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Sn composite preform for high-temperature bonding applications
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Fast formation of Cu-Sn intermetallic joints using pre-annealed Sn/Cu/Sn composite preform for high-temperature bonding applications So-Eun Jeong Visualization; Validation; Investigation , Seung-Boo Jung Writing- Reviewing and Editing , Jeong-Won Yoon Conceptualization; Methodology; Data curation; Writing- Original draft preparation; Supervision PII: DOI: Reference:
S0040-6090(20)30088-2 https://doi.org/10.1016/j.tsf.2020.137873 TSF 137873
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
30 October 2019 19 February 2020 19 February 2020
Please cite this article as: So-Eun Jeong Visualization; Validation; Investigation , Seung-Boo Jung Writing- Reviewing and Editing , Jeong-Won Yoon Conceptualization; Methodology; Data curatio Fast formation of Cu-Sn intermetallic joints using pre-annealed Sn/Cu/Sn composite preform for high-temperature bonding applications, Thin Solid Films (2020), doi: https://doi.org/10.1016/j.tsf.2020.137873
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Highlights We investigated solid-state growth of Cu–Sn intermetallics in composite Sn/Cu/Sn preform. Aged Sn/Cu/Sn preforms compensated for drawback of long bonding time. As the aging time elapsed, the thickness of Cu decreased. The Cu3Sn adjacent to Cu core layer increased, whereas the Cu6Sn5 gradually decreased.
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Fast formation of Cu-Sn intermetallic joints using pre-annealed Sn/Cu/Sn composite preform for high-temperature bonding applications
So-Eun Jeong1,2, Seung-Boo Jung2*, Jeong-Won Yoon1,3*
1
Welding and Joining R&D Group, Korea Institute of Industrial Technology (KITECH), 156 Gaetbeol-ro, Yeonsu-gu, Incheon 21999, Korea 2
School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 16419, Korea 3
Department of Advanced Materials Engineering, Chungbuk National University, 1 Chungdae-ro, Seowon-gu, Cheongju, Chungbuk 28644, Korea
2
*Corresponding author: Tel. +82-31-290-7359; Fax: +82-31-290-7371 E-mail address: [email protected] (S.B. Jung) *Corresponding author: Tel. +82-32-850-0237; Fax: +82-32-850-0210 E-mail address: [email protected] (J.W. Yoon)
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Abstract This paper presents the interfacial reaction and growth rate of Cu-Sn intermetallic compounds (IMCs) for various aging times in a composite preform for a die-attach material at high temperature. The feasibility of the composite preform we fabricated for power electronic applications to reduce the long bonding time of transient liquid phase (TLP) bonding was evaluated in this work. After the plating process, the composite preform was composed of Sn layers at both sides of the Cu core layer and the Cu layer. In terms of the aging treatment, the IMC layer was formed rapidly at the Cu-Sn interface in the composite preform by consuming the Sn layers, and the reduction and growth rates of Cu and Cu3Sn were calculated, respectively. We performed the TLP bonding process at 270 °C using various aged composite preforms. The Cu-Sn IMC was formed rapidly on both sides adjacent to the Cu core layer. The thickness of Cu decreased as the aging time elapsed. Therefore, the amount of Cu3Sn adjacent to the Cu core layer increased and the amount of Cu6Sn5 rapidly increased, simultaneously. We investigated the thicknesses and variations of the phase of the joint in the composite preform during the aging treatment process and sequential TLP bonding process for various times. We fabricated pre-annealed Sn/Cu/Sn preforms and performed fast CuSn TLP bonding for high temperature power electronic applications.
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Keywords: Intermetallic compound; Annealing; Diffusion; Joining; Interfaces
1. Introduction Recently, the use of Pb is limited worldwide because of the environmental regulations on end of life vehicles and restriction of hazardous substances (RoHS). Accordingly, power modules for high temperature applications used in power conversion systems are actively being developed in various fields such as electric vehicles, hybrid electric vehicles, and materials for aerospace application [1]. In particular, the use of power modules used in the power conversion systems of eco-friendly vehicles is rapidly increasing. A power module is a device that converts, distributes, and manages DC power supplied from the battery of an environment-friendly vehicle [2]. As a result, solder that can withstand high temperature is needed instead of Pb solder; furthermore, automobiles are usually used for a long period of time, which leads to increase in temperature. Pb-free solder materials are needed for flip-chip packaging, die attachment, and power semiconductor for high-temperature applications. There are promising power semiconductor devices with wide gap width such as silicon carbide (SiC) and gallium nitride (GaN), with good properties such as high thermal conductivity and high
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breakdown voltage, which are suitable for high-temperature and high power applications [3]. To efficiently utilize these characteristics, the assembly process should minimize the heat treatment [4]. Therefore, advancement in semiconductor devices such as SiC and GaN has resulted in reduced power loss and size of the module. However, since it is possible to drive at a high temperature, the die attachment technology requires a bonding technique that can be used at a high temperature for a long time. Currently, many Pb-free solder alloys have been proposed as a result of research and development of Pb-free solder alloys [5,6]. However, despite the development of several commercial and environmentally-friendly Pb-free solders, Pb-free solder substitutes for high temperature applications are limited. For example, Zn-based or Bi-based solder alloys are easily oxidized and have poor wettability and reliability properties [3,7]. Wu et al. [5] reported that Pb-free solders of ternary alloy or quaternary alloy were developed because additional alloy elements were needed to improve the performance of the alloy and the characteristics of binary Pb-free solder cannot fully meet the requirements for electronic packaging. Some good candidates for die attachment applications at high temperatures include soldering technology using Au-based and ternary high-temperature solders, Ag [8] and Cu [9] sintering technology, and transient liquid phase (TLP) bonding technology. However, the cost of Au-based high-temperature solder and Ag
