Preparation and Thermal Analysis of Sn-Ag Nano Solders

Transcription

1 Materials Transactions, Vol. 1, No. 12 () pp. 214 to 2149 # The Japan Institute of Metals Preparation and Thermal Analysis of -Ag Nano Solders Tran Thai Bao 1; * 1, Yunkyum Kim 1; * 1, Joonho Lee 1; * 2 and Jung-Goo Lee 2 1 Department of Materials Science and Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul , Korea 2 Functional Materials Division, Korea Institute of Materials Science, 66 Sangnam-dong, Changwon, Gyeongnam 641-, Korea In this study, -Ag nano solders of three different compositions (-1. mass%ag, -3. mass%ag and -6. mass%ag) were synthesized via arc-discharge process. The properties of -Ag nano solders were analyzed using X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), and Differential Scanning Calorimetry (DSC). Particle size relatively widely ranged from nm to 34 nm. For -1. mass%ag and -3. mass%ag, average size was 24 nm, and that for -6. mass%ag was 4 nm with some extra-ordinary large particles of nm. The melting points of the prepared Ag nano solders were examined with DSC at different heating rates 1 K, 3 K and K/min. The congruent melting point of -Ag nano solders was found to be 487 K. [doi:.23/matertrans.mj13] (Received April, ; Accepted September 28, ; Published November 17, ) Keywords: arc discharge process, differential scanning calorimetry, nano solder, phase diagram, tin-silver alloy 1. Introduction The melting temperatures of -Ag-Cu and -Cu leadfree solders are in the range between 493 K and K, which are much higher than the melting temperature of conventional -Pb solder (approximately 46 K). Accordingly, the solder reflow temperature becomes as high as 33 K. 1) In order to decrease the melting point, many attempts have been carried out on new alloy design. However, those attempts have not been so successful. Several researchers paid attention to nanoparticles, because the melting point of nanoparticles can be decreased without changing the chemical composition because of the high surface to volume ratio. 1 4) Hsaio and Duh synthesized -3. mass%ag-. mass%cu nanoparticles at room temperature via chemical reduction method. 2) However, without stabilizer, agglomeration and surface oxidization of the nanoparticles easily occurred, and consequently the melting point of this solder was as high as bulk powder (approximately 489 K). Jiang et al. used the chemical reduction method to fabricate - 3. mass%ag nanoparticle alloy in methanol solution at low temperature using 1. phenalthroline as a surfactant. They reported that the melting point of nm -3. mass%ag alloy nanoparticles was decreased by 28.3 K from that of bulk. 3) Pande et al. manufactured -3. mass%ag nanoparticles in silicon oil at approximately 13 K using ethylene glycol as a stabilizer. The nanoparticles have the average size of (8 ) nm and the melting point of 1 K. 1) Zou et al. fabricated -3. mass%ag-. mass%cu nanoparticles using the same method, and decreased the melting temperature by 4.2 K. 4) The chemical reduction method has been considered as relatively cheap and convenient method to prepare nano solders. However, the chemical reduction method has some problem in reproducibility and difficulties in chemical composition control. Therefore, the chemical reduction method is not suitable for mass production. On the other * 1 Graduate Student, Korea University * 2 Corresponding author, joonholee@korea.ac.kr hand, the graphite arc process has been used for mass fabrication of nanoparticles. However, this method also has some problems in the particle size control and poor and irreproducible yield. Recently, several researchers applied the arc discharging method as a new preparation method of nano particles. 8) This convenient method has some unique advantages compared to other methods. Variety systems of binary, ternary, etc. can be easily reproduced with controlled particle size and shape and composition. Recently, Lee and his co-workers studied the thermodynamic properties of bulk and nano solder alloys. 9 16) They calculated the phase diagrams of nanoparticles and pointed out that the melting point decrease may be obtained across the whole composition range for conventional lead-free solder alloys. 13) They also found an interesting behavior of the phase diagram of nanoparticles using a simple regular solution model: when the interaction parameter of solid is positive and that of liquid is negative, all the solidus and liquidus lines and the eutectic point move to the corner of the lower melting point component. 13) -Ag alloy system at high region have a positive interaction parameter in solid ( and are immiscible in solid), and a negative interaction parameter in liquid. 17) Therefore, -Ag system is a good candidate to examine the change in the phase stability of nano system. In this study, the arc discharge method has been applied for the fabrication of -Ag nano solder alloys. The prepared samples were examined with XRD, SEM, TEM, EDX and DSC. The obtained thermal analysis data were compared with the bulk phase diagram to confirm the decrease in melting temperatures. 2. Experimental 2.1 Sample preparation Ag (99.99% purity, 1 3 mm grains, Kojundo Chemical Laboratory) and (99.% purity, mesh powder, Sam Chun Chemicals) were carefully weighed and mixed with different weight percentage of -1.%Ag, -3.8%Ag and -7.%Ag respectively. Then master alloy tabulates were

