture strain. The shear strength data in Fig. 6b shows that the Cu3Sn interconnect exhibited highest shear strength of 71.1 MPa (200 ms), which was approximately 2.5 times higher than that of the Sn-based solder interconnect bonded for 40 ms (28.8 MPa). The shear strength of the Cu3Sn interconnect under the MRSW process was relatively higher than that of the Cu3Sn interconnect fabricated by an ultrasonic bonding process

670

B. Liu et al. / Journal of Alloys and Compounds 687 (2016) 667e673

Fig. 4. Cross-sectional SEM images of the deep-etched Cu/Sn/Cu interconnect under the MRSW process for different times: (a) 80 ms, (b) 120 ms, and (c) 160 ms; (d) EDS spectrum along the line in the inset of magnified image of (c).

Fig. 5. A nearly full Cu3Sn interconnect formed after 200 ms. (a) cross-sectional SEM image of the nearly full Cu3Sn interconnect; (b) the magnified image in (a); (c) TEM image at the white rectangular region of (b); (d) and (e) are the typical diffraction patterns of Cu3Sn and Cu41Sn11, respectively.

(65.8 MPa) [29]. The excellent shear property of the as-fabricated Cu3Sn interconnect was probably attributed to that there were few microvoids in the Cu3Sn layer. Fig. 6cee show the fractographic morphologies at the cathode side of the interconnects bonded for 40, 120, 200 ms. For the interconnects bonded for 40 ms, the Cu6Sn5 and Sn phases were divided over a wide area at both of the cathode and anode side. Thus, the fracture mostly propagated along the interface between Sn and Cu6Sn5 in the Sn-based solder interconnects (Fig. 6c). As reported in previous literature, serious stress concentration was

generated around the Sn/Cu6Sn5 interface during shearing due to the difference of material properties between Sn and Cu6Sn5 [29e31]. For the interconnect with dendritic CueSn IMCs, the fatigue crack mainly occurred around the anode side. As shown in Fig. 6d, the tips of the dendritic Cu6Sn5 at cathode side was broke. Moreover, the voids in the residual Sn solder also played an important role in the nucleation and propagation of cracks. For the Cu3Sn interconnect, the failure mainly occurred inside of the Cu3Sn layer. The cleavage-like morphology with broken Cu3Sn grains was observed in Fig. 6e. Even though Cu3Sn compound is brittle in

B. Liu et al. / Journal of Alloys and Compounds 687 (2016) 667e673

671

Fig. 6. Shear test results of the as-fabricated interconnects under different bonding times showing (a) the shear force versus displacement curves, (b) the corresponding shear strength, and fractographic morphologies at cathode side with the bonding times of (c) 40 ms, (d) 120 ms, (e) 200 ms.

nature (elastic modulus, 108.3 MPa), the Cu3Sn interconnect exhibited a more excellent mechanical property than the Sn-based solder interconnect. 4. Discussion 4.1. Asymmetrical dissolution of Cu substrates under current stressing The results from the extremely fast formation of CueSn IMCs with dendritic morphologies revealed that the joule heat-induced temperature coupled with the passage of strong electric current both significantly influenced the solid/liquid interfacial reaction. When the strong electric current flew through the conductive Cu/ Sn/Cu interconnect, joule heat-induced temperature increased above Sn melting point in a short time (~ms). Meanwhile, both of the Cu dissolution and diffusion were promoted with the passage of electric current in the Cu/molten Sn/Cu system. One basic requirement for the formation of the full Cu3Sn interconnect was the sufficient supplies of Cu atoms during the bonding process. The consumed thicknesses of the Cu substrates in Fig. 7 show that the consumed thickness of Cu at the cathode is clearly larger than that at anode. The thickness of the consumed Cu anode before 80 ms was hard to be measured because the Cu dissolution at anode was quite small. After bonded for 200 ms, the consumed thickness of Cu at cathode was 11.5 mm, which was approximately three times larger than that at anode (3.9 mm). Unlike the solid-state EM, there was no back-stress in liquid solder [19,32]. Thus, the mass flux of Cu species, JCu, in the Cu/ molten Sn/Cu reaction system can be expressed as [33,34]:

JCu ¼ Jchem þ Jem ¼ DCu

dC D þ C Cu z*erj; dt kT

(1)

where DCu is the Cu diffusivity in molten Sn, z* is the effective charge of Cu, e is the charge of an electron, r is the resistivity of Sn, j

Fig. 7. Consumed thickness of Cu at the cathode and anode versus the bonding times.

is the current density, k is the Boltzmann constant, T is the absolute temperature. In Eq. (1), the term Jchem is the chemical diffusion term, whereas the term Jem is the drift term caused by liquid-solid cathode ) and EM. The Cu atomic flux flowing out of the cathode (JCu anode ) can be written as follow, respectively, anode (JCu chem;cathode cathode em JCu ¼ JCu þ JCu ;

(2)

chem;anode anode em ¼ JCu  JCu ; JCu

(3)

chem;cathode chem;anode where JCu and JCu are the Cu atomic flux induced by chemical potential gradient at cathode and anode, respectively. Eqs.

672

B. Liu et al. / Journal of Alloys and Compounds 687 (2016) 667e673

Fig. 8. Magnified SEM images and corresponding schematic diagrams illustrating the CueSn IMCs microstructure evolution during the MRSW process for different times: (a) 40 ms, (b) 80 ms, (c) 120 ms, (d) 160 ms, (e) 200 ms.

(2) and (3) show that the Cu atomic flux flowing out of the cathode cathode ) was larger than that of the anode (J anode ). This was the (JCu Cu reason for the asymmetrical dissolution of Cu at cathode and anode. Accompanying the rapid dissolution of the Cu cathode was the formation of CueSn IMCs. The Cu flux in the molten Sn solder can be considered as the dominating contributor to the raid growth of CueSn IMCs, as Cu was the dominant diffusing specie in molten Sn solder [35]. After 200 ms, the consumed thickness of Cu at the cathode and anode (XCu) totaled 15.4 mm. As 1 mol of Cu3Sn formed in the Cu/Sn/Cu interconnect needs 3 mol of the dissolved Cu atoms, the theoretical thickness of Cu3Sn (XCu3Sn) formed after 200 ms can be expressed as:

XCu3Sn ¼

XCu rCu MCu3Sn ; rCu3Sn MCu

(4)

where r and M with subscripts representing Cu and Cu3Sn are the corresponding densities and molar weights, respectively. The densities of rSn and rCu3Sn were taken as 8.90 and 8.97 g/cm3 [10]. Based on Eq. (4), the calculated thickness of Cu3Sn was 25.0 mm, which was slightly larger than the measured thickness of Cu3Sn (23.6 mm) in Fig. 5. Therefore, sufficient and speedy dissolution of Cu substrates during the MRSW process ensured the ultrafast formation of Cu3Sn interconnect in the Cu/Sn/Cu reaction system. 4.2. Formation of dendritic CueSn IMCs under constitutional supercooling effect The size, shape and the composition of those dendritic CueSn IMCs varied with bonding time. Those variations can be explained by the way in the solidification process under current stressing. The solidification process such as constitutional supercooling [36], nucleation and crystal growth were influenced by electric current. The formation of dendrites in alloys were usually caused by constitutional supercooling whereby solute was rejected into the melt as alloy solidified and a solute-rich layer can be build-up ahead of the solidification front [37]. As reported by Tiller et al., the constitutional supercooling required for dendrite formation was achieved when [38]:

G mC0 ð1  k0 Þ < ; v Dk0

(5)

where G is the temperature gradient, v is the rate of solidification, m is the slope of the liquidus line, C0 is the overall Cu solute concentration, and k0 is the solute distribution coefficient. Eq. (5) shows that the degree of constitutional supercooling can be significantly enhanced by the increase of the rate of solidification (v) and the solute concentration (C0). As reported by Xian et al. [24], for the Sn-4 wt%Cu solder alloy, a significant constitutional supercooling can be generated in the liquid ahead of Cu6Sn5-liquid interfaces under the cooling rate of 1.7
C/s, resulting in the formation of dendritic Cu6Sn5. In the present work, the cooling rate of the Cu/ molten Sn/Cu system can be even up to 820
C/s. Meanwhile, the Cu solute concentration was much larger than 9.0 wt% after bonded for 80 ms, resulting from the continuous dissolution and liquid-solid EM of Cu under current stressing. Therefore, more significant constitutional supercooling was generated in the Cu/molten Sn/Cu system, leading to the formation of dendritic CueSn IMCs during the MRSW process. 4.3. Dramatic microstructure evolution of CueSn IMCs under MRSW process Fig. 8 shows the microstructure evolution of CueSn IMCs during the MRSW process. As shown in Fig. 8a and b, at the initial stage (heating up stage), the bonding temperature was increased above the Sn melting point, and a relatively small quantity of Cu atoms were dissolved into the molten Sn solder. When the localized Cu concentration in the molten Sn near the molten Sn/Cu interfaces increased to a certain level, a small number of dendritic Cu6Sn5 grains were precipitated out under constitutional supercooling conditions. Then the continuous dissolution and the electromigration of Cu atoms in the molten Sn contributed to the continued growth of the dendritic Cu6Sn5 compounds (Fig. 8c). With the bonding time increasing, the joule heat-induced temperature was up to the melting point of Cu6Sn5. During this duration of heating stage, the Cu6Sn5 compound was melted, meanwhile, the Cu solubility in molten Sn solder was further promoted. When the Cu concentration increased to a higher level, the Cu solute atoms precipitate as Cu3Sn compounds. After 160 ms, there were approximately 8.5 mm thick Cu substrates were consumed, and the denser columnar dendritic Cu3Sn grains were formed across the interface (Fig. 8d). Then, during the air-cooling stage but still with some liquid Sn, the electromigrated Cu atoms continually replenished the Cu concentration in the residual Sn

B. Liu et al. / Journal of Alloys and Compounds 687 (2016) 667e673

solder, resulting in the continued growth of Cu3Sn grains. After 200 ms, the residual Sn solder was completely consumed, and transformed into Cu3Sn compound (Fig. 8e). 5. Conclusions In the present work, the dramatic microstructure evolution of CueSn IMCs in Cu/Sn (30 mm)/Cu system was studied by using a MRSW process within 200 ms under 0.08 MPa at ambient temperature. (1) The imposed current density of 1.4  104 A/cm2 sharply increased the overall temperature of the interconnection system above the melting point of Sn. The joule heat-induced temperature and the passage of electric current enhanced the Cu dissolution at cathode into molten Sn solder. (2) At the initial stage, accompanying with the speedy electromigration of Cu atoms in the molten Sn solder, the columnar dendritic Cu6Sn5 compounds were formed due to constitutional supercooling. (3) With the joule heat-induced temperature increasing, the Cu6Sn5 compound was melted and totally transformed into Cu3Sn compounds. After 200 ms, a nearly full Cu3Sn interconnect with few voids was formed. (4) The Cu3Sn interconnect exhibited a higher reliable shear property of 71.1 MPa than the solder interconnects under the loading rate of 200 mm/s. This ultrafast and cost-effective approach offers a full control of fabrication of high-temperature-stable IMC interconnects, which can be a promising tool for the reliable interconnection in hightemperature power electronics. Acknowledgements The authors are grateful for financial support from the National Natural Science Foundation of China (Grant No. 51522503) and support from Program for New Century Excellent Talents in University (NCET-13-0175). References [1] 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 Adv. Funct. Solid-State Mater. 176 (2011) 283e288. [2] K. Suganuma, S.J. Kim, K.S. Kim, High-temperature lead-free solders: properties and possibilities, JOM 61 (2009) 64e71. [3] Y. Zhong, R. An, C. Wang, Z. Zheng, Z.Q. Liu, C.H. Liu, C.F. Li, T.K. Kim, S. Jin, Low temperature sintering Cu6Sn5 nanoparticles for superplastic and superuniform high temperature circuit interconnections, Small 11 (2015) 4097e4103. [4] H. Ji, Y. Qiao, M. Li, Rapid formation of intermetallic joints through ultrasonicassisted die bonding with Sn-0.7Cu solder for high temperature packaging application, Scr. Mater. 110 (2016) 19e23. [5] X. Liu, S. He, H. Nishikawa, Thermally stable Cu3Sn/Cu composite joint for high-temperature power device, Scr. Mater. 110 (2016) 101e104. [6] G. Xiao, X. Yang, G. Yuan, Z. Li, X. Shu, Mechanical properties of intermetallic compounds at the Sn-3.0Ag-0.5Cu/Cu joint interface using nanoindentation, Mater. Des. 88 (2015) 520e527. [7] P.F. Yang, Y.S. Lai, S.R. Jian, J. Chen, R.S. Chen, Nanoindentation identifications of mechanical properties of Cu6Sn5, Cu3Sn, and Ni3Sn4 intermetallic compounds derived by diffusion couples, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process 485 (2008) 305e310. [8] W.L. Chiu, C.M. Liu, Y.S. Haung, C. Chen, Formation of nearly void-free Cu3Sn intermetallic joints using nanotwinned Cu metallization, Appl. Phys. Lett. 104 (2014) 171902. [9] R. Zhang, Y. Tian, C. Hang, B. Liu, C. Wang, Formation mechanism and orientation of Cu3Sn grains in Cu-Sn intermetallic compound joints, Mater. Lett. 110