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sintering technology is high owing to the use of expensive materials, and they require high processing temperature. In addition, Cu sintering technology has drawbacks such as oxidation in the bonding materials. On the other hand, TLP bonding technology has advantages such as low cost and requires low bonding pressure. TLP bonding is a combination of soldering and diffusion joining processes. When a metal with a lowmelting point is placed between metals with a high-melting point, the metal with a lowmelting point temporarily changes to a liquid phase and is bonded by diffusive action. The obtained intermetallic compound (IMC) has a high re-melting point (i.e. 415 °C for Cu6Sn5 and 676 °C for Cu3Sn as shown in Fig. 1 [10]). However, this is dependent on the IMC characteristics and there are also drawbacks such as a long bonding time [11]. Lee et al. [2] reported that the Cu-Sn TLP bonded joint was completely converted into Cu-Sn IMC at 300 °C for 2 h in N2 atmosphere. Thus, new materials for operating at a high-temperature are needed for TLP technology bonding at a relatively lowtemperature for a short bonding time. In addition, the materials in the die attachment must have good thermal and electrical conductivity, which can significantly affect the efficiency and reliability of the power module [3]. Cu-Sn bonded materials are widely used in high temperature power devices for TLP bonding technology. In general, Cu-Sn systems are considered the most promising
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candidates that can be converted to IMC owing to their low cost, low process temperature, and high performance for 3D microelectronic packaging [12-17]. In this study, we verified the Cu-Sn IMC formation step at 150 °C using Cu foil to reduce the bonding time. The overall objective of this study is to verify the change in reaction in the joint using Cu foil under various aging times. The Sn layer reacts to form Cu-Sn IMC with inner Cu and Cu on the chip. This study focused on the conditions for forming microstructure joints for a short time using aged preforms.
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2. Experimental procedures In this work, a Cu foil with a thickness of 5 μm was used as the core layer in the preform. Prior to plating process, the tarnish on the Cu foil was removed by rinsing in 10 % sulfuric acid for 1 min and running deionized water. Immersion Sn plating was performed in a commercial Sn plating solution (ITP-1000, Incheon Chemical Co. Ltd., Korea) bath at 65 ± 2 °C as shown in Fig. 2(a). Various thicknesses of Sn layers in the range of 1 – 4 μm were obtained by controlling the processing conditions of the immersion. The 5 μm Cu foil was immersed in a plating solution for 30 min. After the immersion process, the plated Cu foil was rinsed with running deionized water and dried at room temperature. Heat-treatment was carried out at 150 °C for up to 72 h in an oven to obtain a fast reaction between Sn and Cu. Figure 2(b) shows the schematics of the Cu foils after plating and sequential aging treatment. Si square chips with dimensions of 4 mm x 4 mm were used as a chip and substrate in this work. The Si chips were sputtered in the order of Ti (0.1 μm), Cu (1 μm), and Sn (2 μm). The composite preforms were placed in-between the Si chips. Pressure was applied on the Si chip/composite preform/Si chip at 20 MPa using a thermal compression bonder (SJCWD-100, SJ Co. Ltd., Korea), and the samples were bonded at 270 °C for 5 min and 10 min, respectively in vacuum atmosphere. In this process, graphite sheet was used
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to uniformly deliver pressure to the chip. After completing the aging and bonding step, the samples were mounted in epoxy and polished, then examined using a field emission scanning electron microscope (FESEM, INSPECT F, FEI, USA) equipped with an energy dispersive X-ray spectroscope (EDX), electron probe micro analyzer (EPMA, JXA-8500F, JEOL, USA), and focused ion beam (FIB, NOVA-600, FEI, USA). The operating voltage used in the SEM observations was 20 kV. The thicknesses of the phases were measured quantitatively using micrographs obtained from the cross-sections of the intermetallic layer. The average layer thickness was determined as the total area occupied divided by the length.
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3. Results and discussion Figure 3 shows the cross-sectional SEM micrographs of the Sn plated Cu foil (5 μm thickness) after plating at 65 °C for various aging times at 150 °C. The composite preform that was plated for 30 min at 65 °C has a total thickness of approximately 8–9 μm, with the remaining Cu core layer at the center and two layers of Sn at each side of the Cu foil, as shown in Fig. 3(a). The thickness of the Cu foil was reduced from 5 μm to approximately 3 μm after plating, and Sn was plated with a thickness of 2–3 μm on the side of the Cu core layer. The general substitution plating is as follows: Sn2+ + 2Cu → Sn + 2Cu+ During the aging process, the thickness of the Cu core decreased gradually. It is also clear that the amount of Sn on both sides of the Cu core layer decreased and the thickness of the formed Cu-Sn IMC extended around the Cu core layer. The quantitative EDX analysis shows that the composition of the Cu-Sn IMC phase formed at both interfaces corresponds to the Cu6Sn5 and Cu3Sn phases in the binary Cu-Sn phase diagram as shown in Fig. 1. The thin Cu-Sn IMC phase adjacent to Cu was identified as a Cu3Sn layer using EDX analysis. The chemical compositions of the Cu6Sn5 and Cu3Sn phases were 54~55at.%Cu-45~46at.%Sn and 74~76at.%Cu-24~26at.%Sn, respectively. The remaining Sn on both sides of the composite preform was fully converted to Cu-Sn
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IMC as shown in Fig. 3(f). In other words, the Sn remains until 12 h of aging as shown in Fig. 3(e). As the heat treatment progressed to 72 h, the thickness of the Cu core layer decreased and the proportion of the Cu-Sn IMC layer increased. The composite preform is composed of Cu-Sn IMC (Cu6Sn5 and Cu3Sn) layers around the Cu core layer as shown in Figs. 3(f)-(h). We performed FIB analysis to observe the microstructures and phases of the aged preform samples as it is difficult to clearly distinguish the boundaries between the CuSn IMCs and the plated Sn layers using SEM observations. Figure 4 shows the crosssectional FIB micrographs of the Sn/Cu/Sn interface in the composite preform aged at 150 °C for different aging times. The preform before aging treatment clearly shows the Sn on both sides of the Cu core layer as depicted in Fig. 4(a). The Cu3Sn IMC, Cu6Sn5 IMC, and Sn adjacent to the Cu core layer are formed in sequence in the aged composite preform as shown in Figs. 4(b)-(d). During the aging process, the thicknesses of the Cu and Sn layers significantly decreased. In contrast, the thickness of the Cu-Sn IMCs increased as the Sn and Cu layers are consumed with time. A small amount of Sn remained on both sides of the Cu layer in the composite preform as shown in Fig. 4(d). Figure 4(e) shows that both Sn layers are fully consumed and converted to Cu-Sn IMCs. This indicates that a fast reaction takes place at the interface for a very short aging time.
