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Mechanical properties of SnBi-SnAgCu composition mixed solder joints using bending test
Materials Science & Engineering A 668 (2016) 224–233
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Mechanical properties of SnBi-SnAgCu composition mixed solder joints using bending test Fengjiang Wang a,b,n, Ying Huang a, Chengchao Du a a b
Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang 212003, China State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 April 2016 Received in revised form 17 May 2016 Accepted 18 May 2016 Available online 19 May 2016
A comparative analysis between the microstructure and tensile properties of eutectic Sn-58Bi, Sn-3.0Ag0.5Cu and SnBi-SnAgCu composition mixed solders including 20, 25 or 50 wt% Sn-Ag-Cu in Sn-Bi solder was conducted, and the three point flexural bending properties of the solders as Cu/solder/Cu joints were evaluated with the effect of loading rate. With Sn-Ag-Cu added into Sn-Bi solder, the content of eutectic Sn-Bi phase decreased while Cu6Sn5 IMC and Sn-Ag-Bi eutectic structure were produced in the microstructure. Tensile results on the solder alloys indicated that SB-20SAC and SB-25SAC mixed solders showed an increased ductility with higher elongation and lower tensile strength compared with Sn-Bi solder, while SB-50SAC showed a brittle characteristic with the greatest strength but lowest elongation among all the mixed solders. The bending tests on the solder joints illustrated that composition mixing was helpful to increase the toughness of Sn-Bi solder, and the bending fracture energy of joints was dependent on the loading rate. A relationship between strain rate, crack propagation and fracture toughness was developed. Sn-Bi solder always had a brittle fracture mode. With Sn-Ag-Cu mixed into SnBi solder, the fracture toughness of joints increased. The crack propagation was then decided by the strain rate, in which ductile failure in the solder matrix was occurred in the bending with a lower strain rate while brittle failure along IMC/solder interface was produced in a higher strain rate. & 2016 Elsevier B.V. All rights reserved.
Keywords: Sn-Bi Sn-Ag-Cu Mixed solder alloy Bending test Fracture mode
1. Introduction Sn-Ag-Cu is the most common used Pb-free solder alloy [1,2]. However, due to its higher melting temperature (217–221 °C) and the increasing price on silver, eutectic Sn-58Bi solder is also increasingly attracted with the lower melting temperature (138 °C) and coefficient of thermal expansion [3,4]. However, two urgent problems are existed for the wide using of Sn-Bi solder: ductility decreasing by the inherent embrittlement of Bi element [5] and reliability decreasing by Bi segregation or coarsening at the interface of solder joints [6,7]. Adding alloying elements into Sn-Bi solder is commonly used to improve its properties by various studies. For ductility issue, Ag and Cu are the most common used alloying elements into Sn-Bi solder. McCormack found that the ductility was significantly increased with about 0.25– 0.5 wt% of Ag into Sn-58Bi due to the refinement of microstructure [7], which was also verified by Sakuyama [8] and Suganuma [9]. Li [10] studied the improving effect of adding Ag nanoparticles on the mechanical properties and microstructure of Sn-58Bi solder. Takao n Correspondence to: Jiangsu University of Science and Technology, 2 Mengxi Rd, Zhenjiang, Jiangsu 212003, China. E-mail address: [email protected] (F. Wang).
http://dx.doi.org/10.1016/j.msea.2016.05.072 0921-5093/& 2016 Elsevier B.V. All rights reserved.
et al. [11] investigated the microstructure and properties of Sn-Bi solder with the addition of Cu, and showed a finer microstructure and better ductility. The addition of In [12-14], Ni [15], yttrium oxide [16], Sb [8] and others were also performed to improve the properties of Sn-Bi solder. For reliability issues on Sn-Bi/Cu joint, Bi segregation or coarsening was normally produced at the interface between Cu-Sn intermetallic compounds (IMC) and Cu during thermal aging [17-20], or at anode during electro-migration [21,22]. Addition of third alloying elements into Sn-58Bi solder or Cu substrate provides a solution. Adding Co or Ga into Sn-Bi solder [23,24], or Ag or Zn into Cu substrate [25,26] were effectively proved to prevent Bi atoms from segregation under thermal aging. Bi coarsening occurred in the electromigration was also improved by the addition of Ag nanoparticles [27], Al [28], and Ni [29]. It can be seen that most studies are focused on the effect of small addition of third alloying element into Sn-Bi solder. However, with Sn-Ag-Cu transferring into Sn-Bi solder, soldering between Sn-Bi paste and Sn-Ag-Cu ball provides a lower reflowing temperature on step soldering and a cheaper technique on advanced packaging. Correspondingly, the compatibility between SnBi and Sn-Ag-Cu solder strongly affects the microstructure and properties of joints. In the present study, Sn-3.0Ag-0.5Cu solder was tried to incorporate into Sn-58Bi solder to produce the SnBi-
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SnAgCu composition mixed solder. The microstructure and properties of mixed solder were studied with the effect of different content of Sn-Ag-Cu in Sn-Bi solder.
2. Experimental procedure Sn-58Bi (SB) and Sn-3.0Ag-0.5Cu (SAC) solder alloys were used in this work. These two alloys were prepared from 99.9% Sn, 99.99% Cu, 99.9% Bi and 99.99% Ag. All the raw materials were melted in a ceramic crucible at 600 °C for 30 min. Mechanical stirring was needed to homogenize the solder alloy. To prevent the solder from oxidation during the melting, KCl:LiCl (1.3:1) were used over the surface of liquid solder. The melt was casted as an ingot in a steel mold. Then Sn-58Bi and Sn-3.0Ag-0.5Cu were used as starting materials to produce Sn-Bi and Sn-Ag-Cu mixed solder alloys by remelting them with different ratio. The compositions were SB-20SAC, SB-25SAC and SB-50SAC in weight percent, respectively. The corresponding weight ratio between Sn-58Bi and Sn-3.0Ag-0.5Cu solder was 4:1, 3:1 and 1:1, respectively, and the final compositions of mixed solders were listed in Table 1. Each solder alloy was casted as ingot. A film of 400 mm in thickness, 1 mm in width and 15 mm in length was also supplied for soldering the rectangle type solder joint specimen. Preparation on the solder joint is illustrated in Fig. 1. Oxygen-free Cu (OFC) with
SB SB-20SAC SB-25SAC SB-50SAC SAC
the size of 15*15*1 mm was used as the base metal. The test specimen was soldered at a peak temperature of 250 °C for a holding time of 15 s, and then was cut by electrical discharge machining to get the test pieces with size of 1 mm on the depth and width and 30 mm on the length. Microstructural characterization of the solder alloys was performed. The solder was metallographically polished to a final 0.05 mm using colloidal silica solution, and then examined with a scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) and electron probe micro analyzer (EPMA). The phases in each solder were detected using X-ray diffraction (XRD). The mechanical properties of solder alloys were tensile tested with specimen shown in Fig. 2a, while the properties on solder joint were measured with a three point flexural method shown in Fig. 2b. During bending, the radius of loading pin and two supporting pins was 2 mm, and the support span between two supporting pins was 13 mm. To investigate the effect of strain rate, the loading pin was lowered from above at a constant rate of 0.5, 0.1, 0.05 and 0.01 mm/s, respectively. The crack propagation in solder joints was observed using optical microscope (OM), the fractured surfaces were observed using SEM, and the compositions of phases were characterized by EDX.