673

(2013) 137e140. [10] J. Li, P. Agyakwa, C. Johnson, Interfacial reaction in Cu/Sn/Cu system during the transient liquid phase soldering process, Acta Mater. 59 (2011) 1198e1211. [11] N. Bosco, F. Zok, Critical interlayer thickness for transient liquid phase bonding in the Cu-Sn system, Acta Mater. 52 (2004) 2965e2972. [12] S. Dong, G. Kelkar, Y. Zhou, Electrode sticking during micro-resistance welding of thin metal sheets, IEEE Trans. Electron. Packag. Manuf. 25 (2002) 355e361. [13] Y. Zhou, P. Gorman, W. Tan, K. Ely, Weldability of thin sheet metals during small-scale resistance spot welding using an alternating-current power supply, J. Electron. Mater. 29 (2000) 1090e1099. [14] K. Ely, Y. Zhou, Microresistance spot welding of kovar, steel, and nickel, Sci. Technol. Weld. Join. 6 (2001) 63e72. [15] H. Ma, A. Kunwar, J. Sun, B. Guo, H. Ma, In situ study on the increase of intermetallic compound thickness at anode of molten tin due to electromigration of copper, Scr. Mater. 107 (2015) 88e91. [16] C. Liu, Y. Hu, Y. Liu, H. Tseng, T. Huang, C. Lu, Y. Chuang, S. Cheng, Epitaxial CueSn bulk crystals grown by electric current, Acta Mater. 61 (2013) 5713e5719. [17] Z.A. Munir, D.V. Quach, M. Ohyanagi, Electric current activation of sintering: a review of the pulsed electric current sintering process, J. Am. Ceram. Soc. 94 (2011) 1e19. [18] Y. Gu, P. Shen, N.N. Yang, K.Z. Cao, Effects of direct current on the wetting behavior and interfacial morphology between molten Sn and Cu substrate, J. Alloy. Compd. 586 (2014) 80e86. [19] J. Huang, C. Tsai, Y. Lin, C. Kao, Pronounced electromigration of Cu in molten Sn-based solders, J. Mater. Res. 23 (2008) 250e257. [20] S.W. Chen, C.M. Chen, C.H. Wang, C.M. Hsu, Effects of Electromigration on Electronic Solder Joints, Lead-Free Solders: Materials Reliability for Electronics, 2012, pp. 401e422. [21] R.K. Chinnam, C. Fauteux, J. Neuenschwander, J. Rusch, Evolution of the microstructure of Sn-Ag-Cu solder joints exposed to ultrasonic waves during solidification, Acta Mater. 59 (2011) 1474e1481. [22] Y. Tian, R. Zhang, C. Hang, L. Niu, C. Wang, Relationship between morphologies and orientations of Cu6Sn5 grains in Sn3.0Ag0.5Cu solder joints on different Cu pads, Mater. Charact. 88 (2014) 58e68. [23] H.F. Zou, H.J. Yang, Z.F. Zhang, Morphologies, orientation relationships and evolution of Cu6Sn5 grains formed between molten Sn and Cu single crystals, Acta Mater. 56 (2008) 2649e2662. [24] J.W. Xian, S.A. Belyakov, T.B. Britton, C.M. Gourlay, Heterogeneous nucleation of Cu6Sn5 in Sn-Cu-Al solders, J. Alloys Compd. 