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Cu lead-free solder joint
Accepted Manuscript Intermetallic compound formation and mechanical property of Sn-Cu-xCr/Cu lead-free solder joint Junghwan Bang, Dong-Yurl Yu, Yong-Ho Ko, Min-Su Kim, Hiroshi Nishikawa, ChangWoo Lee PII:
S0925-8388(17)33039-6
DOI:
10.1016/j.jallcom.2017.09.011
Reference:
JALCOM 43067
To appear in:
Journal of Alloys and Compounds
Received Date: 16 March 2017 Revised Date:
29 August 2017
Accepted Date: 1 September 2017
Please cite this article as: J. Bang, D.-Y. Yu, Y.-H. Ko, M.-S. Kim, H. Nishikawa, C.-W. Lee, Intermetallic compound formation and mechanical property of Sn-Cu-xCr/Cu lead-free solder joint, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.09.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Intermetallic compound formation and mechanical property of Sn-Cu-xCr/Cu leadfree solder joint
1,2,3,
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Chang-Woo Lee
Micro-Joining Center, Korea Institute of Industrial Technology, 156 Gaetbeol-ro, Yeonsu-gu, Incheon 406-840, Korea
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*, Dong-Yurl Yu , Yong-Ho Ko , Min-Su Kim , Hiroshi Nishikawa and
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Junghwan Bang
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Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
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Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
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*Corresponding author.
Tel.: +82-32-850-0217; fax: +82-32-850-0210
E-mail address: [email protected] (J.H. Bang)
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ACCEPTED MANUSCRIPT Abstract The shear strength and interface characteristics of a new Pb-free solder, Sn-0.7Cu-0.2Cr (in wt.%) alloy, were estimated to evaluate its suitability for automobile electronics modules. The mother alloy of this environmentally friendly Sn-0.7Cu-0.2Cr solder, which has a moderate melting temperature of about
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231 °C, was fabricated first, and 300 µm-diameter solder balls were made from this mother alloy. Then, prototype modules were fabricated by reflow using the solder balls. A shear test was performed, and the interface microstructures were observed after aging under various conditions. For comparison, the
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commercial Sn-3.0Ag-0.5Cu and Sn-0.7Cu solders were also tested. After aging at 125 and 150 °C, the shear strength of the Sn-0.7Cu-0.2Cr solder joint was lower than that of the Sn-3.0Ag-0.5Cu joint but
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higher than that of the Sn-0.7Cu joint. An irregular intermetallic compound (IMC) layer also grew, and its average thickness was measured for each aging condition. The thickness of the IMC layer of the Sn-0.7Cu0.2Cr solder joint was smaller than those of the other two solder joints. From these results, it is expected
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that the new solder will be more reliable than other commercial solders.
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Keywords: metals and alloys; intermetallics; solid state reactions; mechanical properties; microstructure
ACCEPTED MANUSCRIPT 1. Introduction
Recently, Pb-free soldering in automotive electronic components has become a hot issue because of new restrictions on the use of Pb, such as those in the End of Life Vehicle (ELV) directive, which dictates that
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Pb-bearing solder cannot be used in the automotive electronics after January 1, 2016 [1,2]. In the general electronics industry, Pb-bearing solder was successfully replaced with Pb-free solder. However, it is difficult to use Pb-free solder in the vehicle industry because of reliability and safety issues. In particular,
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the electric control units (ECUs) in vehicles are exposed to more severe thermal shocks and thermal vibration conditions. For example, ECUs around the transmission and engine are subject to vibrations of
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3G to 20G, and their operating temperatures are approximately 130 °C [3]. Therefore, development of a solder material that is more reliable is required. Some Pb-free solders have been suggested to replace Pbbearing solder, like Sn-3.0Ag-0.5Cu, Sn-3.5Ag, Sn-Au, Sn-Sb, Sn-Cu, and Sn-Zn solders. However, these have all had problems under actual operating conditions. For example, Sn-3.0Ag-0.5Cu alloy showed a
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decreased bonding strength and exhibited creep under high-temperature conditions [4]. Sn-Au alloy has a high cost for raw materials due to usage of Au metal. Zn and Al-based alloys exhibit poor wettability due to their strong susceptibility to oxidation [5,6]. In addition, Bi-based alloy has limited uses because of poor
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mechanical properties and a low melting temperature [7]. Cost is another important factor in the adoption
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of solders for practical electronic applications. Eutectic Sn-0.7Cu solder has a low cost, relative high melting temperature and moderate mechanical properties. However, disadvantages of the Sn-0.7Cu solder are a relative low wettability and reliability. It was reported that a minor addition of Cr effectively inhibits the growth of Cu-Sn IMCs after reflow on the Cu substrate [8,9]. In this study, a new Sn-0.7Cu-0.2Cr solder material was investigated for automobile electronics applications Solder balls were fabricated and used to make prototype modules. The shear strength of the solder ball was measured, and its microstructure was observed after aging. Finally, these results were compared with the commercial solders Sn-3.0Ag0.5Cu and Sn-0.7Cu to evaluate the reliability of Sn-0.7Cu-0.2Cr solder.
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The compositions of solder alloys used in this experiment were Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn0.7Cu-0.2Cr (in wt.%). The mother alloys were made by induction heating in a high-vacuum argon
were fabricated by reflow process with the solder balls.
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atmosphere at 1100 °C. Then, solder balls with diameters of 300 µm were fabricated. The test modules
The substrate material of test module was FR4 PCB, and the metal pad was Cu with an organic
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solderability preservative (OSP) surface finish. The thickness of the OSP layer was approximately 0.3 µm. The size of Cu opening pad was approximately 230 µm, and its thickness was about 20 µm. To avoid
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oxidation during the reflow process, flux was coated on the Cu pad. The reflow process was carried out by using an infrared 9-zone reflow machine. The conditions of reflow process are shown in Table 1. The peak temperatures of the Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr solders were 250, 255, and 260.5 °C, respectively. After the reflow process, the test samples were cooled to room temperature in air. Figure 1
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shows the schematic graph of the reflow profile for the Sn-0.7Cu-0.2Cr solder.
Figure 1 Reflow profile for Sn-0.7Cu-0.2Cr solder
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time
Pre-heating above
Solder
Peak reflow
melting temperature(°C)
Time(s) temperature(s) 53
Sn-0.7Cu
73
47
Sn-0.7Cu-0.2Cr
73
47
250
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255
260.5
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Sn-3.0Ag-0.5Cu
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To measure the melting temperatures of the three solder materials, differential scanning calorimetry (DSC) analysis was performed. Approximately 8 mg samples of the three solder alloys were used for the DSC analysis. The measurement was carried out in three steps. First, the sample temperature was stabilized at 50 °C, and then it was increased up to 300 °C at a rate of 10 °C/min. Finally, the temperature was cooled
reactions.