3. Results and discussion 3.1. Microstructure characterization
Table 1. Elemental composition of Solder alloys. Solder
225
Element compositions (wt%) Sn
Ag
Cu
Bi
bal bal bal bal bal
– 0.6 0.75 1.5 3.0
– 0.1 0.125 0.25 0.5
58 46.4 43.5 29 –
Fig. 1. Schematic of solder joint fabrication.
Fig. 3 shows the microstructure of as-prepared Sn-58Bi solder and SnBi-SnAgCu composition mixed solders with Sn-Ag-Cu content of 20, 25 and 50 wt% in Sn-Bi. The bright and dark grey regions were identified as Bi-rich phase and β-Sn phase, respectively. For Sn-58Bi eutectic solder, during solidification eutectic reaction was occurred in liquid solder (L) at 138 °C with the following eutectic reaction from Sn-Bi binary phase diagram [30]:
Fig. 2. Schematics of (a) mini-tensile specimen on solder alloy and (b) three point flexural testing on solder joint.
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Fig. 3. Microstructures of as-prepared solders: (a) Sn-58Bi, (b) SB-20SAC, (c) SB-25SAC and (d) SB-50SAC.
L → (β − Sn) + ( Bi)
(1)
The resulted solder matrix was composed of lamellar eutectic structure where the Bi-rich phases and the Sn-rich phases interlocked with each other alternately, as shown in Fig. 3a. A few primary crystals of Sn dendrites were observed due to the nonequilibrium cooling. With Sn-Ag-Cu mixed into Sn-Bi solder, the microstructure was obviously changed. For solder with 20 or 25 wt% Sn-Ag-Cu, it can be clearly seen that the number of Sn dendrites increased, Sn-Bi eutectic structure was coarsened, and isolated Cu6Sn5 phases were produced, while element Ag was not easily differentiated from the solder matrix. To identify all the existing phases, each solder was examined by XRD, and the results on SB-SAC mixed solders with different content of Sn-Ag-Cu are illustrated in Fig. 4, in which Sn-58Bi eutectic and Sn-3.0Ag-0.5Cu solder were also analyzed. Cu and Ag were existed as Cu6Sn5 and Ag3Sn IMCs, respectively. Research on Sn-Ag-Bi ternary system has also shown that there existed a ternary eutectic reaction at 138.4 °C [31]:
L → ϵ − Ag3Sn + (β − Sn) + ( Bi)
(2)
To confirm the distribution of Ag3Sn and Cu6Sn5 in the solder matrix, the element distribution was investigated using EPMA. Fig. 5 shows the results of elemental mapping on SB-25SAC solder. Ag and Cu atoms were detected on the significant areas, in which Cu was coexisted with Sn, indicating Cu-Sn IMCs were isolated in the solder matrix, while Ag was simultaneously coexisted with Sn and Bi, indicating Ag3Sn IMCs were coexisted with Sn and Bi. Therefore, after Sn-Ag-Cu solder was added into Sn-Bi solder, Sn-Bi was transformed into hypo-eutectic system. During solidification, primary Sn dendrite was firstly precipitated from liquid solder
Fig. 4. XRD patterns of Sn-Bi, Sn-Ag-Cu and SB-SAC mixed solders.
through the following reaction:
L → L + (β − Sn)
(3)
The amounts of primary Sn dendrite increased with the addition of Sn-Ag-Cu solder. Cu was reacted with Sn to form Cu6Sn5 IMC at 227 °C:
Cu + Sn → Cu6 Sn5
(4)
After the temperature reached at 138 °C, (Eqs. (1) and 2) were simultaneously occurred to produce lamellar Sn-Bi eutectic and Sn-Bi-Ag eutectic structure, respectively, as the results shown in Fig. 3b and c. With 50 wt% Sn-Ag-Cu mixed into Sn-Bi solder, as
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Fig. 5. Microstructure and element mapping of SB-25SAC: (a) SEM image, (b) Ag, (c) Sn, (d) Bi and (e) Cu.
illustrated in Fig. 3d, large amounts of primary Sn dendrites were produced, and Cu6Sn5, Ag3Sn and Bi particles were finely distributed in the solder matrix due to decreasing on the Bi composition while increasing on the Sn, Ag and Cu compositions in the solder. 3.2. Mechanical behavior Mechanical properties of solder joints were investigated with three point flexural test considering the effect of strain rate. The typical examples of load-displacement curves obtained from the bending test are shown in Fig. 6, in which the curves in 6a are from different solder compositions under constant loading rate of 0.50 mm/s, and the curves in 6b are from Sn-58Bi eutectic solder joints under different loading rate of 0.01, 0.05, 0.10 and 0.50 mm/ s, respectively. To clearly illustrate the mechanical behavior of
solder joint, fracture energy was calculated by area under the loaddisplacement curve, and described the ability to fracture resistance for solder joint during bending. The detailed results on fracture energy including the effect of solder compositions and loading rate are shown in Fig. 7. It seems that Sn-58Bi solder had the highest bending strength, which increased with the strain rate, but its fracture energy was the lowest among all the solder joints and was independent of loading rate. With Sn-Ag-Cu solder mixed into Sn-Bi solder, the joint strength decreased, while the fracture energy increased and reached the maximum with the mixed composition of 25 wt% Sn-Ag-Cu in Sn-Bi solder, and then decreased with the increasing on content of Sn-Ag-Cu solder. The fracture energy was also influenced by the loading rate, and showed better results under the lower loading rate. From Fig. 6a, it is clearly seen that no fracture was occurred in Sn-3.0Ag-0.5Cu solder joints under bending test, and so the fracture energy of Sn-
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Fig. 6. Bending curves from (a) effect of solder composition under loading rate at 0.5 mm/s, and (b) effect of loading rate on Sn-58Bi eutectic solder.
