Microstructure evolution and properties of rapidly solidified Au-20Sn eutectic solder prepared by single-roll technology

Journal of Alloys and Compounds 781 (2019) 873e882

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Microstructure evolution and properties of rapidly solidified Au-20Sn eutectic solder prepared by single-roll technology Shengfa Liu a, Dongxiao Zhang a, *, Jieran Xiong b, Chen Chen a, Tianjie Song a, Li Liu a, **, Shangyu Huang a a b

School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China Shanwei Bolin Electronic Packaging Material Co., Ltd., Shanwei 516600, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 April 2018 Received in revised form 3 November 2018 Accepted 5 December 2018 Available online 8 December 2018

The current chip-level Au-Sn solder production process is very complex, to simplify the process while improving the quality of the solder, Au-20Sn solder ribbon was prepared by single-roll rapid solidification technology. The microstructure evolution, composition distribution, phase composition, melting characteristics, wettability and mechanical properties of the solder were studied. The rapidly solidified Au-20Sn solder alloy consisted of a small amount of primary z0 -Au5Sn dendrites and eutectic structure (z0 -Au5Snþd-AuSn). The microstructure was refined and the components were evenly distributed, while no new phase was formed during the annealing. With Sn element diffused into the d-AuSn phase from the z0 -Au5Sn phase, the z0 -Au5Sn phase was decomposed and the d-AuSn phase grew up and distributed in the matrix. After rolling, the d-AuSn phase was elongated along the elongation direction of the ribbon, and the elements were further segregated. Ultimately, compared to the cast-rolling Au-20Sn solder, the melting range was narrower, and the wettability and the shear strength of the solder joint improved. © 2018 Elsevier B.V. All rights reserved.

Keywords: Single-roll rapid solidification Au-20Sn eutectic solder Microstructure evolution Properties

1. Introduction With the development of electronic technology, harsh environment electronics calls for devices that are capable of surviving extreme environment conditions such as high ambient temperature, high pressure and others [1,2]. High-lead (Pb) solders are presently granted immunity from the Restriction of Hazardous Substances (RoHS) requirements and Waste Electrical and Electronic Equipment (WEEE) directive for their use in electronic systems operating [3e5]. Au-20Sn eutectic solder is a high temperature solder for chip level package with a melting point of 278  C that is used in high performance optoelectronics, power electronics, MEMS sensors, hermetic sealing, and other applications [6e8]. Au-20Sn is lead-free, which can be used without flux under some specific circumstances, such as chip level and vacuum environment, and it is a hard solder with good creep resistance relative to traditional soft solders such as Sn-Pb alloys [9]. In addition, Au-

* Corresponding author. Wuhan University of Technology, 122 Luoshi Rd, Wuhan, Hubei, 430070, China. ** Corresponding author. Wuhan University of Technology, 122 Luoshi Rd, Wuhan, Hubei, 430070, China. E-mail addresses: [email protected] (D. Zhang), [email protected] (L. Liu). https://doi.org/10.1016/j.jallcom.2018.12.073 0925-8388/© 2018 Elsevier B.V. All rights reserved.

20Sn eutectic solder has favourable mechanical properties, good thermal and electrical conductivity, and high resistance to electromigration, which are important characteristics for highly reliable electronic applications [10]. The commercial production of Au-20Sn solder ribbons is mainly prepared by casting and rolling, which makes it difficult to prepare solder ribbons with various sizes and shapes that meet the requirements of microelectronic devices for chip level package. Moreover, the conventional process is very complicated which will greatly increase the cost because of the requirement of multiple rolling [11]. From AueSn binary phase diagram shown in Fig. 1 [12,15], AueSn eutectic alloy with the component proportion of 80 wt% Au and 20 wt% Sn at the eutectic temperature of 278  C. Au20Sn alloy is composed of z0 -Au5Sn phase and d-AuSn at room temperature. It has been found that the Au-20Sn eutectic alloy produced by conventional casting, are mainly composed of primary phase (z0 -Au5Sn phase) and lamellar eutectic microstructure(z0 Au5Sn þ d-AuSn phase) [7,11]. The coarse primary z0 -Au5Sn phase belongs to hexagonal system, in which the phase has a big brittleness for lack of slip system. Therefore, The presence of coarse primary z0 -Au5Sn phase and the inhomogeneous distribution of it are the main reasons for the poor plasticity which limits the application of Au-20Sn alloy. To overcome the difficulties of plastic