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The thin Cu3Sn IMC phase adjacent to the initially formed Cu core layer reacted with the Cu6Sn5 IMC phase and the Sn layer, and its thickness gradually increased. Once the Sn has been depleted, the Cu6Sn5 IMC reacts with Cu and is converted to Cu3Sn IMC owing to the limitation of the Sn source. Figure 4(f) shows that it was converted to Cu3Sn IMC, leaving a small amount of Cu6Sn5 IMC. As a result, the residual Sn phase with low-melting point can be converted to the IMC phase with high-melting point at a short aging time. The thicknesses of the Cu and the Cu3Sn IMC layers were measured quantitatively, and the result was plotted as the square root of the aging time. Figure 5(a) shows a plot of the square root of the aging time at 150 °C according to the average thicknesses of the Cu and Cu3Sn layers in the composite preform. The relationship between the thickness of each phase in the composite preform and the aging time can be described in a general form as given below [18,19]. W=√(Dt) where W is the thickness of each layer in the composite preform, D is the interdiffusion coefficient, and t is the aging time. According to the results, the thickness of the Cu layer decreased as shown in Fig. 5(a), whereas the thickness of the Cu 3Sn IMC layer increased.
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The average layer thickness was determined as the total area divided by the length as presented in Table 1. The overall slope of the average thicknesses under different aging times was calculated by dividing the average thickness curve of the Cu and Cu 3Sn layers by the square root of the aging time at 150 °C. In other words, the overall slope indicates the growth rate. Cu exhibited a negative slope, whereas Cu3Sn exhibited a positive slope. The results can be used to predict the growth rate of interfacial Cu-Sn IMCs in the composite preform. Figure 5(b) shows the ratio of metal and Cu-Sn IMC layers in the aged composite preform after aging at 150 °C for various aging times. The ratio of metal and IMC thickness was measured quantitatively through the cross-section of the intermetallic layer. As a result, the proportion of Cu6Sn5 IMC gradually decreased, and the proportion of Cu3Sn IMC surged with time. Once the Sn has been depleted, the Cu6Sn5 IMC reacts with Cu and is converted to Cu3Sn IMC. Figure 6 shows the schematics of the bonding machine and test specimens before and after bonding. After the bonding process, the Sn of both the chip and the Sn/Cu/Sn preform reacts to form a Cu-Sn IMC phase at the joint as shown in Fig. 6(b). In other words, Sn with a low-melting point reacts to form Cu6Sn5 and Cu3Sn between the chip and Cu layer. In the TLP bonding process, we used two types of Sn/Cu/Sn preforms
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aged 3 h and 12 h because the Sn layer was completely converted into Cu-Sn IMCs in the 24 h aged preform sample shown in Fig. 4(e). Figure 7 shows the cross-sectional SEM micrographs of joints with the composite preform after TLP bonding at 270 °C for 5 min using the aged preforms under various times. The total thickness of the joint is approximately 9–11 μm. A thin layer of Cu3Sn was formed on both sides adjacent to the Cu. Furthermore, Cu3Sn was formed close to both chips. The Cu3Sn/Cu6Sn5/Cu3Sn was formed in sequence from the Cu core layer. As the aging time elapsed, the amount of reacted Cu increased, and the thickness of Cu decreased. Therefore, the amount of Cu3Sn IMC adjacent to the Cu layer increased, and the amount of Cu6Sn5 increased simultaneously. It is apparent from the reaction that most of the Sn has been depleted, and the amount of Cu3Sn increased owing to the depletion of Cu and Cu6Sn5. To clarify the interfacial microstructures and intermetallic phases of the TLP bonded joint, EPMA analysis was used to examine the Fig. 7(b) sample. The elemental map and line scan results are shown in Figs. 8 and 9. From these results, we clearly confirmed the presence of the remaining Cu layer and sequential Cu3Sn/Cu6Sn5/Cu3Sn IMC layers from the Cu core layer. Figure 10 shows the cross-sectional SEM micrographs of joints with the composite preform after TLP bonding at 270 °C for 10 min using the aged preforms under various
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times. The total thickness of the joint is approximately 8–11 μm. The Cu layer was mostly consumed in the aged preform for 12 h as shown in Fig. 10(c). It can be observed that the amount of Cu6Sn5 IMC changed to Cu3Sn IMC increased compared with bonding for 5 min. Shang et al. [20] reported that Cu3Sn grains nucleated with solid state aging and grew the interfaces of both Cu-Cu3Sn and Cu3Sn-Cu6Sn5. Once the remaining Sn has been fully depleted, Cu3Sn can grow at the expense of the Cu6Sn5 formed at the original boundaries until it has been depleted. Figures 11 and 12 show the EPMA elemental map and line scan results of Fig. 10(c) sample. We clearly observed the complete consumption of the Cu core layer and formation of the sequential Cu3Sn/Cu6Sn5/Cu3Sn/Cu6Sn5/Cu3Sn IMC layers in the joint. In this study, the aged Sn/Cu/Sn preforms compensated for the drawback of long bonding time in the TLP bonding technology. Actually, the TLP bonded joints with the Cu-Sn IMCs were easily fabricated within 5 to 10 min. As a result, we fabricated the pre-annealed Sn/Cu/Sn preforms and Cu-Sn TLP bonded joint in a short bonding time for high-temperature applications. The evaluation of the mechanical strength of the corresponding TLP bonded joints will be carried out in our future work and the mechanical test results will be reported in the future.