Fig. 7. Effect of solder composition and loading rate on fracture energy during bending test.
Ag-Cu joints was not included in Fig. 7, which means Sn-Ag-Cu solder had the excellent fracture toughness. Therefore, with SnAg-Cu solder added into Sn-Bi solder, the fracture energy of solder increased, especially under the lower loading rate, which was attributed to the decrease on lamellar Sn-Bi eutectic phases and the increase on the finely distributed IMC particles in solder matrix as illustrated in Fig. 3. There was no obvious effect with the solder composition on the fracture energy under higher loading rate of 0.5 mm/s. In fact, the fracture toughness was controlled by the crack propagation in bulk solder or along interface during bending test. The solder joints usually showed a brittle fracture under a higher strain rate, and the fracture resistance was mainly decided by the interfacial bonding. 3.3. Fracture mode During reflow soldering, solder reacts with Cu substrate to produce a Cu6Sn5 IMC layer at the interface. Joint mechanical behavior is usually strain rate dependent, in which the joint strength was controlled by the bulk solder at low strain rate and by the brittle IMC layer at high strain rate [32,33]. Correspondingly, three failure modes are existed in solder joints during mechanical test: ductile solder mode, brittle IMC mode and mixed solder/IMC mode
[34]. To investigate the effect of strain rate on the fracture modes of SnBi-SnAgCu composition mixed solder joints during bending test, the crack propagation of solder joints under different loading rate is illustrated in Fig. 8. The complete fracture surfaces of solder joints with selected bending rate of 0.5 mm/s were examined using SEM, and the resulting images are shown in Fig. 9. During three point flexural test, direct tensile and compress stress were produced in the lower (left side in Fig. 9a) and upper region (right side in Fig. 9b) of solder joint, respectively. Crack was always initiated at the tensile region, and then finally propagated into bulk solder. Therefore, the failure mode was mainly determined by the crack in the tensile region and was dependent on the solder composition and strain rate. For Sn-58Bi eutectic solder joints shown in Fig. 8a, the crack was always propagated along the interface between solder and Cu substrate, and was a typical brittle IMC fracture mode independent of loading rate. For SB-20SAC and SB-25SAC solder joints, as shown in Fig. 8b and c, no fracture was produced under a lower loading rate at 0.01 mm/s, while with increasing loading rate, fracture was occurred in the bulk solder under 0.05 and 0.1 mm/s, and then transformed into the IMC failure mode under 0.5 mm/s. However, with content of Sn-Ag-Cu increased to 50 wt% in Sn-Bi solder, the crack was initiated and propagated in the bulk solder under rate of 0.01 mm/s, but was propagated along the interface under rate of 0.05, 0.1 or 0.5 mm/s, as illustrated in Fig. 8d. These brittle IMC fracture modes were easily found in Fig. 9 from the complete fracture surfaces under loading rate of 0.5 mm/s: brittle fracture at IMC/solder interface from the tensile region and ductile fracture in the solder from the compress region, respectively. Some solder residues were also found in the crack. The magnified images from tensile region in the fracture surface for each solder joint are illustrated in Fig. 10. The compositions of typical phases existed at the fracture surface were also identified using EDX with the compositions listed in Table 2. It can be found that all the joints exhibited partly brittle structure. In Sn-58Bi solder joint, Cu6Sn5 IMC layer (a), Bi-rich phase (b) and Sn dimple (c) were simultaneously observed in Fig. 10a, which indicates that the fracture path is partly along the solder/Cu6Sn5 IMC interface and partly within the coarsened Birich phase. With 20 or 25 wt% Sn-Ag-Cu mixed into Sn-Bi solder, the ratio of fracture on IMC layer to fracture within coarsened Birich phase increased, as shown in Fig. 10b and c. With 50 wt% SnAg-Cu in Sn-Bi solder, the fracture surface was mostly composed of Cu6Sn5 layer, and the voids composed of Cu3Sn caused from the peeling of Cu6Sn5 (h in Fig. 10d) IMC were occasionally observed in Fig. 10d.
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Fig. 8. Cross-sectional images of joints showing crack propagation after bending test under different loading rate: (a) Sn-58Bi, (b) SB-20SAC, (c) SB-25SAC and (d) SB-50SAC.
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Fig. 9. Macro fracture images on (a) Sn-58Bi, (b) SB-20SAC, (c) SB-25SAC and (d) SB-50SAC solder joint after bending under rate of 0.5 mm/s.
It seems there was a relationship between loading rate, crack propagation and fracture energy from Figs. 7 and 8. When a crack was occurred along the interface between solder and Cu substrate, there was a lower fracture energy or toughness resistance for solder such as the Sn-58Bi solder under all the loading rates or all SnBi-contained solders under loading rate of 0.5 mm/s. When a crack was occurred in the bulk solder, higher fracture energy was usually produced in the joint, and then the fracture resistance was decided by the inherent ductility of solder. To investigate the effect of strain rate and compositions on the ductile or brittle mode of SnBi-SnAgCu composition mixed solder joint during bending test, three samples on each composition were tested under different loading rate, and the brittle failure along IMC/solder interface or ductile failure in solder is summarized in Fig. 11. Brittle mode for Sn-58Bi and ductile mode for Sn-3.0Ag0.5Cu were always observed independent of loading rate, respectively. With Sn-Ag-Cu content in SB-SAC mixed solder increasing, the fracture mode was transformed from brittle to ductile, and the red line in Fig. 11 should be followed during transformation combined with the effect of loading rate. The brittle characteristic of solder joint under high strain rate was mainly caused by the following reason. At the interface between solder and IMC layer in a solder joint, Sn, Bi and Cu6Sn5 were simultaneously existed. From their lattice constants listed in Table 3, it can be clearly seen that
poor coherent grain boundary was existed among them due to the great difference on the constants. Therefore, during bending on solder joint under a higher strain rate, the dislocation was easily piled up at the boundary between solder and IMC. Meanwhile, stress was also easily concentrated at the interface due to the great difference on modulus of solder and Cu6Sn5. Combined stress concentration with piled dislocation, crack was easily initiated at the interface between solder and IMC layer under a higher loading rate. Under the lower strain rate, the location of crack initiation was mainly determined by the inherent brittle or ductile of solder due to the lower piled dislocation and stress concentration at the interface between solder and IMC layer. Therefore, tensile tests were performed on all five solder alloys with the strain-stress curves shown in Fig. 12a, and the resulted tensile strength and elongation were shown in Fig. 12b. It can be found that Sn-58Bi and SB-50SAC were brittle solders with higher tensile strength and lower elongation, while SB-20SAC and SB-25SAC mixed solders were similar as Sn-3.0Ag-0.5Cu solder, showing a ductile property. For Sn-Bi and SB-50SAC solder joints, the failure between IMC layer and solder was produced before the stress concentration reached the tensile strength of solder during bending test under most loading rates.