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down to 100 °C at a rate of 10 °C/min to check the heat flow and analyze the endothermic and exothermic
Aging tests were carried out at temperatures of 125 °C and 150 °C for up to 1000 h. After the reflow
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process and aging tests, cross-sectional images of the samples were observed by field-emission scanning electron microscope (FE-SEM). The FE-SEM sample was fabricated by polishing and etching. The etchant
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was prepared by mixing of 95% C2H5OH, 3% HNO, and 2% HCl. The thickness and composition of the IMC layers were measured by FE-SEM and energy-dispersive X-ray spectroscopy (EDS). The average thickness of the IMC layer was evaluated from the total area of the IMC layer divided by the total horizontal length using a commercial imaging program. To understand the mechanical properties after aging, a shear test was carried out. In the shear test, the shear speed was 200 µm/s, and the height of the shearing tip was 50 µm. The shear test was repeated 20 times, and the average shear strength was obtained by excluding the maximum and minimum values.
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3.1 Melting temperatures of solder alloys
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Figure 2 DSC results for Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr solders.
The DSC results are shown in Figure 2. The measured melting temperatures of the Sn-3Ag-0.5Cu, Sn0.7Cu, and Sn-0.7Cu-0.2Cr solders were 219, 228, and 231 °C, respectively. Thus, the addition of 0.2 wt.%
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Cr to the Sn-0.7Cu solder increased the melting temperature by about 3 °C.
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3.2 Interfacial microstructure
Figures 3 and 4 show cross-sectional SEM images of Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr solder joints after reflowing and aging at 125 °C and 150°C for up to 1000 h. The images were obtained in the backscattered-electron (BSE) imaging mode of the SEM, which can provide more distinguishable boundaries between the phases and interfacial layers. As shown in Figure 3, a typical scallop-type Cu-Sn IMC layer at the solder/Cu interface was formed after the reflow process for all three solders. The
ACCEPTED MANUSCRIPT compositions of the IMC layer were 54-56 at.% Cu and 44-46 at.% Sn from the EDS analysis, which is equivalent to Cu6Sn5. The IMC was thickest for Sn-3.0Ag-0.5Cu among the three solders. Therefore, Sn diffusion into the Cu pad and IMC growth were faster for Sn-3.0Ag-0.5Cu than for Sn-0.7Cu or Sn-0.7Cu-0.2Cr. The thickness
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of the interfacial IMC layer for the Sn-0.7Cu was thicker than that for the Sn-0.7Cu-0.2Cr solders after aging at 125 °C. As reported previously [10], the grain size of the Cu6Sn5 IMC in Sn-3.0Ag-0.5Cu is very fine compared with that in Sn-0.7Cu. Additionally, it was also previously reported [11] that the length of
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the Cu6Sn5 IMC column was shortened by addition of Cr, which changed the IMC into a rounded crystalline shape.
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After aging under severe conditions of 150 °C for 1000 h, the IMC growth rate was nearly same for the Sn-3.0Ag-0.5Cu and Sn-0.7Cu solders. Aging under these conditions coarsened the Cu6Sn5 IMC in Sn3.0Ag-0.5Cu. On the other hand, the addition of Cr to Sn-0.7Cu solder retarded the growth of the Cu6Sn5 IMC layer at the Sn-0.7Cu-0.2Cr/Cu interface.
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Figure 4 also shows that a very thin Cu3Sn IMC layer was formed at the interface between the Cu6Sn5 IMC layer and the Cu pad after aging at 150 °C for 100 h for Sn-3.0Ag-0.5Cu and Sn-0.7Cu solders. For Sn0.7Cu solder, the Cu3Sn IMC was the thickest [11-13]. However, a Cu3Sn IMC was not observed in the Sn-
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0.7Cu-0.2Cr solder alloy before aging at 150 °C for 500h. After aging at 150 °C, the IMC thicknesses in
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the Sn-3.0Ag-0.5Cu and Sn-0.7Cu alloys were similar. The Cu-Sn IMC changed gradually from a scalloptype morphology to a layer-type morphology during aging at 150 °C. The Sn-0.7Cu-0.2Cr solder had the thinnest IMC layer among the three solders [14-17]. Although Cr addition was studied in some previous papers [12-15], the mechanism of Cr addition effect is not clear. It is assumed that Cr addition prevented IMC growth. The total and Cu3Sn IMC thicknesses are shown in Figures 5 and 6 The thickness of total IMC layer are following order : Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr. A diffusion of metal atoms under solid-state reaction is accelerated by grain boundaries and defects, which can serve as fast diffusion paths or channels [18]. Thermodynamically, the formation of Cu3Sn IMCs between Cu6Sn5 and
ACCEPTED MANUSCRIPT Cu is favorable, but it can be controlled by a kinetic parameter such as the diffusion of Sn atoms into Cu6Sn5 IMCs or Cu atoms under bump metallurgy (UBM) structures [18]. Therefore, the thickness of total and Cu3Sn IMC layers for the Sn-0.7Cu-0.2Cr solder joint was the thinnest among three solder joints in this study.
Figure 7 shows top views of the Cu6Sn5 IMCs formed at the interfaces of the three solder
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joints. The grain size of the Cu6Sn5 IMC in the Sn-0.7Cu was the largest, while that in the Sn-0.7Cu-0.2Cr was the smallest. As a result, Cr addition to the Sn-0.7Cu solder reduced the size of the interfacial Cu6Sn5 IMCs.
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As shown in Figure 8(a), Cr compounds can be confirmed near the Sn/Cu6Sn5 interface in EPMA mapping. The Cr K signal is detected with Sn L signal together, thus it seems Cr-Sn compounds. There are
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no intermetallic phases in an equilibrium phase diagram of Cr-Sn binary system [19]. Some reports have suggested meta-stable intermetallic compounds between Cr and Sn, such as CrSn2 [20] and Cr2Sn3 [19]. Koo et al. [8] also presented the presence of CrSn2 intermetallic compounds phase near the interface in the minor Cr and Ca adopted Sn-0.7Cu solder using an EPMA analysis.
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In our cases, the size of Cr compounds exhibits approximately under 0.5 µm, which is smaller than a spatial resolution of SEM based spectroscopy analysis. For more precise analysis on this Cr compound, we utilized a TEM technique, which can offer a spectroscopic data with the very fine spatial resolution as well
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as a diffraction pattern on the same position. An in-situ lift-out method using a focused ion beam (FIB,
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JIB-4500, JEOL) was employed in order to prepare a specific-sited TEM lamellar sample including Cr compounds, where marked with a dotted box in BSE image of Figure 8(a). A carbon layer with the thickness of 2 µm was deposited on the surface of cross-section sample prior to FIB milling, and the exact position of Cr was marked by FIB line milling, as shown in Figure 8(b). The direction of TEM lamellar sample was denoted as that “A” is a solder side and “B” is a Cu pad side in EPMA (Figure 8(a)), FIB (Figure 8(b)) and TEM (Figure 9) images. Figure 9 shows a scanning TEM (STEM) bright field (BF) image and EDS mapping results on the Sn L, Cu K and Cr L. Cr compound located at the interface of Cu6Sn5/solder and near the interface of it. We can
ACCEPTED MANUSCRIPT infer that this compound is a CrSn2 intermetallic compound through the EDS spot analysis on these compounds (Sn: 69.3 %, Cr: 29.1 % and Cu 1.6 % in atomic percent). A selected area electron diffraction (SAED) pattern of Cr-Sn compound, obtained from denoted area with dotted circle in Figure 10, shows good agreement with the reported crystal structure of an orthorhombic CrSn2 (space group: Fddd; proto
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type: Mg2Cu; a=541.7 pm, b=944.0 pm and c=1839.6 pm) intermetallic compound [19]. Thus, it can be
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identified as CrSn2 with the combination of EDS point analysis and SAED pattern.