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4. Conclusions In this study, the growth rate of Cu-Sn IMC in a composite Sn/Cu/Sn preform was investigated during aging treatment process at 150 °C for various times, and the aged composite preform of various times was designed to compensate for the drawback of TLP bonding technology. It was generally observed in the composite preform that the Cu-Sn IMC layer at both interfaces is double structure, including the Cu6Sn5 layer and Cu3Sn layer at the plated Cu-Sn interface. During the heat treatment process, the Cu3Sn phase grew more than the Cu6Sn5 phase in contact with the residual Sn. As the Cu and Sn reacted and decreased, the amount of Cu3Sn increased with the increase in the aging time. However, the thickness of Cu6Sn5 gradually decreased. Once the residual Sn has been fully depleted, Cu3Sn could grow at the expense of the Cu6Sn5 formed at the original interface until it was depleted. The total thickness of the Cu-Sn IMC layer at both interfaces in the composite preform was calculated using the square root of the aging time. The calculated value is helpful to predict the total thickness of forming the interfacial IMC layer in the composite preform under various thermal conditions for different times. The Sn/Cu/Sn preform was prepared to evaluate the feasibility to reduce the TLP bonding time and was bonded using a thermal compression bonder as the bonding material. After bonding, the Sn of the Si chip and both sides of the composite
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preform reacted with Cu to form Cu-Sn IMC. As the heat treatment time elapsed, the amount of Cu6Sn5 and Cu3Sn increased. The reaction was indicated to be fast owing to the effect of the Cu3Sn phase already formed adjacent to the Cu core layer in the aged composite preform. Furthermore, as the bonding time increased, the amount of Cu decreased sharply, which shows that the rate of change to the Cu3Sn phase is very fast. The results presented in this paper verify the feasibility of the TLP bonding process using aged composite preforms with various times. In this study, we fabricated preannealed Sn/Cu/Sn preforms and Cu-Sn TLP bonded joint in a short bonding time of 10 min.
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Credit Author Statement
So-Eun
Jeong:
Reviewing Won
Yoon:
Visualization,
Validation,
Investigation.:
and Conceptualization,
Seung-Boo
Jung:
Editing.: Methodology,
Original draft preparation, Supervision.
19
Data
WritingJeong-
curation,
Writing-
Declaration of Interest Statement
This manuscript has not been published or presented elsewhere in part or in enti rety and is not under consideration by another journal. We have read and unders tood your journal’s policies, and we believe that neither the manuscript nor the st udy violates any of these. There are no conflicts of interest to declare.
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List of figure captions
Fig. 1. Cu-Sn binary phase diagram [10].
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Fig. 2. Schematics of (a) plating bath and (b) Cu foils after plating and aging treatment.
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Fig. 3. Cross-sectional SEM micrographs of Sn/Cu/Sn preforms after aging at 150 °C for different aging times: (a) initial, (b) 3 h, (c) 5 h, (d) 8 h, (e) 12 h, (f) 24 h, (g) 48 h, and (h) 72 h.
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Fig. 4. Cross-sectional FIB micrographs of Sn/Cu/Sn preforms after aging at 150 °C for different aging times: (a) initial, (b) 3 h, (c) 5 h, (d) 8 h, (e) 12 h, (f) 24 h, (g) 48 h, and (h) 72 h.
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Fig. 5. (a) Variations in the Cu and Cu3Sn IMC thicknesses and (b) ratio of metal and Cu-Sn IMC layers in the composite preform after aging at 150 °C for various times.
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Fig. 6. Schematics of bonding machine and test specimens (a) before and (b) after TLP bonding.
30
Fig. 7. Cross-sectional SEM micrographs of joints bonded with Sn/Cu/Sn preforms after bonding at 270 °C for 5 min.
31
Fig. 8. EPMA elemental mapping results of the sample shown in Fig. 7(b).
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Fig. 9. EPMA line scan analysis results of the sample shown in Fig. 7(b).
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Fig. 10. Cross-sectional SEM micrographs of joints bonded with Sn/Cu/Sn preforms after bonding at 270 °C for 10 min.
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Fig. 11. EPMA elemental mapping results of the sample shown in Fig. 10(c).
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Fig. 12. EPMA line scan analysis results of the sample shown in Fig. 10(c).