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solidification technology has been systematically studied and proposed. 2. Experimental procedures

Fig. 1. Au-Sn binary phase diagram [12,15].

processing, many scholars have done a lot of research on the preparation process of brittle alloys. The typical preparation techniques include mechanical alloying method, overlap rolling method, electroplating deposition method, element or nonmetallic oxide particles addition method and rapid solidification method [13e21]. Rapid solidification is an important non-equilibrium processing technology and has been frequently employed to improve the properties and performance of brittle alloys and also for the development of entirely new compositions. It involves high cooling rates (>103 K/s) and results in significant microstructural and constitutional changes. Rapid solidification solders usually have many unique advantages [22e25]: Firstly, the microstructure of the rapidly solidified solder is uniform, and can be melted in a relatively narrow range during heating. The segregation is significantly reduced, which improves the strength and corrosion of the joint. Secondly, since the rapidly solidified solder is in an unsteady state, there is a tendency to precipitate crystals when it is nearly melted, a large amount of heat is released at the moment of melting, which will improve the wettability of the solder. Thirdly, the rapidly solidified solder has a good formability, and it is possible to form a hard and brittle material into a ribbon to meet the needs of the chip level package. Finally, Compared with using as-cast solders, the interfacial IMC layer of the joint using rapidly solidified solders was more compact and uniform, which depressed the formation and growth of the interfacial IMC during the high-temperature aging and improved the high-temperature stability of the solder joint [26e31]. However, there are few reports on the preparation of Au20Sn solders by rapid solidification process and the subsequent preparation of finished solder ribbon by annealing and rolling. Lee et al. have successfully prepared a good performance of Au-Sn eutectic solder by rapid solidification [32]. They annealed for a long time (70 h) below the peritectoid reaction temperature of the primary z0 -Au5Sn phase. Although the plasticity was also improved, the coarse primary z0 -Au5Sn phase may not be completely eliminated, and the process takes too much time. In this work, Au-20Sn eutectic solder was rapidly prepared by single-roll solidification technique. The rapidly solidified ribbon was processed into finished ribbon annealing and rolling. The microstructure evolution, phase composition and processing properties of rapidly solidified Au20Sn eutectic solder were analysed. The influence of rapid solidification and the effect of annealing and rolling on the microstructure and properties of Au-20Sn eutectic solder was investigated. Therefore, a method for efficiently preparing high-performance Au20Sn eutectic solder ribbon for chip level by single-roll rapid

In this work, Au and Sn ingots with the purity of 99.99% were used to prepare Au-20Sn solder ribbons of 120 mm thickness and 6 mm width directly by a self-developed rapid solidification experimental device (see Fig. 2). The parameters of this rapid solidification tests are listed in Table 1. The ribbons were then annealed in a tube vacuum furnace (HeFei KeJing, GSL-1100X-S) at an annealing temperature of 240  C for 1 h, 2 h, 4 h and 8 h, respectively. Finally, the ribbon annealed at 240  C for 4 h was hotrolled at 210  C into a thinner ribbon with a final thickness of 50 mm. To observe the microstructure of the solder ribbons, the samples were etched slightly by 75 vol% HCl þ25 vol% HNO3 solution after mounted, mechanical polishing, and then studied using field emission scanning electron microscopy (FESEM; Zeiss Ultra Plus). The chemical composition and element distribution were analysed by wavelength dispersive x-ray spectroscopy (WDS, JXA-8230). Its phase constitutions were determined using x-ray diffraction (XRD; D8 Adwance X). The particle size of d-AuSn in the microstructure was quantitatively analysed by Image-J image analysis software. A Mettler-Toledo TGA/DSC thermal analysis device was used to determine the liquidus temperatures of the solder using a heating rate of 10  C/min under N2 atmosphere from 20 to 320  C. After annealed, the ribbon was cut into pieces with a length of 5 mm. Tensile test was performed on the annealed ribbons by the plate bending method. The fracture strain values of the ribbons was characterized by the flexural strain εf , which is calculated by Luborsky method as follows [33]:

εf ¼

t Dt

(1)

Where t is ribbon thickness, D is the distance between the two parallel plates on the measuring device when the sample is broken. The wetting behaviour of Au-20Sn eutectic solder (8 mm  5 mm, with the thickness of 5 mm) was tested on Au plated Cu substrate (10 mm  22 mm, with ~5 mm Au), which was mechanically ground with 800 grit SiC papers and subsequently cleaned in acetone and anhydrous alcohol. Firstly, the samples were heated to 310  C and held at peak temperature for 60s under N2 atmosphere. Then, the spreading area was measured using a twodimensional image measuring instrument (VMS-1510F). The soldering experiment was carried out with the same parameters as the spreading experiment, and then the shear strength of the solder

Fig. 2. Schematic diagram of experimental device: 1 chilling roller, 2 nozzle, 3 heater, 4 crucible and 5 argon gas flushing.

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Table 1 Process parameters for rapid solidification of the Au-20Sn solder ribbon. Nozzle pressure/MPa

Roller diameter/mm

Roller line speed /m s1

Nozzleeroller distance/mm

Heating temperature/ C

Nozzle angle/

Nozzle Size/mm

Cooling rate/K$s1

0.05

150

8

2

320

90

F2.0

7.1  103

joints was measured by a MTS ceramic testing system under a tensile strain rate of 1 mm/min at 20  C in air (see Fig. 3). The shear strain was calculated by firstly subtracting the elastic displacement of the Au plated Cu substrate from the total displacement and then dividing the result by the thicknesses of the solders [34]. The properties of rapid solidification-rolling Au-20Sn solder were compared with the properties of commercially available castingrolling Au-20Sn solder (Shanwei Bolin Electronic Package Material Co.) under the same conditions. 3. Results and discussion 3.1. The microstructure evolution of rapidly solidified Au-20Sn solder 3.1.1. The microstructure after melt-spinning According to the Au-Sn alloy equilibrium phase diagram, the equilibrium solidification structure of the Au-Sn eutectic alloy Au20Sn at room temperature should be a eutectic structure composed of two phases of z0 -Au5Sn phase and d-AuSn phase. However, in the as-cast process, the alloy solidifies into nonequilibrium solidification, and the z0 -Au5Sn phase with higher melting point and closer component to the molten will be precipitated first, and the nucleation will grow into the primary phase. When the alloy solution reaches the eutectic reaction temperature, a eutectic reaction occurs, and the z0 -Au5Sn and d-AuSn phases formed will grow synergistically in a layered manner. The encapsulation reaction then occurs when the temperature drops to around 190  C (a small portion of the d-AuSn is consumed). The primary z0 -Au5Sn phase and the eutectic z0 -Au5Sn phase eventually form the primary z0 -Au5Sn phase and the eutectic z0 -Au5Sn phase, respectively, resulting in the formation of dendritic z0 -Au5Sn primary phase, and the remaining eutectic layer of z0 -Au5Sn and dAuSn phases. The microstructure of Au-20Sn eutectic solder alloy prepared by as-cast and single-roll rapid solidification is shown in Fig. 4, the microstructure of the as-cast solder contains a large number of coarse cellular dendritic primary phases, which are about 50 mm in size, and secondary dendrite are about 2 mme5 mm. However, Compared with the as-cast Au-20Sn microstructure in Fig. 4(a) and (b), the amount of primary phase of rapidly solidified Au-20Sn is obviously reduced and refined, because the rapid cooling rate suppresses the segregation and growth of the primary phase caused by “constitutional supercooling”. The dendrite length of primary phase is 3~5 mm and the secondary dendrite spacing is 0.3e0.5 mm. In Fig. 4 (d), the size of d-AuSn phase in the eutectic microstructure is 100e300 nm, and it is distributed in granular or