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Figure 3 Cross-sectional SEM images of IMC layers after aging at 125 °C for up to 1000 h
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Figure 4 Cross-sectional SEM images of IMC layers after aging at 150 °C for up to 1000 h
Figure 5 Total IMC layer thickness after aging at (a) 125 °C and (b) 150 °C
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Figure 6 Cu3Sn IMC layer thickness after aging at (a) 125 °C and (b) 150 °C
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(a)
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(b)
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Figure 7 Top views of Cu6Sn5 IMCs formed at the interfaces in three solder joints aged at (a) 125 and (b) 150 °C for up to 1000 h
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Figure 8 (a) EPMA element mapping of the as-reflowed Sn-0.7Cu-0.2Cr solder joint and (b)
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FIB image of TEM sample preparation position
Figure 9 STEM BF image and EDS mapping results of Cu6Sn5/solder interface
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Figure 10 (a) STEM BF image and (b) SAED pattern of Cr-Sn compound
3.3 Shear strength of solder joints.
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The ball shear test was carried out to evaluate effect of the Cr addition on the mechanical reliability of the solder joints after aging under different conditions. Figures 11 shows the shear
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strength of these solder joints and the variations in the decreasing rates with aging test at 125 °C and 150 °C, respectively. The as-reflowed joints had comparatively high shear forces, but the
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shear forces decreased rapidly during the initial 100 h of aging and then decreased more slowly up to 1000 h of aging.
Sn-3.0Ag-0.5Cu solder exhibited biggest drop in shear force, of 250 gf in as-reflowed condition and 196 gf after 1000 h of aging at 125 °C. The shear force of the Sn-0.7Cu solder was 190 gf in as-reflowed condition and 156 gf after 1000 h of aging at 125°C. The shear force of the Sn-0.7Cu-0.2Cr solder was higher than that of the Sn-0.7Cu solder by about 10 gf for all aging times. Under harsher aging conditions, the shear forces of Sn-3.0Ag-0.5Cu and Sn-0.7Cu solders
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decreased to 189 gf and 156 gf, respectively, after 1000 h of aging at 150 °C. As was the case for aging at 125°C, the shear force of the Sn-0.7Cu-0.2Cr solder was higher than that of the Sn0.7Cu solder by about 10 gf for all aging times at 150 °C. We assume that the shear force was
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slightly decreased from its initial value after 100 h of aging because of softening of the solder alloy [10, 18, 21, 22]. As a whole, the shear force for three solder joints slightly decreased with
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increasing aging time irrespective of different aging temperature.
Figure 11 Shear strength and decreasing rate of Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu0.2Cr during aging at (a)(b) 125 °C and (c)(d) 150 °C
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To verify the variation of the shear strength, the top views of the fracture surfaces after shear test were examined by SEM. Figure 12 shows the fracture surfaces of the samples aged at different aging temperatures and times. Regardless of the aging condition, the fracture always
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occurred in the bulk solder. In all samples, the fracture surfaces showed a ductile failure. Generally, in the solder ball shear test, a fracture occurs at the interface or in the solder region with the lowest strength [23, 24]. Over the years many studies have been published concerning
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the mechanical properties of solder joints. It was known that excessively thick reaction layers formed between solder and substrate could significantly degrade the mechanical properties of the
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solder joints [24]. However, in this study, fracture analysis indicated that the shear strength could not be related to the thicknesses of the IMC layers formed at the interfaces between three solder and Cu substrate. At all conditions, fracture occurred in the bulk solder regardless of the aging conditions. And the decrease in reliability occurs at the interface between the solder and
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substrate when a thick IMC layer is formed at the interface [25]. The results of interfacial reaction and mechanical tests showed that the new Sn-0.7Cu-0.2Cr/Cu solder joint has the higher strength, lower decreasing rate of shear strength, and a small IMC compared to the conventional
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Sn-0.7Cu solder.
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4. Discussion
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Figure 12 Fracture surfaces of three solder joints aged under different conditions
Based on the results above, such minor amount of Cr greatly retarded the interfacial IMC
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growth and enhanced the joint mechanical property. Therefore, it is worthwhile to figure out the effect of Cr on the interfacial IMC growth. Fundamentally, the formation and growth of the
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interfacial IMC growth is caused by the Cu and Sn diffusion through the interface. Thus, we evaluate the effects of Cr on the Cu and Sn diffusion separately by calculating the amounts of Cu and Sn atoms in the interfacial IMC layers as follows [26, 27] (1) (2)
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in which M is the total weight of the Cu or Sn element per unit area, T is the thickness of the Cu6Sn5 or Cu3Sn layers, ν is the weight fraction of Cu or Sn in the Cu6Sn5 or Cu3Sn molecule and ρ is the density of the Cu6Sn5 or Cu3Sn IMCs, which is 8.45 or 8.97 g·cm3, respectively [28].
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The results listed in Figure 13 show that both the Sn and Cu diffusion rates increase with improved reaction temperature, and more importantly, it is clear that the addition of Cr slightly increased Sn diffusion but significantly repressed the Cu diffusion under thermal aging test.
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From these results, we believe that Cr inhibited the solid state interfacial IMC growth primarily through slowing down the Cu diffusion through the interface. It is well known that only the
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Cu3Sn and Cu6Sn5 phases are formed at the SAC/Cu interface. Therefore, under the similar Sn diffusion condition, a lower Cu diffusion flux leads to repression of the Cu3Sn layer growth at the interface.
Another problematic issue is how Cr affects the diffusion of the interfacial atoms and their
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combination. Unlike Ni atoms, Cr atoms cannot dissolve into the CuSn IMC to form a solid solution based on the TEM and EPMA findings (Fig. 8 and 9), many CrSn2 particles located at the interface and these particles should experience ripening growth with prolonged aging test
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according to the thermodynamics [26]. The inhibiting behavior is probably related to the CrSn2 particles at the interface. It is likely these tiny particles and the flow of the Cr and Sn at the
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interface affects the Cu diffusion and the combination of Sn and Cu atoms. However, the root mechanism still requires further investigation.
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28
24
Mass (10-3ug/mm2)
22
26
Cu diffusion rate (125 OC) SAC305 D=9.7 SC07 D=15.6 SC-Cr D=3.4
24 22
Mass (10-3ug/mm2)
26
20 18 16 14
Sn diffusion rate (125 OC) SAC305 D=4.9 SC07 D=1.1 SC-Cr D=5.3
20 18 16 14
12
12
10
10 8
8 0
200
400
600
800
0
1000
200
50
50
Cu diffusion rate (150 OC) SAC305 D=0.0327 SC07 D=0.0342 SC-Cr D=0.0198
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Mass (10-3ug/mm2)
Mass (10-3ug/mm2)
40
600
800
1000
800
1000
Sn diffusion rate (150 OC) SAC305 D=0.0186 SC07 D=0.0154 SC-Cr D=0.0159
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400
Thermal aging (hours)
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Thermal aging (hours)
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35 30 25 20
35 30 25 20 15
15
10
10 200
400
600
800
1000
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Thermal aging (hours)
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200
400
600
Thermal aging (hours)
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5. Conclusions
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Figure 13 Plot of diffusivities of Cu and Sn versus thermal aging time.