619 (2015) 345e355. [25] Q. Yin, F. Gao, Z. Gu, E.A. Stach, G. Zhou, In situ visualization of metallurgical reactions in nanoscale Cu/Sn diffusion couples, Nanoscale 7 (2015) 4984e4994. [26] N.S. Bosco, F.W. Zok, Strength of joints produced by transient liquid phase bonding in the Cu-Sn system, Acta Mater. 53 (2005) 2019e2027. [27] J.H. Ke, H.Y. Chuang, W.L. Shih, C.R. Kao, Mechanism for serrated cathode dissolution in Cu/Sn/Cu interconnect under electron current stressing, Acta Mater. 60 (2012) 2082e2090. [28] Y. Wang, Y. Lin, C. Tu, C. Kao, Effects of minor Fe, Co, and Ni additions on the reaction between SnAgCu solder and Cu, J. Alloys Compd. 478 (2009) 121e127. [29] M. Li, Z. Li, Y. Xiao, C. Wang, Rapid formation of Cu/Cu3Sn/Cu joints using ultrasonic bonding process at ambient temperature, Appl. Phys. Lett. 102 (2013) 094104. [30] J. Zhao, C.Q. Cheng, L. Qi, C.Y. Chi, Kinetics of intermetallic compound layers and shear strength in Bi-bearing SnAgCu/Cu soldering couples, J. Alloys Compd. 473 (2009) 382e388. [31] H.L.J. Pang, K.H. Tan, X.Q. Shi, Z.P. Wang, Microstructure and intermetallic growth effects on shear and fatigue strength of solder joints subjected to thermal cycling aging, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process 307 (2001) 42e50. [32] M. Huang, Z. Zhang, N. Zhao, Q. Zhou, A synchrotron radiation real-time in situ imaging study on the reverse polarity effect in Cu/Sn-9Zn/Cu interconnect during liquid-solid electromigration, Scr. Mater. 68 (2013) 853e856. [33] M. Huang, Z. Zhang, N. Zhao, F. Yang, In situ study on reverse polarity effect in Cu/Sn-9Zn/Ni interconnect undergoing liquid-solid electromigration, J. Alloys Compd. 619 (2015) 667e675. [34] M. Huang, Z. Zhang, H. Ma, L. Chen, Different diffusion behavior of Cu and Ni undergoing liquid-solid electromigration, J. Mater. Sci. Technol. 30 (2014) 1235e1242. [35] N. Zhao, Y. Zhong, M. Huang, H. Ma, W. Dong, Growth kinetics of Cu6Sn5 intermetallic compound at liquid-solid interfaces in Cu/Sn/Cu interconnects under temperature gradient, Sci. Rep. 5 (2015). [36] S.K. Kang, M.G. Cho, P. Lauro, D.Y. Shih, Study of the undercooling of Pb-free, flip-chip solder bumps and in situ observation of solidification process, J. Mater. Res. 22 (2007) 557e560. [37] T. Cheng, The mechanism of grain refinement in TiAl alloys by boron additionan alternative hypothesis, Intermetallics 8 (2000) 29e37. [38] W.A. Tiller, K.A. Jackson, J.W. Rutter, B. Chalmers, The redistribution of solute atoms during the solidification of metals, Acta Mater. 1 (1953) 428e437.