lamellar fragmentation in the z0 -Au5Sn phase matrix. The point component analysis using WDS shows that the composition of the primary phase z0 -Au5Sn is 75 at.% Au and 25 at.% Sn, in which the composition of Sn is higher than the theoretical value. On the one hand, According to Aziz solute retention model [35], during solidification, the solvent and solute atoms in the liquid phase solidify instantaneously to obtain a supersaturated layer, then the solute atoms diffuse in the opposite direction to the liquid. If the diffusion is not completed before the next liquid atom solidifies, the diffusion process will end and the supersaturated solute atoms will remain in the solid state. Under the condition of rapid solidification, the moving speed of the solid-liquid interface is increasing and the solute atoms do not have time to diffuse and the “solute trapping” phenomenon occurs. On the other hand, since the eutectic background of the thin dendrites are also excited, the relative Sn content is higher in the eutectic phase that could increase the Sn content in the measurement of primary phases. Therefore, the content of Sn element in the z0 -Au5Sn phase exceeds the maximum allowed by the phase diagram. By WDS micro-composition analysis, the composition of eutectic structure is 71 at.% Au and 29 at.% Sn. According to the XRD analysis (in Fig. 7), only z0 -Au5Sn and d-AuSn exist in the solder, the same as the ordinary solder solidification contains phase. It shows that the rapid solidification process of Au20Sn eutectic solder ribbon does not produce a new phase. As primary phases formed in the rapid solidification process is few, the number of Au elements that can be accommodated is limited, and which have little influence on the eutectic matrix composition, so the rapid solidification eutectic structure is closer to the theoretical eutectic composition. The d-phase particle size of the rapidly solidified Au-20Sn eutectic solder ribbon is very small compared to that of the ascast microstructure, resulting in the effect of grain refinement. The generation mechanism can be explained by the classical nucleation theory [36]:

    DG Q exp  І ¼ nS εnL nexp  KT KT

(2)

where I is the nucleation rate, n is the number of atoms per unit volume of the liquid, DG (the unit: J) is an activation energy for the nucleation of a critical number of the clustered atoms, nS is the number of atoms on the surface of the critical nucleus, ε is the probability of the atom's transition in a given direction, nL (the unit: s1) is the atomic vibration frequency in the liquid, ε and nL depend on the activity of atoms, Q (the unit: J) is an activation free energy for diffusion across the solideliquid interface, Κ is the Boltzman's

Fig. 3. Schematic diagram of tensile shear strength tests of the solder joints.

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Fig. 4. Microstructure of Au-20Sn solder(FESEM-BES detector): (a) as-cast solder(1000  ), (b) as-cast solder(3000  ), (c) rapidly solidified solder(3000  ) (d) enlarged view of the marked portion of Fig. 3 (c) (50000  ).

constant, and Τ (the unit: K) is the thermodynamic temperature. The general formula for the solidification rate R is:

   DGm R ¼ R0 1  exp  Rg Ti

(3)

where Rg is the gas constant, R0 is a constant of the same order of magnitude that sound travels in pure metals, Ti (the unit: K)is the solidification interface temperature, DGm (the unit: J) is the change of the molar free enthalpy in the solidification process (driving force), which is proportional to the kinetic undercooling DTk (the unit: K) and the friction phase change quotient DSm (the unit: J). The number of grains per unit volume Ζ, nucleation rate І and solidification rate R, the relationship between the three are as follows [37]:

Ζ¼

 3 І 4 R

(4)

From equation (4), it can be concluded that the grain size is proportional to the growth rate and inversely proportional to the nucleation rate. Rapid solidification process produces a large degree of undercooling, І and R are increased, but the rate of increase of І is greater than R, so the number of nuclei per unit volume increases, thus refining the solidified structure. When using rapid solidification process to prepare Au-20Sn eutectic solder ribbon, because of the large degree of supercooling, the increase of nucleation rate greatly increases the number of inhomogeneous nucleation. Although the growth rate is also increasing, when a large number of nuclei have not grown up after the formation, the solidification process has ended. The number of crystal grains per unit area increases significantly, and finally the d phase particles in the microstructure are very small.

3.1.2. The microstructure after annealing As shown in Fig. 5, the microstructure of Au-20Sn rapidly

solidified solder ribbon is annealed at 240  C for 1 h, 2 h, 4 h and 8 h. According to the Au-Sn phase diagram, annealing above 190  C will eliminate the coarse primary phase in a short time. After annealed at 240  C for 1 h, Au-20Sn solder has typical eutectic structure with d-AuSn in dark phase and z0 -Au5Sn in bright phase. The d-AuSn phase is distributed irregularly in the z0 -Au5Sn matrix. With the increase of annealing time, the d-AuSn phase continuously grows. After annealing for 4 h, the primary phase completely disappears, and the final microstructure is entirely composed of (z0 -Au5SnþdAuSn) eutectic structure. However, the growth rate of d-AuSn phase slows down after more than 4 h. The reason is that during the growth of d-AuSn phase, Sn atoms should be absorbed from the supersaturated z0 -Au5Sn phase. The growth of d-AuSn phase leads to the decrease of the Sn content in the periphery, resulting in a low content of Sn. Eventually, It causes the growth of d-AuSn phase to be inhibited. Fig. 6 shows the distribution of elements in the solder ribbons annealing for different time. The results are listed in Table 2. After annealing for 1 h, the ribbon composition is relatively uniform due to the fine microstructure. After annealing for 4 h, the d-AuSn phase grows and the elements are segregated, resulting in the presence of Au-rich zone and the Sn-rich zone, but the overall composition remains uniform. However, when the annealing time is up to 8 h, the composition is not uniform due to the coarse structure. The XRD diffraction patterns for Au-20Sn eutectic solder by rapid solidification after annealing at 240  C are shown in Fig. 6 K2, K3 and K4. It was found that only z0 -Au5Sn and d-AuSn exist in the solder, and no new phase was produced during the annealing process. The morphology of the XRD diffraction pattern changed with certainty. Compared with the XRD phase analysis of the rapid solidification without annealing(see Fig. 6 K1), the width of the main peak of the solder after annealing was narrow, indicating that the microstructure was coarser during the annealing process. Fig. 8 shows the changes of Sn element contents of z0 -Au5Sn phase and d-AuSn phase during annealing. With the increase of annealing time and annealing temperature, the content of Sn in z0 -

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Fig. 5. Microstructure of rapidly solidified Au-20Sn solder ribbon under different annealing parameters: (a) 240  C  1 h; (b) 240  C  2 h; (c) 240  C  4 h; (d) 240  C  8 h.

Fig. 6. The electron probe micrograph and element distribution of rapidly solidified Au-20Sn solder ribbons under annealing parameters: (a), (b), (c) 240  C  1 h; (d), (e), (f) 240  C  4 h; (g), (h), (i) 240  C  8 h.