Sn-0.7Cu-0.2Cr solder with a moderate melting temperature was fabricated for automotive applications. The changes in its microstructure and mechanical properties during hightemperature aging were evaluated, and the results were compared with those obtained for commercial solder alloys. The melting temperature was changed from 228 °C to 231 °C by the addition of 0.2 wt.% Cr to Sn-0.7Cu solder. The total thickness of the IMC layer in Sn-0.7Cu0.2Cr solder was less than those in Sn-0.7Cu or Sn-3.0Ag-0.5Cu solders after aging. According
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to the EPMA result, Cr compound was evenly distributed in the Sn-0.7Cu-0.2Cr solder matrix. Furthermore, the decreasing rate of Sn-0.7Cu-0.2Cr solder was lower than those of the Sn3.0Ag-0.5Cu and Sn-0.7Cu solders. The results of interfacial reaction and mechanical tests
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showed that the new Sn-0.7Cu-0.2Cr/Cu solder joint has the lower decreasing rate of shear
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strength and IMC growth rate.
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20. T. Wölpl, W. Jeitschko, “Crystal structures of VSn2, NbSn2 and CrSn2 with Mg2Cu-type
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structure and NbSnSb with CuAl2-type structure, Journal of Alloys and Compounds, 210
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(1994) 185-190.
21. J. Podzemský, V. Papež, J. Urbánek, K. Dušek, Influence of intermetallic compounds on RF resistance of joints soldered with lead free alloys, Radioengineering 21(2) 2012. 22. G. Zhao, G. M. Sheng, H. Xue, X. Yuan, Improved mechanical property of a Cu/Sn-9Zn0.1Cr/Cu joint using a rapidly solidified solder, Mater. Lett. 68 (2012) 129.
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23. D. G. Kin, S. B. Jung, Microstructure and Mechanical Properties of Sn–0.7Cu Flip Chip Solder Bumps Using Stencil Printing Method, Materials Transactions, Vol. 46, No. 11 (2005) pp. 2366 to 2371.
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24. C. B. Lee, S. B. Jung, Y. E. Shin, C. C. Shur, Effect of Isothermal Aging on Ball Shear Strength in BGA Joints with Sn-3.5Ag-0.75Cu Solder, Materials Transactions, Vol. 43, No. 8 (2002) pp. 1858 to 1863.
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25. J. W. Yoon, B. I. Noh, Y. H. Lee, H. S. Lee, S. B. Jung, Effects of isothermal aging and temperature–humidity treatment of substrate on joint reliability of Sn–3.0Ag–0.5Cu/OSP-
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finished Cu CSP solder joint, Microelectronics Reliability 48 (2008) 1864–1874 26. M. Yang, Y.H. Ko, J. Bang, T.S. Kim, C.W. Lee, S. Zhang, M. Li, Effects of Ag addition on solid–state interfacial reactions between Sn–Ag–Cu solder and Cu substrate, Materials Characterization, 124 (2017), 250-259
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27. M. Yang, H. Ji, S. Wang, Y. H. Ko, C. W. Lee, J. Wu, M. Li, Effects of Ag content on the interfacial reactions between liquid Sn–Ag–Cu solders and Cu substrates during soldering, Journal of Alloys and Compounds, 679 (2016), 18-25
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28. M. Yang, Y. Cao, S. Joo, H. Chen, X. Ma, M. Li, Cu6Sn5 precipitation during Sn-based solder/Cu joint solidification and its effects on the growth of interfacial intermetallic
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compounds, Journal of Alloys and Compounds, 582 (2014), 688-695
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Figure Captions
Figure 1 Reflow profile for Sn-0.7Cu-0.2Cr solder
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Figure 2 DSC results for Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr solders. Figure 3 Cross-sectional SEM images of IMC layers after aging at 125 °C for up to 1000 h. Figure 4 Cross-sectional SEM images of IMC layers after aging at 150 °C for up to 1000 h.
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Figure 5 Total IMC layer thickness after aging at (a) 125 °C and (b) 150 °C.
Figure 6 Cu3Sn IMC layer thickness after aging at (a) 125 °C and (b) 150 °C.
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Figure 7 Top views of Cu6Sn5 IMCs formed at the interfaces in three solder joints aged at (a) 125 and (b) 150 °C for up to 1000 h.
Figure 8 (a) EPMA element mapping of the as-reflowed Sn-0.7Cu-0.2Cr solder joint and (b) FIB image of TEM sample preparation position.
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Figure 9 STEM BF image and EDS mapping results of Cu6Sn5/solder interface Figure 10 (a) STEM BF image and (b) SAED pattern of Cr-Sn compound Figure 11 Shear strength and decreasing rate of Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-
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0.2Cr during aging at (a)(b) 125 °C and (c)(d) 150 °C Figure 12 Fracture surfaces of three solder joints aged under different conditions.
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Figure 13 Plot of diffusivities of Cu and Sn versus thermal aging time.
Highlights
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1. Sn-3.0Ag-0.5Cu alloy showed a decreased bonding strength and exhibited creep under hightemperature conditions. 2. Sn-0.7Cu-0.2Cr solder with a moderate melting temperature was fabricated for automotive
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applications. 3. The changes in its microstructure and mechanical properties during high-temperature aging were evaluated.
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4. The results were compared with those obtained for commercial solder alloys.
S0925-8388(17)33039-6
DOI:
10.1016/j.jallcom.2017.09.011
Reference:
JALCOM 43067
To appear in:
Journal of Alloys and Compounds
Received Date: 16 March 2017 Revised Date:
29 August 2017
Accepted Date: 1 September 2017
Please cite this article as: J. Bang, D.-Y. Yu, Y.-H. Ko, M.-S. Kim, H. Nishikawa, C.-W. Lee, Intermetallic compound formation and mechanical property of Sn-Cu-xCr/Cu lead-free solder joint, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.09.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Intermetallic compound formation and mechanical property of Sn-Cu-xCr/Cu leadfree solder joint
1,2,3,
1
1
2
1
Chang-Woo Lee
Micro-Joining Center, Korea Institute of Industrial Technology, 156 Gaetbeol-ro, Yeonsu-gu, Incheon 406-840, Korea
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*, Dong-Yurl Yu , Yong-Ho Ko , Min-Su Kim , Hiroshi Nishikawa and
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Junghwan Bang
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Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
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Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
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*Corresponding author.