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Fig. 10. Magnified fracture surfaces on the tensile region from Fig. 9(b): (a) Sn-58Bi, (b) SB-20SAC, (c) SB-25SAC, (d) SB-50SAC; (e) and (f) are magnified boxes in (c) and (d).
4. Conclusions In this paper, different content of Sn-3.0Ag-0.5Cu solder was mixed into Sn-58Bi solder to increase the ductility of Sn-Bi solder.
The microstructure and tensile properties of SB-20SAC, SB-25SAC and SB-50SAC mixed solder alloys, and their bending properties with Cu/solder/Cu joints were investigated and compared with Sn-58Bi and Sn-3.0Ag-0.5Cu solder. In addition, the crack
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propagation, fracture surface and failure mode of the couples were examined. The results are summarized as follows: 2) 1) The addition of 20 or 25 wt% Sn-Ag-Cu solder in Sn-Bi solder increased the elongation property of the Sn-Bi alloy because of 3) Table 2. EDX results on points shown in Fig. 10. Location
Composition on at%
a b c d e f g h
note
Sn
Cu
Bi
42.15 4.05 77.89 45.07 3.16 10.77 41.77 15.73
57.16 2.02 11.89 50.98 20.96 25.52 55.58 83.06
0.69 93.93 10.22 3.94 75.88 63.71 2.65 1.21
Cu6Sn5 Bi Sn Cu6Sn5 Bi Bi Cu6Sn5 Cu3Sn
4)
5)
the formation of Cu6Sn5 IMC phases and Sn-Ag-Bi eutectic phases in the solder matrix and the decrease of Bi brittle phases. The tensile results showed that SB-50SAC mixed solder exhibited a brittle characteristic as same as Sn-58Bi solder, but SB20SAC and SB-25SAC mixed solders had improved ductility compared with Sn-58Bi solder. The fracture toughness of solder joints was dependent on the loading rate during three point flexural test, and SB-25SAC mixed solder joint exhibited the greatest toughness among all the bending tested solder joints. A relationship between loading rate, crack propagation and fracture toughness was developed. The joint with crack occurred along the interface between solder and IMC layer showed lower fracture energy, which was mainly existed under the bending on a higher loading rate or in a brittle solder, and the joint with crack produced in the solder matrix showed a higher fracture resistance. The distribution on fracture mode of SB-SAC composition mixed solder joints was plotted with the effect of strain rate and solder compositions.
Conflict of interest No.
Acknowledgement
Fig. 11. Ductile and brittle fracture distribution on solder joints. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
This research is supported by projects funded by National Natural Science Foundation of China (Grant No. 51541104), Jiangsu Planning Project of Science and Technology (Grant Nos. BK20 12163, BK20150466) and the State Key Lab of Advanced Welding & Joining, Harbin Institute of Technology (Grant No. AWJ-M13-10).
Table 3. Lattice constants of Sn, Bi and Cu6Sn5. Lattice constant
a*b*c (Å)
α*β*γ (deg.)
Cu6Sn5 Sn Bi
11.03*7.29*9.83 5.831*5.831*3.182 4.547*4.547*11.86
90.0*98.82*90.0 90.0*90.0*90.0 90.0*90.0*12 0.0
Fig. 12. Tensile results on solder alloys: (a) strain-stress curves, (b) relationship between content of Sn-Ag-Cu and strength/elongation.
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[18] X. Wu, Y. Li, Z. Min, Interfacial reaction and growth behavior of IMCs layer between Sn–58Bi solders and a Cu substrate, J. Mater. Sci. Mater. Electron. 24 (2013) 2027–2034. [19] C.Z. Liu, W. Zhang, Bismuth redistribution induced by intermetallic compound growth in SnBi/Cu microelectronic interconnect, J. Mater. Sci. 44 (2009) 149–153. [20] T.Y. Kang, Y.Y. Xiu, C.Z. Liu, L. Hui, J.J. Wang, W.P. Tong, Bismuth segregation enhances intermetallic compound growth in SnBi/Cu microelectronic interconnect, J. Alloy. Compd. 509 (2011) 1785–1789. [21] X. Gu, Y.C. Chan, Thermomigration and electromigration in Sn58Bi solder joints, J. Appl. Phys. 105 (2009) 093537. [22] X. Gu, K.C. Yung, Y.C. Chan, Thermomigration and electromigration in Sn58Bi ball grid array solder joints, J. Mater. Sci. Mater. Electron. 21 (2010) 1090–1098. [23] Y.-C. Huang, S.-W. Chen, Effects of Co alloying and size on solidification and interfacial reactions in Sn–57wt%Bi–(Co)/Cu couples, J. Electron. Mater. 40 (2011) 62–69. [24] S.K. Lin, T.L. Nguyen, S.C. Wu, Y.H. Wang, Effective suppression of interfacial intermetallic compound growth between Sn–58wt% Bi solders and Cu substrates by minor Ga addition, J. Alloy. Compd. 586 (2014) 319–327. [25] Q.K. Zhang, H.F. Zou, Z.F. Zhang, Influences of substrate alloying and reflow temperature on Bi segregation behaviors at Sn-Bi/Cu interface, J. Electron. Mater. 40 (2011) 2320–2328. [26] Q.K. Zhang, H.F. Zou, Z.F. Zhang, Improving tensile and fatigue properties of Sn–58Bi/Cu solder joints through alloying substrate, J. Mater. Res. 25 (2010) 303–314. [27] S. Ismathullakhan, H. Lau, Y.C. Chan, Enhanced electromigration reliability via Ag nanoparticles modified eutectic Sn–58Bi solder joint, Microsyst. Technol. 19 (2013) 1069–1080. [28] I. Shafiq, Y.C. Chan, S. Xu, Q.Q. Li, Electro-migration study of nano Al doped lead–free Sn-58Bi on Cu and Au/Ni/Cu ball grid array (BGA) packages, in: Proceedings of the 18th European Microelectronics and Packaging Conference (EMPC), 2011, pp. 1–7. [29] G. Xu, F. Guo, X. Wang, Retarding the electromigration effects to the eutectic SnBi solder joints by micro–sized Ni–particles reinforcement approach, J. Alloy. Compd. 509 (2011) 878–884. [30] U.R. Kattner, Phase diagrams for lead–free solder alloys, JOM 12 (2002) 45–51. [31] S. Hassam, E. Dichi, B. Legendre, Experimental equilibrium phase diagram of the Ag–Bi–Sn system, J. Alloy. Compd. 268 (1998) 199–206. [32] K.E. Yazzie, H.X. Xie, J.J. Williams, N. Chawla, On the relationship between solder–controlled and intermetallic compound (IMC)–controlled fracture in Sn-based solder joints, Scr. Mater. 66 (2012) 586–589. [33] K.E. Yazzie, H. Fei, H. Jiang, N. Chawla, Rate–dependent behavior of Sn alloy–Cu couples: Effects of microstructure and composition on mechanical shock resistance, Acta Mater. 60 (2012) 4336–4348. [34] H.-T. Lee, S.-Y. Hu, T.-F. Hong, Y.-F. Chen, The shear strength and fracture behavior of Sn–Ag–xSb solder joints with Au/Ni–P/Cu UBM, J. Electron. Mater. 37 (2008) 867–873.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties of SnBi-SnAgCu composition mixed solder joints using bending test Fengjiang Wang a,b,n, Ying Huang a, Chengchao Du a a b
Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang 212003, China State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 April 2016 Received in revised form 17 May 2016 Accepted 18 May 2016 Available online 19 May 2016
A comparative analysis between the microstructure and tensile properties of eutectic Sn-58Bi, Sn-3.0Ag0.5Cu and SnBi-SnAgCu composition mixed solders including 20, 25 or 50 wt% Sn-Ag-Cu in Sn-Bi solder was conducted, and the three point flexural bending properties of the solders as Cu/solder/Cu joints were evaluated with the effect of loading rate. With Sn-Ag-Cu added into Sn-Bi solder, the content of eutectic Sn-Bi phase decreased while Cu6Sn5 IMC and Sn-Ag-Bi eutectic structure were produced in the microstructure. Tensile results on the solder alloys indicated that SB-20SAC and SB-25SAC mixed solders showed an increased ductility with higher elongation and lower tensile strength compared with Sn-Bi solder, while SB-50SAC showed a brittle characteristic with the greatest strength but lowest elongation among all the mixed solders. The bending tests on the solder joints illustrated that composition mixing was helpful to increase the toughness of Sn-Bi solder, and the bending fracture energy of joints was dependent on the loading rate. A relationship between strain rate, crack propagation and fracture toughness was developed. Sn-Bi solder always had a brittle fracture mode. With Sn-Ag-Cu mixed into SnBi solder, the fracture toughness of joints increased. The crack propagation was then decided by the strain rate, in which ductile failure in the solder matrix was occurred in the bending with a lower strain rate while brittle failure along IMC/solder interface was produced in a higher strain rate. & 2016 Elsevier B.V. All rights reserved.
Keywords: Sn-Bi Sn-Ag-Cu Mixed solder alloy Bending test Fracture mode
1. Introduction Sn-Ag-Cu is the most common used Pb-free solder alloy [1,2]. However, due to its higher melting temperature (217–221 °C) and the increasing price on silver, eutectic Sn-58Bi solder is also increasingly attracted with the lower melting temperature (138 °C) and coefficient of thermal expansion [3,4]. However, two urgent problems are existed for the wide using of Sn-Bi solder: ductility decreasing by the inherent embrittlement of Bi element [5] and reliability decreasing by Bi segregation or coarsening at the interface of solder joints [6,7]. Adding alloying elements into Sn-Bi solder is commonly used to improve its properties by various studies. For ductility issue, Ag and Cu are the most common used alloying elements into Sn-Bi solder. McCormack found that the ductility was significantly increased with about 0.25– 0.5 wt% of Ag into Sn-58Bi due to the refinement of microstructure [7], which was also verified by Sakuyama [8] and Suganuma [9]. Li [10] studied the improving effect of adding Ag nanoparticles on the mechanical properties and microstructure of Sn-58Bi solder. Takao n Correspondence to: Jiangsu University of Science and Technology, 2 Mengxi Rd, Zhenjiang, Jiangsu 212003, China. E-mail address: [email protected] (F. Wang).
http://dx.doi.org/10.1016/j.msea.2016.05.072 0921-5093/& 2016 Elsevier B.V. All rights reserved.
et al. [11] investigated the microstructure and properties of Sn-Bi solder with the addition of Cu, and showed a finer microstructure and better ductility. The addition of In [12-14], Ni [15], yttrium oxide [16], Sb [8] and others were also performed to improve the properties of Sn-Bi solder. For reliability issues on Sn-Bi/Cu joint, Bi segregation or coarsening was normally produced at the interface between Cu-Sn intermetallic compounds (IMC) and Cu during thermal aging [17-20], or at anode during electro-migration [21,22]. Addition of third alloying elements into Sn-58Bi solder or Cu substrate provides a solution. Adding Co or Ga into Sn-Bi solder [23,24], or Ag or Zn into Cu substrate [25,26] were effectively proved to prevent Bi atoms from segregation under thermal aging. Bi coarsening occurred in the electromigration was also improved by the addition of Ag nanoparticles [27], Al [28], and Ni [29]. It can be seen that most studies are focused on the effect of small addition of third alloying element into Sn-Bi solder. However, with Sn-Ag-Cu transferring into Sn-Bi solder, soldering between Sn-Bi paste and Sn-Ag-Cu ball provides a lower reflowing temperature on step soldering and a cheaper technique on advanced packaging. Correspondingly, the compatibility between SnBi and Sn-Ag-Cu solder strongly affects the microstructure and properties of joints. In the present study, Sn-3.0Ag-0.5Cu solder was tried to incorporate into Sn-58Bi solder to produce the SnBi-
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SnAgCu composition mixed solder. The microstructure and properties of mixed solder were studied with the effect of different content of Sn-Ag-Cu in Sn-Bi solder.