Au5Sn phase decreased continuously, while the content of Sn in dAuSn phase increased continuously. According to our analysis, this phenomenon may be explained as follows: In rapid solidification,

z0 -Au5Sn deviates from the equilibrium composition, containing supersaturated Sn atoms, and during the annealing process, the Sn atoms diffuse from the z0 -Au5Sn phase to the d-AuSn phase. On the

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As shown in Fig. 9 (a), after annealing, the volume fraction of

d phase in the solder ribbon increases, and the longer the time, the more obvious the increase is. Without annealing, the d-phase vol-

ume fraction in the solder ribbon is 30.1%. After annealing at 240  C for 1 h, the volume fraction of d phase exceeded 33%, which is increased compared with that before annealing. After annealing for 2 h, the d-phase volume fraction in the solder ribbon reaches over 39%, but the growth rate slows down. After annealing at 240  C for 8 h, the d-phase volume fraction in the solder ribbon has reached over 40%, which is obviously higher than that before annealing. During the annealing process, the microstructure of the solder ribbon tends to be transformed to the equilibrium state. As shown in Fig. 9 (b), after annealing, the diameter of the d-phase particles in the solder ribbon increases, and the longer the time, the larger the particle diameter increases. Prior to annealing, the d-phase particle average diameter in the solder ribbon is 75.8 nm. After annealing at 240  C for 1 h, the average diameter of the particles reaches 1 mm or more, which is significantly larger than that when they are not annealed. Then, the growth of particle size slows down. After for 8 h annealing, the average diameter of the d-phase particles in the solder ribbon reaches 1.5 mm or more, which is much higher than that of the unannealed particles. The growth of the d-AuSn phase in rapidly solidified Au-20Sn solder ribbon can be explained by the Ostwald ripening mechanism. Select two second-phase particles of radius r1, r2 (r1
Table 2 Chemical analyses at points shown in Fig. 6a, d and g. Annealing parameters

Test point

Au(wt.%)

Sn(wt.%)

phase

240  C  1 h

A1 B1 A2 B2 A3 B3

88.8 66.6 89.7 65.3 90.4 64.1

11.2 33.4 10.3 34.7 9.6 35.9

z0 -Au5Sn d-AuSn z0 -Au5Sn d-AuSn z0 -Au5Sn d-AuSn

240  C  4 h 240  C  8 h

Fig. 8. Changes of Sn element content in z0 -Au5Sn phase and d-AuSn phase in rapidly solidified Au-20Sn solder during annealing.

one hand, the two-phase components gradually approach the equilibrium state, and on the other hand, the d-AuSn phase grows continuously.

ln

ca ðrÞ 2snB 1 ¼ , ca ð∞Þ kB T r

(5)

Where ca ðrÞ is the equilibrium concentration of solute atoms at the interface, ca ð∞Þ is the concentration of solute atoms far from the second phase particles, s is the interfacial tension, kB is the Boltzmann constant, nB is the atomic volume. From Equation (5), it can be concluded that the equilibrium atom concentration at the interface is related to the radius of the second phase. The smaller the radius is, the higher the concentration of solute atoms is, and the concentration difference exists between the second phase of large particles and the second phase of small particles. The solute atoms diffuse from small particles to large particles, resulting in the disappearance of small particles and the growth of large particles. Fig. 10 shows the growth model of the d-AuSn phase. The Sn content around the d-AuSn phase with diameter r1 is c(r1), and the Sn content around the d-AuSn phase with diameter r2 is c(r2), c(r1)>c(r2), which leads to the diffusion of Sn atoms from small particles to large particles. Therefore, in the rapidly solidified Au-20Sn solder during the annealing process, the small particle size of the d-AuSn phase decreases, the large particle size grows. The total number of particles decreases, and the average particle size increases. The d-AuSn phase exists around the primary phase'-Au5Sn. As d-AuSn grows up, it takes up the position of the primary phase z0 -Au5Sn, and the primary phase tends to disappear. The final microstructure consists of eutectic structures(z0 -Au5SnþdAuSn). Fig. 11 shows the relationship between different annealing times and fracture strain values. The annealing process improves the toughness of the ribbon, and the toughening of the solder ribbon can be achieved in a relatively short time. However, as the time further increases, the d-AuSn phase coarsens, resulting in increasing solder brittleness and decreasing the fracture strain values. When the ribbons are annealed at 240  C for 4 h, the dendrite primary phase z0 -Au5Sn disappeared, and the islandshaped d-AuSn phase is distributed in the continuous z0 -Au5Sn phase matrix, and the fracture strain values reaches a maximum of 2.82%.