Tel.: +82-32-850-0217; fax: +82-32-850-0210
E-mail address: [email protected] (J.H. Bang)
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ACCEPTED MANUSCRIPT Abstract The shear strength and interface characteristics of a new Pb-free solder, Sn-0.7Cu-0.2Cr (in wt.%) alloy, were estimated to evaluate its suitability for automobile electronics modules. The mother alloy of this environmentally friendly Sn-0.7Cu-0.2Cr solder, which has a moderate melting temperature of about
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231 °C, was fabricated first, and 300 µm-diameter solder balls were made from this mother alloy. Then, prototype modules were fabricated by reflow using the solder balls. A shear test was performed, and the interface microstructures were observed after aging under various conditions. For comparison, the
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commercial Sn-3.0Ag-0.5Cu and Sn-0.7Cu solders were also tested. After aging at 125 and 150 °C, the shear strength of the Sn-0.7Cu-0.2Cr solder joint was lower than that of the Sn-3.0Ag-0.5Cu joint but
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higher than that of the Sn-0.7Cu joint. An irregular intermetallic compound (IMC) layer also grew, and its average thickness was measured for each aging condition. The thickness of the IMC layer of the Sn-0.7Cu0.2Cr solder joint was smaller than those of the other two solder joints. From these results, it is expected
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that the new solder will be more reliable than other commercial solders.
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Keywords: metals and alloys; intermetallics; solid state reactions; mechanical properties; microstructure
ACCEPTED MANUSCRIPT 1. Introduction
Recently, Pb-free soldering in automotive electronic components has become a hot issue because of new restrictions on the use of Pb, such as those in the End of Life Vehicle (ELV) directive, which dictates that
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Pb-bearing solder cannot be used in the automotive electronics after January 1, 2016 [1,2]. In the general electronics industry, Pb-bearing solder was successfully replaced with Pb-free solder. However, it is difficult to use Pb-free solder in the vehicle industry because of reliability and safety issues. In particular,
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the electric control units (ECUs) in vehicles are exposed to more severe thermal shocks and thermal vibration conditions. For example, ECUs around the transmission and engine are subject to vibrations of
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3G to 20G, and their operating temperatures are approximately 130 °C [3]. Therefore, development of a solder material that is more reliable is required. Some Pb-free solders have been suggested to replace Pbbearing solder, like Sn-3.0Ag-0.5Cu, Sn-3.5Ag, Sn-Au, Sn-Sb, Sn-Cu, and Sn-Zn solders. However, these have all had problems under actual operating conditions. For example, Sn-3.0Ag-0.5Cu alloy showed a
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decreased bonding strength and exhibited creep under high-temperature conditions [4]. Sn-Au alloy has a high cost for raw materials due to usage of Au metal. Zn and Al-based alloys exhibit poor wettability due to their strong susceptibility to oxidation [5,6]. In addition, Bi-based alloy has limited uses because of poor
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mechanical properties and a low melting temperature [7]. Cost is another important factor in the adoption
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of solders for practical electronic applications. Eutectic Sn-0.7Cu solder has a low cost, relative high melting temperature and moderate mechanical properties. However, disadvantages of the Sn-0.7Cu solder are a relative low wettability and reliability. It was reported that a minor addition of Cr effectively inhibits the growth of Cu-Sn IMCs after reflow on the Cu substrate [8,9]. In this study, a new Sn-0.7Cu-0.2Cr solder material was investigated for automobile electronics applications Solder balls were fabricated and used to make prototype modules. The shear strength of the solder ball was measured, and its microstructure was observed after aging. Finally, these results were compared with the commercial solders Sn-3.0Ag0.5Cu and Sn-0.7Cu to evaluate the reliability of Sn-0.7Cu-0.2Cr solder.
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The compositions of solder alloys used in this experiment were Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn0.7Cu-0.2Cr (in wt.%). The mother alloys were made by induction heating in a high-vacuum argon
were fabricated by reflow process with the solder balls.
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atmosphere at 1100 °C. Then, solder balls with diameters of 300 µm were fabricated. The test modules
The substrate material of test module was FR4 PCB, and the metal pad was Cu with an organic
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solderability preservative (OSP) surface finish. The thickness of the OSP layer was approximately 0.3 µm. The size of Cu opening pad was approximately 230 µm, and its thickness was about 20 µm. To avoid
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oxidation during the reflow process, flux was coated on the Cu pad. The reflow process was carried out by using an infrared 9-zone reflow machine. The conditions of reflow process are shown in Table 1. The peak temperatures of the Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr solders were 250, 255, and 260.5 °C, respectively. After the reflow process, the test samples were cooled to room temperature in air. Figure 1
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shows the schematic graph of the reflow profile for the Sn-0.7Cu-0.2Cr solder.
Figure 1 Reflow profile for Sn-0.7Cu-0.2Cr solder
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time
Pre-heating above
Solder
Peak reflow
melting temperature(°C)
Time(s) temperature(s) 53
Sn-0.7Cu
73
47
Sn-0.7Cu-0.2Cr
73
47
250
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255
260.5
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Sn-3.0Ag-0.5Cu
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To measure the melting temperatures of the three solder materials, differential scanning calorimetry (DSC) analysis was performed. Approximately 8 mg samples of the three solder alloys were used for the DSC analysis. The measurement was carried out in three steps. First, the sample temperature was stabilized at 50 °C, and then it was increased up to 300 °C at a rate of 10 °C/min. Finally, the temperature was cooled
reactions.
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down to 100 °C at a rate of 10 °C/min to check the heat flow and analyze the endothermic and exothermic
Aging tests were carried out at temperatures of 125 °C and 150 °C for up to 1000 h. After the reflow
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process and aging tests, cross-sectional images of the samples were observed by field-emission scanning electron microscope (FE-SEM). The FE-SEM sample was fabricated by polishing and etching. The etchant
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was prepared by mixing of 95% C2H5OH, 3% HNO, and 2% HCl. The thickness and composition of the IMC layers were measured by FE-SEM and energy-dispersive X-ray spectroscopy (EDS). The average thickness of the IMC layer was evaluated from the total area of the IMC layer divided by the total horizontal length using a commercial imaging program. To understand the mechanical properties after aging, a shear test was carried out. In the shear test, the shear speed was 200 µm/s, and the height of the shearing tip was 50 µm. The shear test was repeated 20 times, and the average shear strength was obtained by excluding the maximum and minimum values.
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3.1 Melting temperatures of solder alloys
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Figure 2 DSC results for Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr solders.
The DSC results are shown in Figure 2. The measured melting temperatures of the Sn-3Ag-0.5Cu, Sn0.7Cu, and Sn-0.7Cu-0.2Cr solders were 219, 228, and 231 °C, respectively. Thus, the addition of 0.2 wt.%
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Cr to the Sn-0.7Cu solder increased the melting temperature by about 3 °C.