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Table 1. Thicknesses and growth rates of Cu and Cu3Sn layers for various aging times
Cu Cu3Sn
Initial
3h
8h
12 h
24 h
72 h
Growth rate
3.209 μm 0
1.881 μm 0.972 μm
1.345 μm 1.330 μm
1.258 μm 1.884 μm
1.101 μm 2.592 μm
0.210 μm 4.233 μm
-0.3175 μm·h-1/2 0.4998 μm·h-1/2
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Fast formation of Cu-Sn intermetallic joints using pre-annealed Sn/Cu/Sn composite preform for high-temperature bonding applications So-Eun Jeong Visualization; Validation; Investigation , Seung-Boo Jung Writing- Reviewing and Editing , Jeong-Won Yoon Conceptualization; Methodology; Data curation; Writing- Original draft preparation; Supervision PII: DOI: Reference:
S0040-6090(20)30088-2 https://doi.org/10.1016/j.tsf.2020.137873 TSF 137873
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
30 October 2019 19 February 2020 19 February 2020
Please cite this article as: So-Eun Jeong Visualization; Validation; Investigation , Seung-Boo Jung Writing- Reviewing and Editing , Jeong-Won Yoon Conceptualization; Methodology; Data curatio Fast formation of Cu-Sn intermetallic joints using pre-annealed Sn/Cu/Sn composite preform for high-temperature bonding applications, Thin Solid Films (2020), doi: https://doi.org/10.1016/j.tsf.2020.137873
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Highlights We investigated solid-state growth of Cu–Sn intermetallics in composite Sn/Cu/Sn preform. Aged Sn/Cu/Sn preforms compensated for drawback of long bonding time. As the aging time elapsed, the thickness of Cu decreased. The Cu3Sn adjacent to Cu core layer increased, whereas the Cu6Sn5 gradually decreased.
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Fast formation of Cu-Sn intermetallic joints using pre-annealed Sn/Cu/Sn composite preform for high-temperature bonding applications
So-Eun Jeong1,2, Seung-Boo Jung2*, Jeong-Won Yoon1,3*
1
Welding and Joining R&D Group, Korea Institute of Industrial Technology (KITECH), 156 Gaetbeol-ro, Yeonsu-gu, Incheon 21999, Korea 2
School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 16419, Korea 3
Department of Advanced Materials Engineering, Chungbuk National University, 1 Chungdae-ro, Seowon-gu, Cheongju, Chungbuk 28644, Korea
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*Corresponding author: Tel. +82-31-290-7359; Fax: +82-31-290-7371 E-mail address: [email protected] (S.B. Jung) *Corresponding author: Tel. +82-32-850-0237; Fax: +82-32-850-0210 E-mail address: [email protected] (J.W. Yoon)
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Abstract This paper presents the interfacial reaction and growth rate of Cu-Sn intermetallic compounds (IMCs) for various aging times in a composite preform for a die-attach material at high temperature. The feasibility of the composite preform we fabricated for power electronic applications to reduce the long bonding time of transient liquid phase (TLP) bonding was evaluated in this work. After the plating process, the composite preform was composed of Sn layers at both sides of the Cu core layer and the Cu layer. In terms of the aging treatment, the IMC layer was formed rapidly at the Cu-Sn interface in the composite preform by consuming the Sn layers, and the reduction and growth rates of Cu and Cu3Sn were calculated, respectively. We performed the TLP bonding process at 270 °C using various aged composite preforms. The Cu-Sn IMC was formed rapidly on both sides adjacent to the Cu core layer. The thickness of Cu decreased as the aging time elapsed. Therefore, the amount of Cu3Sn adjacent to the Cu core layer increased and the amount of Cu6Sn5 rapidly increased, simultaneously. We investigated the thicknesses and variations of the phase of the joint in the composite preform during the aging treatment process and sequential TLP bonding process for various times. We fabricated pre-annealed Sn/Cu/Sn preforms and performed fast CuSn TLP bonding for high temperature power electronic applications.
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Keywords: Intermetallic compound; Annealing; Diffusion; Joining; Interfaces
1. Introduction Recently, the use of Pb is limited worldwide because of the environmental regulations on end of life vehicles and restriction of hazardous substances (RoHS). Accordingly, power modules for high temperature applications used in power conversion systems are actively being developed in various fields such as electric vehicles, hybrid electric vehicles, and materials for aerospace application [1]. In particular, the use of power modules used in the power conversion systems of eco-friendly vehicles is rapidly increasing. A power module is a device that converts, distributes, and manages DC power supplied from the battery of an environment-friendly vehicle [2]. As a result, solder that can withstand high temperature is needed instead of Pb solder; furthermore, automobiles are usually used for a long period of time, which leads to increase in temperature. Pb-free solder materials are needed for flip-chip packaging, die attachment, and power semiconductor for high-temperature applications. There are promising power semiconductor devices with wide gap width such as silicon carbide (SiC) and gallium nitride (GaN), with good properties such as high thermal conductivity and high
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breakdown voltage, which are suitable for high-temperature and high power applications [3]. To efficiently utilize these characteristics, the assembly process should minimize the heat treatment [4]. Therefore, advancement in semiconductor devices such as SiC and GaN has resulted in reduced power loss and size of the module. However, since it is possible to drive at a high temperature, the die attachment technology requires a bonding technique that can be used at a high temperature for a long time. Currently, many Pb-free solder alloys have been proposed as a result of research and development of Pb-free solder alloys [5,6]. However, despite the development of several commercial and environmentally-friendly Pb-free solders, Pb-free solder substitutes for high temperature applications are limited. For example, Zn-based or Bi-based solder alloys are easily oxidized and have poor wettability and reliability properties [3,7]. Wu et al. [5] reported that Pb-free solders of ternary alloy or quaternary alloy were developed because additional alloy elements were needed to improve the performance of the alloy and the characteristics of binary Pb-free solder cannot fully meet the requirements for electronic packaging. Some good candidates for die attachment applications at high temperatures include soldering technology using Au-based and ternary high-temperature solders, Ag [8] and Cu [9] sintering technology, and transient liquid phase (TLP) bonding technology. However, the cost of Au-based high-temperature solder and Ag