2. Experimental procedure Sn-58Bi (SB) and Sn-3.0Ag-0.5Cu (SAC) solder alloys were used in this work. These two alloys were prepared from 99.9% Sn, 99.99% Cu, 99.9% Bi and 99.99% Ag. All the raw materials were melted in a ceramic crucible at 600 °C for 30 min. Mechanical stirring was needed to homogenize the solder alloy. To prevent the solder from oxidation during the melting, KCl:LiCl (1.3:1) were used over the surface of liquid solder. The melt was casted as an ingot in a steel mold. Then Sn-58Bi and Sn-3.0Ag-0.5Cu were used as starting materials to produce Sn-Bi and Sn-Ag-Cu mixed solder alloys by remelting them with different ratio. The compositions were SB-20SAC, SB-25SAC and SB-50SAC in weight percent, respectively. The corresponding weight ratio between Sn-58Bi and Sn-3.0Ag-0.5Cu solder was 4:1, 3:1 and 1:1, respectively, and the final compositions of mixed solders were listed in Table 1. Each solder alloy was casted as ingot. A film of 400 mm in thickness, 1 mm in width and 15 mm in length was also supplied for soldering the rectangle type solder joint specimen. Preparation on the solder joint is illustrated in Fig. 1. Oxygen-free Cu (OFC) with
SB SB-20SAC SB-25SAC SB-50SAC SAC
the size of 15*15*1 mm was used as the base metal. The test specimen was soldered at a peak temperature of 250 °C for a holding time of 15 s, and then was cut by electrical discharge machining to get the test pieces with size of 1 mm on the depth and width and 30 mm on the length. Microstructural characterization of the solder alloys was performed. The solder was metallographically polished to a final 0.05 mm using colloidal silica solution, and then examined with a scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) and electron probe micro analyzer (EPMA). The phases in each solder were detected using X-ray diffraction (XRD). The mechanical properties of solder alloys were tensile tested with specimen shown in Fig. 2a, while the properties on solder joint were measured with a three point flexural method shown in Fig. 2b. During bending, the radius of loading pin and two supporting pins was 2 mm, and the support span between two supporting pins was 13 mm. To investigate the effect of strain rate, the loading pin was lowered from above at a constant rate of 0.5, 0.1, 0.05 and 0.01 mm/s, respectively. The crack propagation in solder joints was observed using optical microscope (OM), the fractured surfaces were observed using SEM, and the compositions of phases were characterized by EDX.
3. Results and discussion 3.1. Microstructure characterization
Table 1. Elemental composition of Solder alloys. Solder
225
Element compositions (wt%) Sn
Ag
Cu
Bi
bal bal bal bal bal
– 0.6 0.75 1.5 3.0
– 0.1 0.125 0.25 0.5
58 46.4 43.5 29 –
Fig. 1. Schematic of solder joint fabrication.
Fig. 3 shows the microstructure of as-prepared Sn-58Bi solder and SnBi-SnAgCu composition mixed solders with Sn-Ag-Cu content of 20, 25 and 50 wt% in Sn-Bi. The bright and dark grey regions were identified as Bi-rich phase and β-Sn phase, respectively. For Sn-58Bi eutectic solder, during solidification eutectic reaction was occurred in liquid solder (L) at 138 °C with the following eutectic reaction from Sn-Bi binary phase diagram [30]:
Fig. 2. Schematics of (a) mini-tensile specimen on solder alloy and (b) three point flexural testing on solder joint.
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Fig. 3. Microstructures of as-prepared solders: (a) Sn-58Bi, (b) SB-20SAC, (c) SB-25SAC and (d) SB-50SAC.
L → (β − Sn) + ( Bi)
(1)
The resulted solder matrix was composed of lamellar eutectic structure where the Bi-rich phases and the Sn-rich phases interlocked with each other alternately, as shown in Fig. 3a. A few primary crystals of Sn dendrites were observed due to the nonequilibrium cooling. With Sn-Ag-Cu mixed into Sn-Bi solder, the microstructure was obviously changed. For solder with 20 or 25 wt% Sn-Ag-Cu, it can be clearly seen that the number of Sn dendrites increased, Sn-Bi eutectic structure was coarsened, and isolated Cu6Sn5 phases were produced, while element Ag was not easily differentiated from the solder matrix. To identify all the existing phases, each solder was examined by XRD, and the results on SB-SAC mixed solders with different content of Sn-Ag-Cu are illustrated in Fig. 4, in which Sn-58Bi eutectic and Sn-3.0Ag-0.5Cu solder were also analyzed. Cu and Ag were existed as Cu6Sn5 and Ag3Sn IMCs, respectively. Research on Sn-Ag-Bi ternary system has also shown that there existed a ternary eutectic reaction at 138.4 °C [31]:
L → ϵ − Ag3Sn + (β − Sn) + ( Bi)
(2)
To confirm the distribution of Ag3Sn and Cu6Sn5 in the solder matrix, the element distribution was investigated using EPMA. Fig. 5 shows the results of elemental mapping on SB-25SAC solder. Ag and Cu atoms were detected on the significant areas, in which Cu was coexisted with Sn, indicating Cu-Sn IMCs were isolated in the solder matrix, while Ag was simultaneously coexisted with Sn and Bi, indicating Ag3Sn IMCs were coexisted with Sn and Bi. Therefore, after Sn-Ag-Cu solder was added into Sn-Bi solder, Sn-Bi was transformed into hypo-eutectic system. During solidification, primary Sn dendrite was firstly precipitated from liquid solder
Fig. 4. XRD patterns of Sn-Bi, Sn-Ag-Cu and SB-SAC mixed solders.