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3.2 Interfacial microstructure
Figures 3 and 4 show cross-sectional SEM images of Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr solder joints after reflowing and aging at 125 °C and 150°C for up to 1000 h. The images were obtained in the backscattered-electron (BSE) imaging mode of the SEM, which can provide more distinguishable boundaries between the phases and interfacial layers. As shown in Figure 3, a typical scallop-type Cu-Sn IMC layer at the solder/Cu interface was formed after the reflow process for all three solders. The
ACCEPTED MANUSCRIPT compositions of the IMC layer were 54-56 at.% Cu and 44-46 at.% Sn from the EDS analysis, which is equivalent to Cu6Sn5. The IMC was thickest for Sn-3.0Ag-0.5Cu among the three solders. Therefore, Sn diffusion into the Cu pad and IMC growth were faster for Sn-3.0Ag-0.5Cu than for Sn-0.7Cu or Sn-0.7Cu-0.2Cr. The thickness
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of the interfacial IMC layer for the Sn-0.7Cu was thicker than that for the Sn-0.7Cu-0.2Cr solders after aging at 125 °C. As reported previously [10], the grain size of the Cu6Sn5 IMC in Sn-3.0Ag-0.5Cu is very fine compared with that in Sn-0.7Cu. Additionally, it was also previously reported [11] that the length of
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the Cu6Sn5 IMC column was shortened by addition of Cr, which changed the IMC into a rounded crystalline shape.
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After aging under severe conditions of 150 °C for 1000 h, the IMC growth rate was nearly same for the Sn-3.0Ag-0.5Cu and Sn-0.7Cu solders. Aging under these conditions coarsened the Cu6Sn5 IMC in Sn3.0Ag-0.5Cu. On the other hand, the addition of Cr to Sn-0.7Cu solder retarded the growth of the Cu6Sn5 IMC layer at the Sn-0.7Cu-0.2Cr/Cu interface.
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Figure 4 also shows that a very thin Cu3Sn IMC layer was formed at the interface between the Cu6Sn5 IMC layer and the Cu pad after aging at 150 °C for 100 h for Sn-3.0Ag-0.5Cu and Sn-0.7Cu solders. For Sn0.7Cu solder, the Cu3Sn IMC was the thickest [11-13]. However, a Cu3Sn IMC was not observed in the Sn-
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0.7Cu-0.2Cr solder alloy before aging at 150 °C for 500h. After aging at 150 °C, the IMC thicknesses in
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the Sn-3.0Ag-0.5Cu and Sn-0.7Cu alloys were similar. The Cu-Sn IMC changed gradually from a scalloptype morphology to a layer-type morphology during aging at 150 °C. The Sn-0.7Cu-0.2Cr solder had the thinnest IMC layer among the three solders [14-17]. Although Cr addition was studied in some previous papers [12-15], the mechanism of Cr addition effect is not clear. It is assumed that Cr addition prevented IMC growth. The total and Cu3Sn IMC thicknesses are shown in Figures 5 and 6 The thickness of total IMC layer are following order : Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr. A diffusion of metal atoms under solid-state reaction is accelerated by grain boundaries and defects, which can serve as fast diffusion paths or channels [18]. Thermodynamically, the formation of Cu3Sn IMCs between Cu6Sn5 and
ACCEPTED MANUSCRIPT Cu is favorable, but it can be controlled by a kinetic parameter such as the diffusion of Sn atoms into Cu6Sn5 IMCs or Cu atoms under bump metallurgy (UBM) structures [18]. Therefore, the thickness of total and Cu3Sn IMC layers for the Sn-0.7Cu-0.2Cr solder joint was the thinnest among three solder joints in this study.
Figure 7 shows top views of the Cu6Sn5 IMCs formed at the interfaces of the three solder
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joints. The grain size of the Cu6Sn5 IMC in the Sn-0.7Cu was the largest, while that in the Sn-0.7Cu-0.2Cr was the smallest. As a result, Cr addition to the Sn-0.7Cu solder reduced the size of the interfacial Cu6Sn5 IMCs.
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As shown in Figure 8(a), Cr compounds can be confirmed near the Sn/Cu6Sn5 interface in EPMA mapping. The Cr K signal is detected with Sn L signal together, thus it seems Cr-Sn compounds. There are
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no intermetallic phases in an equilibrium phase diagram of Cr-Sn binary system [19]. Some reports have suggested meta-stable intermetallic compounds between Cr and Sn, such as CrSn2 [20] and Cr2Sn3 [19]. Koo et al. [8] also presented the presence of CrSn2 intermetallic compounds phase near the interface in the minor Cr and Ca adopted Sn-0.7Cu solder using an EPMA analysis.
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In our cases, the size of Cr compounds exhibits approximately under 0.5 µm, which is smaller than a spatial resolution of SEM based spectroscopy analysis. For more precise analysis on this Cr compound, we utilized a TEM technique, which can offer a spectroscopic data with the very fine spatial resolution as well
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as a diffraction pattern on the same position. An in-situ lift-out method using a focused ion beam (FIB,
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JIB-4500, JEOL) was employed in order to prepare a specific-sited TEM lamellar sample including Cr compounds, where marked with a dotted box in BSE image of Figure 8(a). A carbon layer with the thickness of 2 µm was deposited on the surface of cross-section sample prior to FIB milling, and the exact position of Cr was marked by FIB line milling, as shown in Figure 8(b). The direction of TEM lamellar sample was denoted as that “A” is a solder side and “B” is a Cu pad side in EPMA (Figure 8(a)), FIB (Figure 8(b)) and TEM (Figure 9) images. Figure 9 shows a scanning TEM (STEM) bright field (BF) image and EDS mapping results on the Sn L, Cu K and Cr L. Cr compound located at the interface of Cu6Sn5/solder and near the interface of it. We can
ACCEPTED MANUSCRIPT infer that this compound is a CrSn2 intermetallic compound through the EDS spot analysis on these compounds (Sn: 69.3 %, Cr: 29.1 % and Cu 1.6 % in atomic percent). A selected area electron diffraction (SAED) pattern of Cr-Sn compound, obtained from denoted area with dotted circle in Figure 10, shows good agreement with the reported crystal structure of an orthorhombic CrSn2 (space group: Fddd; proto
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type: Mg2Cu; a=541.7 pm, b=944.0 pm and c=1839.6 pm) intermetallic compound [19]. Thus, it can be
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identified as CrSn2 with the combination of EDS point analysis and SAED pattern.
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Figure 3 Cross-sectional SEM images of IMC layers after aging at 125 °C for up to 1000 h
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Figure 4 Cross-sectional SEM images of IMC layers after aging at 150 °C for up to 1000 h
Figure 5 Total IMC layer thickness after aging at (a) 125 °C and (b) 150 °C
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Figure 6 Cu3Sn IMC layer thickness after aging at (a) 125 °C and (b) 150 °C
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(b)
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Figure 7 Top views of Cu6Sn5 IMCs formed at the interfaces in three solder joints aged at (a) 125 and (b) 150 °C for up to 1000 h
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Figure 8 (a) EPMA element mapping of the as-reflowed Sn-0.7Cu-0.2Cr solder joint and (b)
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FIB image of TEM sample preparation position
Figure 9 STEM BF image and EDS mapping results of Cu6Sn5/solder interface
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Figure 10 (a) STEM BF image and (b) SAED pattern of Cr-Sn compound
3.3 Shear strength of solder joints.
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The ball shear test was carried out to evaluate effect of the Cr addition on the mechanical reliability of the solder joints after aging under different conditions. Figures 11 shows the shear
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strength of these solder joints and the variations in the decreasing rates with aging test at 125 °C and 150 °C, respectively. The as-reflowed joints had comparatively high shear forces, but the
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shear forces decreased rapidly during the initial 100 h of aging and then decreased more slowly up to 1000 h of aging.