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Fig. 9. The d-AuSn phase volume fraction (a) and particle diameter (b) change in rapidly solidified Au-20Sn during annealing.

eutectic melting point temperature, and the melting range is narrower. Because the primary phase is eliminated after annealing, the eutectic structure is close to equilibrium and evenly distributed.

Fig. 10. The growth model of the d-AuSn phase.

3.1.3. The microstructure after rolling Fig. 13 shows the microstructure and distribution of elements of the solder ribbon prepared by the rapid solidification-rolling process. The d-AuSn phase is elongated along the elongation direction of the ribbon and the amount of deformation is small. The composition of the z0 -Au5Sn phase is 90.0 wt% Au and 10.0 wt% Sn, and the d-AuSn composition is 64.9 wt% Au and 35.1 wt% Sn, which are close to the equilibrium component. Compared with the distribution of elements after annealing, the micro-zones of the rapid solidification-rolling solders are further segregated, Au-rich areas and Sn-rich areas appear, but the overall composition remains evenly distributed. 3.2. The properties of rapid solidification-rolling Au-20Sn eutectic solder

Fig. 11. Relationship between annealing time and fracture strain values.

During the annealing process, the ribbon structure is coarsened, which affects the melting characteristics. Therefore, it is necessary to confirm the effect of different annealing times on the melting characteristics of the rapidly solidified solder. Fig. 12 shows the DSC with different annealing time. Only one endothermic can be found in the curve of rapidly solidified solders, which referred to melting reaction. After annealing of the Au-20Sn rapid solidification solder ribbon, the solid phase line of the Au-20Sn ribbon is increased, the liquidus is lowered, the melting point is closer to the theoretical

3.2.1. Melting characteristics The melting characteristics of the solder ribbons by rapid solidification-rolling and cast-rolling are measured using differential scanning calorimetry (DSC). As shown in Fig. 14, the solidus temperature of the rapidly solidified solder is 278.7  C, the liquidus temperature is 281.2  C, and the melting range is 2.5  C. The solidus temperature of the cast-rolling solder is 282.0  C, the liquidus temperature is 289.3  C, the liquid solid phase temperature difference is 7.3  C. It can be found that both the liquidus temperature and the solidus temperature have some difference, and the melting range of the rapid solidification-rolling ribbon is smaller than that of the cast-rolling ribbon. The main reasons are: (1) the finer microstructure of the rapidly solidified solder increases the specific surface area and surface energy, leading to the decrease of the melting temperature [39]; (2) the microstructure of the rapidly solidified solder is formed by crystallizing under short-range diffusion, and can be quickly converted into liquid at a temperature above the melting point; and (3) subsequent annealing-rolling further releases excessive free energy stored in the rapidly solidified state, causing grain growth and further elemental segregation. 3.2.2. Wetting behaviour As shown in Fig. 15, the spreading area of casting-rolling Au20Sn solder ribbon is obviously smaller than that of rapid solidification-rolling solder ribbon and the surface is not shiny, with an oxidized layer attached. The spreading area of cast-rolling

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Fig. 12. The differential scanning calorimetry of (a) rapidly solidified Au-20Sn solder ribbons, (b) annealed at 240  C for 1 h, (c) annealed at 240  C for 4 h and (d) annealed at 240  C for 8 h.

Fig. 13. The electron probe micrograph and element distribution of Au-20Sn solder ribbons prepared by the rapid solidification-rolling process.