through the following reaction:
L → L + (β − Sn)
(3)
The amounts of primary Sn dendrite increased with the addition of Sn-Ag-Cu solder. Cu was reacted with Sn to form Cu6Sn5 IMC at 227 °C:
Cu + Sn → Cu6 Sn5
(4)
After the temperature reached at 138 °C, (Eqs. (1) and 2) were simultaneously occurred to produce lamellar Sn-Bi eutectic and Sn-Bi-Ag eutectic structure, respectively, as the results shown in Fig. 3b and c. With 50 wt% Sn-Ag-Cu mixed into Sn-Bi solder, as
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227
Fig. 5. Microstructure and element mapping of SB-25SAC: (a) SEM image, (b) Ag, (c) Sn, (d) Bi and (e) Cu.
illustrated in Fig. 3d, large amounts of primary Sn dendrites were produced, and Cu6Sn5, Ag3Sn and Bi particles were finely distributed in the solder matrix due to decreasing on the Bi composition while increasing on the Sn, Ag and Cu compositions in the solder. 3.2. Mechanical behavior Mechanical properties of solder joints were investigated with three point flexural test considering the effect of strain rate. The typical examples of load-displacement curves obtained from the bending test are shown in Fig. 6, in which the curves in 6a are from different solder compositions under constant loading rate of 0.50 mm/s, and the curves in 6b are from Sn-58Bi eutectic solder joints under different loading rate of 0.01, 0.05, 0.10 and 0.50 mm/ s, respectively. To clearly illustrate the mechanical behavior of
solder joint, fracture energy was calculated by area under the loaddisplacement curve, and described the ability to fracture resistance for solder joint during bending. The detailed results on fracture energy including the effect of solder compositions and loading rate are shown in Fig. 7. It seems that Sn-58Bi solder had the highest bending strength, which increased with the strain rate, but its fracture energy was the lowest among all the solder joints and was independent of loading rate. With Sn-Ag-Cu solder mixed into Sn-Bi solder, the joint strength decreased, while the fracture energy increased and reached the maximum with the mixed composition of 25 wt% Sn-Ag-Cu in Sn-Bi solder, and then decreased with the increasing on content of Sn-Ag-Cu solder. The fracture energy was also influenced by the loading rate, and showed better results under the lower loading rate. From Fig. 6a, it is clearly seen that no fracture was occurred in Sn-3.0Ag-0.5Cu solder joints under bending test, and so the fracture energy of Sn-
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Fig. 6. Bending curves from (a) effect of solder composition under loading rate at 0.5 mm/s, and (b) effect of loading rate on Sn-58Bi eutectic solder.
Fig. 7. Effect of solder composition and loading rate on fracture energy during bending test.
Ag-Cu joints was not included in Fig. 7, which means Sn-Ag-Cu solder had the excellent fracture toughness. Therefore, with SnAg-Cu solder added into Sn-Bi solder, the fracture energy of solder increased, especially under the lower loading rate, which was attributed to the decrease on lamellar Sn-Bi eutectic phases and the increase on the finely distributed IMC particles in solder matrix as illustrated in Fig. 3. There was no obvious effect with the solder composition on the fracture energy under higher loading rate of 0.5 mm/s. In fact, the fracture toughness was controlled by the crack propagation in bulk solder or along interface during bending test. The solder joints usually showed a brittle fracture under a higher strain rate, and the fracture resistance was mainly decided by the interfacial bonding. 3.3. Fracture mode During reflow soldering, solder reacts with Cu substrate to produce a Cu6Sn5 IMC layer at the interface. Joint mechanical behavior is usually strain rate dependent, in which the joint strength was controlled by the bulk solder at low strain rate and by the brittle IMC layer at high strain rate [32,33]. Correspondingly, three failure modes are existed in solder joints during mechanical test: ductile solder mode, brittle IMC mode and mixed solder/IMC mode
[34]. To investigate the effect of strain rate on the fracture modes of SnBi-SnAgCu composition mixed solder joints during bending test, the crack propagation of solder joints under different loading rate is illustrated in Fig. 8. The complete fracture surfaces of solder joints with selected bending rate of 0.5 mm/s were examined using SEM, and the resulting images are shown in Fig. 9. During three point flexural test, direct tensile and compress stress were produced in the lower (left side in Fig. 9a) and upper region (right side in Fig. 9b) of solder joint, respectively. Crack was always initiated at the tensile region, and then finally propagated into bulk solder. Therefore, the failure mode was mainly determined by the crack in the tensile region and was dependent on the solder composition and strain rate. For Sn-58Bi eutectic solder joints shown in Fig. 8a, the crack was always propagated along the interface between solder and Cu substrate, and was a typical brittle IMC fracture mode independent of loading rate. For SB-20SAC and SB-25SAC solder joints, as shown in Fig. 8b and c, no fracture was produced under a lower loading rate at 0.01 mm/s, while with increasing loading rate, fracture was occurred in the bulk solder under 0.05 and 0.1 mm/s, and then transformed into the IMC failure mode under 0.5 mm/s. However, with content of Sn-Ag-Cu increased to 50 wt% in Sn-Bi solder, the crack was initiated and propagated in the bulk solder under rate of 0.01 mm/s, but was propagated along the interface under rate of 0.05, 0.1 or 0.5 mm/s, as illustrated in Fig. 8d. These brittle IMC fracture modes were easily found in Fig. 9 from the complete fracture surfaces under loading rate of 0.5 mm/s: brittle fracture at IMC/solder interface from the tensile region and ductile fracture in the solder from the compress region, respectively. Some solder residues were also found in the crack. The magnified images from tensile region in the fracture surface for each solder joint are illustrated in Fig. 10. The compositions of typical phases existed at the fracture surface were also identified using EDX with the compositions listed in Table 2. It can be found that all the joints exhibited partly brittle structure. In Sn-58Bi solder joint, Cu6Sn5 IMC layer (a), Bi-rich phase (b) and Sn dimple (c) were simultaneously observed in Fig. 10a, which indicates that the fracture path is partly along the solder/Cu6Sn5 IMC interface and partly within the coarsened Birich phase. With 20 or 25 wt% Sn-Ag-Cu mixed into Sn-Bi solder, the ratio of fracture on IMC layer to fracture within coarsened Birich phase increased, as shown in Fig. 10b and c. With 50 wt% SnAg-Cu in Sn-Bi solder, the fracture surface was mostly composed of Cu6Sn5 layer, and the voids composed of Cu3Sn caused from the peeling of Cu6Sn5 (h in Fig. 10d) IMC were occasionally observed in Fig. 10d.