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sintering technology is high owing to the use of expensive materials, and they require high processing temperature. In addition, Cu sintering technology has drawbacks such as oxidation in the bonding materials. On the other hand, TLP bonding technology has advantages such as low cost and requires low bonding pressure. TLP bonding is a combination of soldering and diffusion joining processes. When a metal with a lowmelting point is placed between metals with a high-melting point, the metal with a lowmelting point temporarily changes to a liquid phase and is bonded by diffusive action. The obtained intermetallic compound (IMC) has a high re-melting point (i.e. 415 °C for Cu6Sn5 and 676 °C for Cu3Sn as shown in Fig. 1 [10]). However, this is dependent on the IMC characteristics and there are also drawbacks such as a long bonding time [11]. Lee et al. [2] reported that the Cu-Sn TLP bonded joint was completely converted into Cu-Sn IMC at 300 °C for 2 h in N2 atmosphere. Thus, new materials for operating at a high-temperature are needed for TLP technology bonding at a relatively lowtemperature for a short bonding time. In addition, the materials in the die attachment must have good thermal and electrical conductivity, which can significantly affect the efficiency and reliability of the power module [3]. Cu-Sn bonded materials are widely used in high temperature power devices for TLP bonding technology. In general, Cu-Sn systems are considered the most promising
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candidates that can be converted to IMC owing to their low cost, low process temperature, and high performance for 3D microelectronic packaging [12-17]. In this study, we verified the Cu-Sn IMC formation step at 150 °C using Cu foil to reduce the bonding time. The overall objective of this study is to verify the change in reaction in the joint using Cu foil under various aging times. The Sn layer reacts to form Cu-Sn IMC with inner Cu and Cu on the chip. This study focused on the conditions for forming microstructure joints for a short time using aged preforms.
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2. Experimental procedures In this work, a Cu foil with a thickness of 5 μm was used as the core layer in the preform. Prior to plating process, the tarnish on the Cu foil was removed by rinsing in 10 % sulfuric acid for 1 min and running deionized water. Immersion Sn plating was performed in a commercial Sn plating solution (ITP-1000, Incheon Chemical Co. Ltd., Korea) bath at 65 ± 2 °C as shown in Fig. 2(a). Various thicknesses of Sn layers in the range of 1 – 4 μm were obtained by controlling the processing conditions of the immersion. The 5 μm Cu foil was immersed in a plating solution for 30 min. After the immersion process, the plated Cu foil was rinsed with running deionized water and dried at room temperature. Heat-treatment was carried out at 150 °C for up to 72 h in an oven to obtain a fast reaction between Sn and Cu. Figure 2(b) shows the schematics of the Cu foils after plating and sequential aging treatment. Si square chips with dimensions of 4 mm x 4 mm were used as a chip and substrate in this work. The Si chips were sputtered in the order of Ti (0.1 μm), Cu (1 μm), and Sn (2 μm). The composite preforms were placed in-between the Si chips. Pressure was applied on the Si chip/composite preform/Si chip at 20 MPa using a thermal compression bonder (SJCWD-100, SJ Co. Ltd., Korea), and the samples were bonded at 270 °C for 5 min and 10 min, respectively in vacuum atmosphere. In this process, graphite sheet was used
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to uniformly deliver pressure to the chip. After completing the aging and bonding step, the samples were mounted in epoxy and polished, then examined using a field emission scanning electron microscope (FESEM, INSPECT F, FEI, USA) equipped with an energy dispersive X-ray spectroscope (EDX), electron probe micro analyzer (EPMA, JXA-8500F, JEOL, USA), and focused ion beam (FIB, NOVA-600, FEI, USA). The operating voltage used in the SEM observations was 20 kV. The thicknesses of the phases were measured quantitatively using micrographs obtained from the cross-sections of the intermetallic layer. The average layer thickness was determined as the total area occupied divided by the length.
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3. Results and discussion Figure 3 shows the cross-sectional SEM micrographs of the Sn plated Cu foil (5 μm thickness) after plating at 65 °C for various aging times at 150 °C. The composite preform that was plated for 30 min at 65 °C has a total thickness of approximately 8–9 μm, with the remaining Cu core layer at the center and two layers of Sn at each side of the Cu foil, as shown in Fig. 3(a). The thickness of the Cu foil was reduced from 5 μm to approximately 3 μm after plating, and Sn was plated with a thickness of 2–3 μm on the side of the Cu core layer. The general substitution plating is as follows: Sn2+ + 2Cu → Sn + 2Cu+ During the aging process, the thickness of the Cu core decreased gradually. It is also clear that the amount of Sn on both sides of the Cu core layer decreased and the thickness of the formed Cu-Sn IMC extended around the Cu core layer. The quantitative EDX analysis shows that the composition of the Cu-Sn IMC phase formed at both interfaces corresponds to the Cu6Sn5 and Cu3Sn phases in the binary Cu-Sn phase diagram as shown in Fig. 1. The thin Cu-Sn IMC phase adjacent to Cu was identified as a Cu3Sn layer using EDX analysis. The chemical compositions of the Cu6Sn5 and Cu3Sn phases were 54~55at.%Cu-45~46at.%Sn and 74~76at.%Cu-24~26at.%Sn, respectively. The remaining Sn on both sides of the composite preform was fully converted to Cu-Sn
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IMC as shown in Fig. 3(f). In other words, the Sn remains until 12 h of aging as shown in Fig. 3(e). As the heat treatment progressed to 72 h, the thickness of the Cu core layer decreased and the proportion of the Cu-Sn IMC layer increased. The composite preform is composed of Cu-Sn IMC (Cu6Sn5 and Cu3Sn) layers around the Cu core layer as shown in Figs. 3(f)-(h). We performed FIB analysis to observe the microstructures and phases of the aged preform samples as it is difficult to clearly distinguish the boundaries between the CuSn IMCs and the plated Sn layers using SEM observations. Figure 4 shows the crosssectional FIB micrographs of the Sn/Cu/Sn interface in the composite preform aged at 150 °C for different aging times. The preform before aging treatment clearly shows the Sn on both sides of the Cu core layer as depicted in Fig. 4(a). The Cu3Sn IMC, Cu6Sn5 IMC, and Sn adjacent to the Cu core layer are formed in sequence in the aged composite preform as shown in Figs. 4(b)-(d). During the aging process, the thicknesses of the Cu and Sn layers significantly decreased. In contrast, the thickness of the Cu-Sn IMCs increased as the Sn and Cu layers are consumed with time. A small amount of Sn remained on both sides of the Cu layer in the composite preform as shown in Fig. 4(d). Figure 4(e) shows that both Sn layers are fully consumed and converted to Cu-Sn IMCs. This indicates that a fast reaction takes place at the interface for a very short aging time.