Fig. 14. The differential scanning calorimetry of Au-20Sn solder ribbons prepared by (a) the cast-rolling and (b) the rapid solidification-rolling.

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Fig. 15. Typical images of two kinds of Au-20Sn eutectic solder ribbons after the spreading experiment: (a) cast-rolling, (b) rapid solidification-rolling.

Au-20Sn solder is 13.05 mm2. The area of the rapid solidificationrolling Au-20Sn solder is 14.48 mm2, and the average spreading area increases by 10.96%. The reasons are: Firstly, cast-rolling solder structure is coarser, in which there is a large segregation of components, and the melting points of the phases are inconsistent. When heated to the liquidus temperature, the low-melting eutectic phase first melted, then primary phase of the high melting point and the intermetallic compound melted. The high-melting phase hindered the spreading of the low-melting liquid phase, and the solder did not easily form a precursor film when melted. Therefore, the wettability was lowered and the high melting point portion was accumulated due to the slow dispersion, which caused delamination. The rapidly solidified solder inherited the uniform distribution of elements in the melting furnace and had a narrow melting range. When heated to the liquidus temperature, it simultaneously and uniformly melted and spread, and the precursor film was easily formed, which lead to high wettability. Secondly, under the same wetting conditions, the lower liquidus temperature of the rapidly solidified solder increases the superheat and fluidity, and thus improves wettability. Thirdly, the wettability is determined by the physical wetting effect. According to the YoungeDupre equation [40]:

cosq ¼ ðsSG  sSL Þ=sLG

Fig. 16. Typical stressestrain curves of castingerolling and rapidly solidified solder joints.

(6)

Where sSG , sSL and sLG are solidegas, liquidesolid and gaseliquid surface energy, respectively, and q is the wetting angle. The highly reactive atoms in the more uniform and disordered melt of the rapidly solidified solder are easier to combine with the atoms on the substrates, and decrease the surface energy sLG and wetting angle q, and thus increase the wettability. 3.2.3. Shear strength The shear strength of the solder joints was conducted by MTS ceramic testing system and compared with casting-rolling Au-20Sn solder joints. Stressestrain curves of castingerolling and rapidly solidified solder joints are shown in Fig. 16. The average shear strength increases by 19.02% from 32.91 ± 1.50 MPa to 39.17 ± 0.70 MPa.The main reasons are that the coarser and strongly directional microstructure is not beneficial for enhancing the shear strength of castingerolling solder joints, while the finer and more uniform microstructure with finely dispersed d-AuSn particles contributes to the improvement of the shear strength of the rapidly solidified solder joints. 4. Conclusion The microstructure evolution and its mechanism of rapidly solidified Au-20Sn solder are systematically studied under melt-

spinning, annealing and rolling condition. The properties of finished rapid solidification-rolling Au-20Sn eutectic solder ribbon are investigated, and the casting-rolling Au-20Sn eutectic solders ribbon under the same condition are tested for comparison. The following results can be concluded: 1) Compared to as-cast solder, in rapid solidification-rolling Au20Sn eutectic solder, the composition distribution was more uniform, the number of dendrite primary phases z0 -Au5Sn in the microstructure was reduced and refined, and the eutectic structure (z0 -Au5Snþd-AuSn) was finer, and the size of d-AuSn phase was 100e300 nm. Furthermore, in the process of rapid solidification, the refinement mechanism of primary phase and eutectic structure was discussed in detail. 2) No new phase was formed during annealing of the rapidly solidified solder. With Sn element diffused into the d-AuSn phase from the z0 -Au5Sn phase to reach an equilibrium, the z0 -Au5Sn phase was decomposed and the d-AuSn phase grew up and distributed in the matrix, which the volume fraction and the particle size were significantly increases. After annealed at 240  C for 4 h, the coarse primary phase disappeared, and the eutectic structure was close to equilibrium and evenly distributed, which led to the increase of toughness and the decrease of melting process.

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