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Fig. 8. Cross-sectional images of joints showing crack propagation after bending test under different loading rate: (a) Sn-58Bi, (b) SB-20SAC, (c) SB-25SAC and (d) SB-50SAC.
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Fig. 9. Macro fracture images on (a) Sn-58Bi, (b) SB-20SAC, (c) SB-25SAC and (d) SB-50SAC solder joint after bending under rate of 0.5 mm/s.
It seems there was a relationship between loading rate, crack propagation and fracture energy from Figs. 7 and 8. When a crack was occurred along the interface between solder and Cu substrate, there was a lower fracture energy or toughness resistance for solder such as the Sn-58Bi solder under all the loading rates or all SnBi-contained solders under loading rate of 0.5 mm/s. When a crack was occurred in the bulk solder, higher fracture energy was usually produced in the joint, and then the fracture resistance was decided by the inherent ductility of solder. To investigate the effect of strain rate and compositions on the ductile or brittle mode of SnBi-SnAgCu composition mixed solder joint during bending test, three samples on each composition were tested under different loading rate, and the brittle failure along IMC/solder interface or ductile failure in solder is summarized in Fig. 11. Brittle mode for Sn-58Bi and ductile mode for Sn-3.0Ag0.5Cu were always observed independent of loading rate, respectively. With Sn-Ag-Cu content in SB-SAC mixed solder increasing, the fracture mode was transformed from brittle to ductile, and the red line in Fig. 11 should be followed during transformation combined with the effect of loading rate. The brittle characteristic of solder joint under high strain rate was mainly caused by the following reason. At the interface between solder and IMC layer in a solder joint, Sn, Bi and Cu6Sn5 were simultaneously existed. From their lattice constants listed in Table 3, it can be clearly seen that
poor coherent grain boundary was existed among them due to the great difference on the constants. Therefore, during bending on solder joint under a higher strain rate, the dislocation was easily piled up at the boundary between solder and IMC. Meanwhile, stress was also easily concentrated at the interface due to the great difference on modulus of solder and Cu6Sn5. Combined stress concentration with piled dislocation, crack was easily initiated at the interface between solder and IMC layer under a higher loading rate. Under the lower strain rate, the location of crack initiation was mainly determined by the inherent brittle or ductile of solder due to the lower piled dislocation and stress concentration at the interface between solder and IMC layer. Therefore, tensile tests were performed on all five solder alloys with the strain-stress curves shown in Fig. 12a, and the resulted tensile strength and elongation were shown in Fig. 12b. It can be found that Sn-58Bi and SB-50SAC were brittle solders with higher tensile strength and lower elongation, while SB-20SAC and SB-25SAC mixed solders were similar as Sn-3.0Ag-0.5Cu solder, showing a ductile property. For Sn-Bi and SB-50SAC solder joints, the failure between IMC layer and solder was produced before the stress concentration reached the tensile strength of solder during bending test under most loading rates.
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Fig. 10. Magnified fracture surfaces on the tensile region from Fig. 9(b): (a) Sn-58Bi, (b) SB-20SAC, (c) SB-25SAC, (d) SB-50SAC; (e) and (f) are magnified boxes in (c) and (d).
4. Conclusions In this paper, different content of Sn-3.0Ag-0.5Cu solder was mixed into Sn-58Bi solder to increase the ductility of Sn-Bi solder.
The microstructure and tensile properties of SB-20SAC, SB-25SAC and SB-50SAC mixed solder alloys, and their bending properties with Cu/solder/Cu joints were investigated and compared with Sn-58Bi and Sn-3.0Ag-0.5Cu solder. In addition, the crack
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propagation, fracture surface and failure mode of the couples were examined. The results are summarized as follows: 2) 1) The addition of 20 or 25 wt% Sn-Ag-Cu solder in Sn-Bi solder increased the elongation property of the Sn-Bi alloy because of 3) Table 2. EDX results on points shown in Fig. 10. Location
Composition on at%
a b c d e f g h
note
Sn
Cu
Bi
42.15 4.05 77.89 45.07 3.16 10.77 41.77 15.73
57.16 2.02 11.89 50.98 20.96 25.52 55.58 83.06
0.69 93.93 10.22 3.94 75.88 63.71 2.65 1.21
Cu6Sn5 Bi Sn Cu6Sn5 Bi Bi Cu6Sn5 Cu3Sn
4)
5)
the formation of Cu6Sn5 IMC phases and Sn-Ag-Bi eutectic phases in the solder matrix and the decrease of Bi brittle phases. The tensile results showed that SB-50SAC mixed solder exhibited a brittle characteristic as same as Sn-58Bi solder, but SB20SAC and SB-25SAC mixed solders had improved ductility compared with Sn-58Bi solder. The fracture toughness of solder joints was dependent on the loading rate during three point flexural test, and SB-25SAC mixed solder joint exhibited the greatest toughness among all the bending tested solder joints. A relationship between loading rate, crack propagation and fracture toughness was developed. The joint with crack occurred along the interface between solder and IMC layer showed lower fracture energy, which was mainly existed under the bending on a higher loading rate or in a brittle solder, and the joint with crack produced in the solder matrix showed a higher fracture resistance. The distribution on fracture mode of SB-SAC composition mixed solder joints was plotted with the effect of strain rate and solder compositions.
Conflict of interest No.
Acknowledgement
Fig. 11. Ductile and brittle fracture distribution on solder joints. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
This research is supported by projects funded by National Natural Science Foundation of China (Grant No. 51541104), Jiangsu Planning Project of Science and Technology (Grant Nos. BK20 12163, BK20150466) and the State Key Lab of Advanced Welding & Joining, Harbin Institute of Technology (Grant No. AWJ-M13-10).
Table 3. Lattice constants of Sn, Bi and Cu6Sn5. Lattice constant
a*b*c (Å)
α*β*γ (deg.)
Cu6Sn5 Sn Bi
11.03*7.29*9.83 5.831*5.831*3.182 4.547*4.547*11.86
90.0*98.82*90.0 90.0*90.0*90.0 90.0*90.0*12 0.0
Fig. 12. Tensile results on solder alloys: (a) strain-stress curves, (b) relationship between content of Sn-Ag-Cu and strength/elongation.
F. Wang et al. / Materials Science & Engineering A 668 (2016) 224–233
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