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The thin Cu3Sn IMC phase adjacent to the initially formed Cu core layer reacted with the Cu6Sn5 IMC phase and the Sn layer, and its thickness gradually increased. Once the Sn has been depleted, the Cu6Sn5 IMC reacts with Cu and is converted to Cu3Sn IMC owing to the limitation of the Sn source. Figure 4(f) shows that it was converted to Cu3Sn IMC, leaving a small amount of Cu6Sn5 IMC. As a result, the residual Sn phase with low-melting point can be converted to the IMC phase with high-melting point at a short aging time. The thicknesses of the Cu and the Cu3Sn IMC layers were measured quantitatively, and the result was plotted as the square root of the aging time. Figure 5(a) shows a plot of the square root of the aging time at 150 °C according to the average thicknesses of the Cu and Cu3Sn layers in the composite preform. The relationship between the thickness of each phase in the composite preform and the aging time can be described in a general form as given below [18,19]. W=√(Dt) where W is the thickness of each layer in the composite preform, D is the interdiffusion coefficient, and t is the aging time. According to the results, the thickness of the Cu layer decreased as shown in Fig. 5(a), whereas the thickness of the Cu 3Sn IMC layer increased.
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The average layer thickness was determined as the total area divided by the length as presented in Table 1. The overall slope of the average thicknesses under different aging times was calculated by dividing the average thickness curve of the Cu and Cu 3Sn layers by the square root of the aging time at 150 °C. In other words, the overall slope indicates the growth rate. Cu exhibited a negative slope, whereas Cu3Sn exhibited a positive slope. The results can be used to predict the growth rate of interfacial Cu-Sn IMCs in the composite preform. Figure 5(b) shows the ratio of metal and Cu-Sn IMC layers in the aged composite preform after aging at 150 °C for various aging times. The ratio of metal and IMC thickness was measured quantitatively through the cross-section of the intermetallic layer. As a result, the proportion of Cu6Sn5 IMC gradually decreased, and the proportion of Cu3Sn IMC surged with time. Once the Sn has been depleted, the Cu6Sn5 IMC reacts with Cu and is converted to Cu3Sn IMC. Figure 6 shows the schematics of the bonding machine and test specimens before and after bonding. After the bonding process, the Sn of both the chip and the Sn/Cu/Sn preform reacts to form a Cu-Sn IMC phase at the joint as shown in Fig. 6(b). In other words, Sn with a low-melting point reacts to form Cu6Sn5 and Cu3Sn between the chip and Cu layer. In the TLP bonding process, we used two types of Sn/Cu/Sn preforms
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aged 3 h and 12 h because the Sn layer was completely converted into Cu-Sn IMCs in the 24 h aged preform sample shown in Fig. 4(e). Figure 7 shows the cross-sectional SEM micrographs of joints with the composite preform after TLP bonding at 270 °C for 5 min using the aged preforms under various times. The total thickness of the joint is approximately 9–11 μm. A thin layer of Cu3Sn was formed on both sides adjacent to the Cu. Furthermore, Cu3Sn was formed close to both chips. The Cu3Sn/Cu6Sn5/Cu3Sn was formed in sequence from the Cu core layer. As the aging time elapsed, the amount of reacted Cu increased, and the thickness of Cu decreased. Therefore, the amount of Cu3Sn IMC adjacent to the Cu layer increased, and the amount of Cu6Sn5 increased simultaneously. It is apparent from the reaction that most of the Sn has been depleted, and the amount of Cu3Sn increased owing to the depletion of Cu and Cu6Sn5. To clarify the interfacial microstructures and intermetallic phases of the TLP bonded joint, EPMA analysis was used to examine the Fig. 7(b) sample. The elemental map and line scan results are shown in Figs. 8 and 9. From these results, we clearly confirmed the presence of the remaining Cu layer and sequential Cu3Sn/Cu6Sn5/Cu3Sn IMC layers from the Cu core layer. Figure 10 shows the cross-sectional SEM micrographs of joints with the composite preform after TLP bonding at 270 °C for 10 min using the aged preforms under various
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times. The total thickness of the joint is approximately 8–11 μm. The Cu layer was mostly consumed in the aged preform for 12 h as shown in Fig. 10(c). It can be observed that the amount of Cu6Sn5 IMC changed to Cu3Sn IMC increased compared with bonding for 5 min. Shang et al. [20] reported that Cu3Sn grains nucleated with solid state aging and grew the interfaces of both Cu-Cu3Sn and Cu3Sn-Cu6Sn5. Once the remaining Sn has been fully depleted, Cu3Sn can grow at the expense of the Cu6Sn5 formed at the original boundaries until it has been depleted. Figures 11 and 12 show the EPMA elemental map and line scan results of Fig. 10(c) sample. We clearly observed the complete consumption of the Cu core layer and formation of the sequential Cu3Sn/Cu6Sn5/Cu3Sn/Cu6Sn5/Cu3Sn IMC layers in the joint. In this study, the aged Sn/Cu/Sn preforms compensated for the drawback of long bonding time in the TLP bonding technology. Actually, the TLP bonded joints with the Cu-Sn IMCs were easily fabricated within 5 to 10 min. As a result, we fabricated the pre-annealed Sn/Cu/Sn preforms and Cu-Sn TLP bonded joint in a short bonding time for high-temperature applications. The evaluation of the mechanical strength of the corresponding TLP bonded joints will be carried out in our future work and the mechanical test results will be reported in the future.