Sn-3.0Ag-0.5Cu solder exhibited biggest drop in shear force, of 250 gf in as-reflowed condition and 196 gf after 1000 h of aging at 125 °C. The shear force of the Sn-0.7Cu solder was 190 gf in as-reflowed condition and 156 gf after 1000 h of aging at 125°C. The shear force of the Sn-0.7Cu-0.2Cr solder was higher than that of the Sn-0.7Cu solder by about 10 gf for all aging times. Under harsher aging conditions, the shear forces of Sn-3.0Ag-0.5Cu and Sn-0.7Cu solders
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decreased to 189 gf and 156 gf, respectively, after 1000 h of aging at 150 °C. As was the case for aging at 125°C, the shear force of the Sn-0.7Cu-0.2Cr solder was higher than that of the Sn0.7Cu solder by about 10 gf for all aging times at 150 °C. We assume that the shear force was
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slightly decreased from its initial value after 100 h of aging because of softening of the solder alloy [10, 18, 21, 22]. As a whole, the shear force for three solder joints slightly decreased with
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increasing aging time irrespective of different aging temperature.
Figure 11 Shear strength and decreasing rate of Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu0.2Cr during aging at (a)(b) 125 °C and (c)(d) 150 °C
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To verify the variation of the shear strength, the top views of the fracture surfaces after shear test were examined by SEM. Figure 12 shows the fracture surfaces of the samples aged at different aging temperatures and times. Regardless of the aging condition, the fracture always
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occurred in the bulk solder. In all samples, the fracture surfaces showed a ductile failure. Generally, in the solder ball shear test, a fracture occurs at the interface or in the solder region with the lowest strength [23, 24]. Over the years many studies have been published concerning
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the mechanical properties of solder joints. It was known that excessively thick reaction layers formed between solder and substrate could significantly degrade the mechanical properties of the
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solder joints [24]. However, in this study, fracture analysis indicated that the shear strength could not be related to the thicknesses of the IMC layers formed at the interfaces between three solder and Cu substrate. At all conditions, fracture occurred in the bulk solder regardless of the aging conditions. And the decrease in reliability occurs at the interface between the solder and
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substrate when a thick IMC layer is formed at the interface [25]. The results of interfacial reaction and mechanical tests showed that the new Sn-0.7Cu-0.2Cr/Cu solder joint has the higher strength, lower decreasing rate of shear strength, and a small IMC compared to the conventional
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Sn-0.7Cu solder.
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4. Discussion
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Figure 12 Fracture surfaces of three solder joints aged under different conditions
Based on the results above, such minor amount of Cr greatly retarded the interfacial IMC
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growth and enhanced the joint mechanical property. Therefore, it is worthwhile to figure out the effect of Cr on the interfacial IMC growth. Fundamentally, the formation and growth of the
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interfacial IMC growth is caused by the Cu and Sn diffusion through the interface. Thus, we evaluate the effects of Cr on the Cu and Sn diffusion separately by calculating the amounts of Cu and Sn atoms in the interfacial IMC layers as follows [26, 27] (1) (2)
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in which M is the total weight of the Cu or Sn element per unit area, T is the thickness of the Cu6Sn5 or Cu3Sn layers, ν is the weight fraction of Cu or Sn in the Cu6Sn5 or Cu3Sn molecule and ρ is the density of the Cu6Sn5 or Cu3Sn IMCs, which is 8.45 or 8.97 g·cm3, respectively [28].
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The results listed in Figure 13 show that both the Sn and Cu diffusion rates increase with improved reaction temperature, and more importantly, it is clear that the addition of Cr slightly increased Sn diffusion but significantly repressed the Cu diffusion under thermal aging test.
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From these results, we believe that Cr inhibited the solid state interfacial IMC growth primarily through slowing down the Cu diffusion through the interface. It is well known that only the
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Cu3Sn and Cu6Sn5 phases are formed at the SAC/Cu interface. Therefore, under the similar Sn diffusion condition, a lower Cu diffusion flux leads to repression of the Cu3Sn layer growth at the interface.
Another problematic issue is how Cr affects the diffusion of the interfacial atoms and their
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combination. Unlike Ni atoms, Cr atoms cannot dissolve into the CuSn IMC to form a solid solution based on the TEM and EPMA findings (Fig. 8 and 9), many CrSn2 particles located at the interface and these particles should experience ripening growth with prolonged aging test
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according to the thermodynamics [26]. The inhibiting behavior is probably related to the CrSn2 particles at the interface. It is likely these tiny particles and the flow of the Cr and Sn at the
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interface affects the Cu diffusion and the combination of Sn and Cu atoms. However, the root mechanism still requires further investigation.
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28
24
Mass (10-3ug/mm2)
22
26
Cu diffusion rate (125 OC) SAC305 D=9.7 SC07 D=15.6 SC-Cr D=3.4
24 22
Mass (10-3ug/mm2)
26
20 18 16 14
Sn diffusion rate (125 OC) SAC305 D=4.9 SC07 D=1.1 SC-Cr D=5.3
20 18 16 14
12
12
10
10 8
8 0
200
400
600
800
0
1000
200
50
50
Cu diffusion rate (150 OC) SAC305 D=0.0327 SC07 D=0.0342 SC-Cr D=0.0198
45 40
Mass (10-3ug/mm2)
Mass (10-3ug/mm2)
40
600
800
1000
800
1000
Sn diffusion rate (150 OC) SAC305 D=0.0186 SC07 D=0.0154 SC-Cr D=0.0159
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400
Thermal aging (hours)
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35 30 25 20
35 30 25 20 15
15
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600
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1000
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Thermal aging (hours)
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600
Thermal aging (hours)
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5. Conclusions
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Figure 13 Plot of diffusivities of Cu and Sn versus thermal aging time.
Sn-0.7Cu-0.2Cr solder with a moderate melting temperature was fabricated for automotive applications. The changes in its microstructure and mechanical properties during hightemperature aging were evaluated, and the results were compared with those obtained for commercial solder alloys. The melting temperature was changed from 228 °C to 231 °C by the addition of 0.2 wt.% Cr to Sn-0.7Cu solder. The total thickness of the IMC layer in Sn-0.7Cu0.2Cr solder was less than those in Sn-0.7Cu or Sn-3.0Ag-0.5Cu solders after aging. According
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to the EPMA result, Cr compound was evenly distributed in the Sn-0.7Cu-0.2Cr solder matrix. Furthermore, the decreasing rate of Sn-0.7Cu-0.2Cr solder was lower than those of the Sn3.0Ag-0.5Cu and Sn-0.7Cu solders. The results of interfacial reaction and mechanical tests
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showed that the new Sn-0.7Cu-0.2Cr/Cu solder joint has the lower decreasing rate of shear
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strength and IMC growth rate.