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4. Conclusions In this study, the growth rate of Cu-Sn IMC in a composite Sn/Cu/Sn preform was investigated during aging treatment process at 150 °C for various times, and the aged composite preform of various times was designed to compensate for the drawback of TLP bonding technology. It was generally observed in the composite preform that the Cu-Sn IMC layer at both interfaces is double structure, including the Cu6Sn5 layer and Cu3Sn layer at the plated Cu-Sn interface. During the heat treatment process, the Cu3Sn phase grew more than the Cu6Sn5 phase in contact with the residual Sn. As the Cu and Sn reacted and decreased, the amount of Cu3Sn increased with the increase in the aging time. However, the thickness of Cu6Sn5 gradually decreased. Once the residual Sn has been fully depleted, Cu3Sn could grow at the expense of the Cu6Sn5 formed at the original interface until it was depleted. The total thickness of the Cu-Sn IMC layer at both interfaces in the composite preform was calculated using the square root of the aging time. The calculated value is helpful to predict the total thickness of forming the interfacial IMC layer in the composite preform under various thermal conditions for different times. The Sn/Cu/Sn preform was prepared to evaluate the feasibility to reduce the TLP bonding time and was bonded using a thermal compression bonder as the bonding material. After bonding, the Sn of the Si chip and both sides of the composite
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preform reacted with Cu to form Cu-Sn IMC. As the heat treatment time elapsed, the amount of Cu6Sn5 and Cu3Sn increased. The reaction was indicated to be fast owing to the effect of the Cu3Sn phase already formed adjacent to the Cu core layer in the aged composite preform. Furthermore, as the bonding time increased, the amount of Cu decreased sharply, which shows that the rate of change to the Cu3Sn phase is very fast. The results presented in this paper verify the feasibility of the TLP bonding process using aged composite preforms with various times. In this study, we fabricated preannealed Sn/Cu/Sn preforms and Cu-Sn TLP bonded joint in a short bonding time of 10 min.
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Credit Author Statement
So-Eun
Jeong:
Reviewing Won
Yoon:
Visualization,
Validation,
Investigation.:
and Conceptualization,
Seung-Boo
Jung:
Editing.: Methodology,
Original draft preparation, Supervision.
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Data
WritingJeong-
curation,
Writing-
Declaration of Interest Statement
This manuscript has not been published or presented elsewhere in part or in enti rety and is not under consideration by another journal. We have read and unders tood your journal’s policies, and we believe that neither the manuscript nor the st udy violates any of these. There are no conflicts of interest to declare.
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List of figure captions
Fig. 1. Cu-Sn binary phase diagram [10].
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Fig. 2. Schematics of (a) plating bath and (b) Cu foils after plating and aging treatment.
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Fig. 3. Cross-sectional SEM micrographs of Sn/Cu/Sn preforms after aging at 150 °C for different aging times: (a) initial, (b) 3 h, (c) 5 h, (d) 8 h, (e) 12 h, (f) 24 h, (g) 48 h, and (h) 72 h.
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Fig. 4. Cross-sectional FIB micrographs of Sn/Cu/Sn preforms after aging at 150 °C for different aging times: (a) initial, (b) 3 h, (c) 5 h, (d) 8 h, (e) 12 h, (f) 24 h, (g) 48 h, and (h) 72 h.
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Fig. 5. (a) Variations in the Cu and Cu3Sn IMC thicknesses and (b) ratio of metal and Cu-Sn IMC layers in the composite preform after aging at 150 °C for various times.
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Fig. 6. Schematics of bonding machine and test specimens (a) before and (b) after TLP bonding.
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Fig. 7. Cross-sectional SEM micrographs of joints bonded with Sn/Cu/Sn preforms after bonding at 270 °C for 5 min.
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Fig. 8. EPMA elemental mapping results of the sample shown in Fig. 7(b).
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Fig. 9. EPMA line scan analysis results of the sample shown in Fig. 7(b).
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Fig. 10. Cross-sectional SEM micrographs of joints bonded with Sn/Cu/Sn preforms after bonding at 270 °C for 10 min.
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Fig. 11. EPMA elemental mapping results of the sample shown in Fig. 10(c).
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Fig. 12. EPMA line scan analysis results of the sample shown in Fig. 10(c).
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Table 1. Thicknesses and growth rates of Cu and Cu3Sn layers for various aging times
Cu Cu3Sn
Initial
3h
8h
12 h
24 h
72 h
Growth rate
3.209 μm 0
1.881 μm 0.972 μm
1.345 μm 1.330 μm
1.258 μm 1.884 μm
1.101 μm 2.592 μm
0.210 μm 4.233 μm
-0.3175 μm·h-1/2 0.4998 μm·h-1/2
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