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References
1. S. K. Kang, A. K. Sharkel, Pb-free solders for electronic packaging, J. Electron. Mater. 23(8) (1994) 701.
2. E. P. Leng, M. Ding, W T. Ling, N. Amin, I. Ahmad, M. Y. Lee, A. S. M. A. Haseeb, A
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study of SnAgNi Co vs Sn3.8AgO.7Cu C5 lead free solder alloy on mechanical strength of BGA solder joint, 10th Electronics Packaging Technology Conference, 2008. 3. R. W. Johnson, J. L. Evans, P. Jacobsen, J. R. Thompson, M. Christopher, The changing
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automotive environment: high-temperature electronics, IEEE Trans. Electron. Pack Manu. 27(3) (2004) 164.
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4. P. Limaye, R. Labie, B. Vandevelde, D. Vandepitte, B. Verlinden, Creep behavior of mixed SAC 405/SnPb soldered assemblies in shear loading, 9th Electronics Packaging Technology Conference, 2007.
5. X. Chen, M. Li, X. Ren, D. Mao, Effects of alloying elements on the characteristics of SnZn lead-free solder, 6th International Conference on Electronics Packaging Technology, 2005.
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6. K. L. Lin, P. C. Liu, J. M. Song, Wetting interaction between Pb-free Sn-Zn series solders and Cu, Ag substrates, Electronic Compound and Technology Conference, 2003. 7. J. Wang, L. Wen, J. Zhou, M. Chung, Mechanical properties and joint reliability
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improvement of Sn-Bi alloy, 13th Electronics Packaging Technology Conference, 2011. 8. J. Koo, J. Chang, Y.W. Lee, S.J. Hong, K.S. Kim, H.M. Lee, New Sn–0.7Cu-based solder alloys with minor alloying additions of Pd, Cr and Ca, Journal of Alloys and Compounds
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alloy on the intermetallic growth with Cu substrate during isothermal aging, Materials Characterization 61 (2010) 355-361.
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thermal aging, J. Electron. Mater. 36(11) (2007).
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Technology (ICEPT 2007), IEEE, 2007.
12. H. Wang, A. Hu, C. C. M. Li, D. Mao, Oxidation behavior and intermetallic compounds
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growth of Sn-Ag-Bi-Cr lead-free solder, 8th International Conference on Electronic Packaging Technology (ICEPT 2007), IEEE, 2007. 13. G. Su, Y. Han, H. Wang, X. Wei, Effect of 0.05 Cr on intermetallic compound layer growth for Sn-Ag-Cu lead-free solder joint during isothermal aging, Proc. 16th IPEA, 2009, China.
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14. I. E. Anderson, T. E. Bloomer, J. C. Foley, R. L. Terpstra, Development of eutectic and near-eutectic Sn-Ag-Cu solder alloys for lead-Free electronic assemblies. 15. C. B. Lee, S. B. Jung, Y. E. Shin, C. C. Shur, Effect of isothermal aging on ball shear
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strength in BGA joints with Sn-3.5Ag-0.75Cu solder, Mater. Trans. 43(8) (2002) 1858 16. D. G. Kim, S. B. Jung, Microstructure and mechanical properties of Sn-0.7Cu flip chip solder bumps using stencil printing method, Mater. Trans. 46(11) (2005) 2366.
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17. X. Shen, M. Li, X. Ren, D. Mao, Effect of alloying elements on the characteristics of SnZn lead-free solder, 6th International Conference on Electronic Packaging Technology,
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2005.
18. M. G. Cho, Y. S. Park, S. K. Seo, K. W. Paik, H. M. Lee, Effect of Ag on ripening growth of Cu6Sn5 grains formed between molten Sn-xAg-0.5Cu solders and Cu, EPTC Proceeding, 2010, 170.
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structure and NbSnSb with CuAl2-type structure, Journal of Alloys and Compounds, 210
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23. D. G. Kin, S. B. Jung, Microstructure and Mechanical Properties of Sn–0.7Cu Flip Chip Solder Bumps Using Stencil Printing Method, Materials Transactions, Vol. 46, No. 11 (2005) pp. 2366 to 2371.
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24. C. B. Lee, S. B. Jung, Y. E. Shin, C. C. Shur, Effect of Isothermal Aging on Ball Shear Strength in BGA Joints with Sn-3.5Ag-0.75Cu Solder, Materials Transactions, Vol. 43, No. 8 (2002) pp. 1858 to 1863.
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25. J. W. Yoon, B. I. Noh, Y. H. Lee, H. S. Lee, S. B. Jung, Effects of isothermal aging and temperature–humidity treatment of substrate on joint reliability of Sn–3.0Ag–0.5Cu/OSP-
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finished Cu CSP solder joint, Microelectronics Reliability 48 (2008) 1864–1874 26. M. Yang, Y.H. Ko, J. Bang, T.S. Kim, C.W. Lee, S. Zhang, M. Li, Effects of Ag addition on solid–state interfacial reactions between Sn–Ag–Cu solder and Cu substrate, Materials Characterization, 124 (2017), 250-259
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27. M. Yang, H. Ji, S. Wang, Y. H. Ko, C. W. Lee, J. Wu, M. Li, Effects of Ag content on the interfacial reactions between liquid Sn–Ag–Cu solders and Cu substrates during soldering, Journal of Alloys and Compounds, 679 (2016), 18-25
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28. M. Yang, Y. Cao, S. Joo, H. Chen, X. Ma, M. Li, Cu6Sn5 precipitation during Sn-based solder/Cu joint solidification and its effects on the growth of interfacial intermetallic
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compounds, Journal of Alloys and Compounds, 582 (2014), 688-695
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Figure Captions
Figure 1 Reflow profile for Sn-0.7Cu-0.2Cr solder
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Figure 2 DSC results for Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-0.2Cr solders. Figure 3 Cross-sectional SEM images of IMC layers after aging at 125 °C for up to 1000 h. Figure 4 Cross-sectional SEM images of IMC layers after aging at 150 °C for up to 1000 h.
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Figure 5 Total IMC layer thickness after aging at (a) 125 °C and (b) 150 °C.
Figure 6 Cu3Sn IMC layer thickness after aging at (a) 125 °C and (b) 150 °C.
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Figure 7 Top views of Cu6Sn5 IMCs formed at the interfaces in three solder joints aged at (a) 125 and (b) 150 °C for up to 1000 h.
Figure 8 (a) EPMA element mapping of the as-reflowed Sn-0.7Cu-0.2Cr solder joint and (b) FIB image of TEM sample preparation position.
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Figure 9 STEM BF image and EDS mapping results of Cu6Sn5/solder interface Figure 10 (a) STEM BF image and (b) SAED pattern of Cr-Sn compound Figure 11 Shear strength and decreasing rate of Sn-3.0Ag-0.5Cu, Sn-0.7Cu, and Sn-0.7Cu-
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0.2Cr during aging at (a)(b) 125 °C and (c)(d) 150 °C Figure 12 Fracture surfaces of three solder joints aged under different conditions.
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Figure 13 Plot of diffusivities of Cu and Sn versus thermal aging time.
Highlights
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1. Sn-3.0Ag-0.5Cu alloy showed a decreased bonding strength and exhibited creep under hightemperature conditions. 2. Sn-0.7Cu-0.2Cr solder with a moderate melting temperature was fabricated for automotive
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applications. 3. The changes in its microstructure and mechanical properties during high-temperature aging were evaluated.
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4. The results were compared with those obtained for commercial solder alloys.