2 2146 T. T. Bao, Y. Kim, J. Lee and J.-G. Lee Table 1 Chemical composition of the prepared nanoparticles. Sample number Mater alloy -1. mass%ag -3.8 mass%ag -7. mass%ag Nanoparticles -1:ð:Þ mass%ag -3:ð1:Þ mass%ag -6:ð1:Þ mass%ag Cathode (tungsten) Reaction chamber to vacuum pump performed at different heating rates of 1, 3, and K/min in the temperature range of 3 to 623 K under N 2 (99.999% purity) atmosphere. Fig. 1 Gas outlet Sample alloy Anode (copper) Gas inlet A schematic illustration of the present experimental apparatus. prepared with an induction melting furnace in an Ar atmosphere. The solidified samples weighted approximately 2 g (2-mm diameter, 8-mm height). Figure 1 shows a schematic diagram of the arc discharge apparatus. The system consists of a power supply, an arc chamber, a particle collector and a gas circulator. A sample alloy was placed on a water-cooled copper anode plate in the center of the reaction chamber. The upper tungsten rod served as the cathode. The distance between the two electrodes can be controlled from outside during operation. Once a master alloy tabulate was placed at the center of the reaction chamber, it was sealed, evacuated, and introduced Ar gas with a pressure of kpa. The electric voltage and electric current were V and 9 A, respectively. The arc discharge process was carried out for about 12 h, which generated large amount of g nano powders. The chamber was cooled under Ar gas for 2 h before collecting alloy nanoparticles on the inner surface of the chamber. 2.2 Analysis method X-Ray Diffraction (18 kw, Rigaku Model D/MAX-2V XRD) was used to study the crystal structure of Ag alloy nanoparticles. Transmission Electron Microscope (TEM, Tecnai, kv), High Resolution Transmission Electron Microscope (HR-TEM, Technai, kv) and Field Emission Scanning Electron Microscope (FE-SEM, Hitachi S-4, kv) were used to observe the morphologies of the synthesized nanoparticles. TEM specimens were prepared by using ultrasound to disperse manufactured Ag alloy nanoparticles in ethanol and then one drop of the Ag alloy nanoparticles solution was placed onto a carbon film supported by copper grids. SEM specimens were prepared as the same way on a silicon wafer. Energy Dispersive X-ray spectroscopy (EDX) (Horiba EX-) was used to analyze the composition of Ag nanoparticle alloys. The melting temperature of the nanoparticles alloys were determined by mean of Differential Scanning Calorimetry (DSC). DSC analysis was firstly carefully calibrated with % purity (Kojundo Chemical Laboratory). For DSC analysis, the nanoparticles were dispersed in alcohol and then the mixture was dropped into an alumina crucible. The analysis was 3. Results and Discussion 3.1 EDX study The compositions of the prepared nanoparticles are given in Table 1. It was found that the Ag concentrations of nanoparticles were slightly lower than that of mother alloys. Nevertheless, the compositions were close to the master alloy, so it is considered that the arc discharge method can fabricate Ag nano solders with fairly well controlled composition. 3.2 X-ray diffraction (XRD) patterns of Ag alloys Figure 2 shows XRD patterns of the Ag nanoparticle alloys. It is clear that prominent peaks in XRD patterns of Ag nano alloys are attributed to and phases. Therefore, it is considered that the phase was successfully alloyed. From the XRD patterns, oxides peaks were not investigated. 3.3 Transmission electron microscopic (TEM) study Figure 3 shows the TEM and FE-SEM images of the prepared nanoparticles. From these images, it was found that spherically shaped nanoparticles were partially necked with each other. The observed size distribution of - 1. mass%ag nanoparticles was between 828 nm. Few larger particles of approximately nm were also investigated. The particle size distribution of -3. mass%ag was in the range of 34 nm. For these two samples, the average size was 24 nm. For -6. mass%ag alloy nanoparticles, some parts are approximately nm, but most of them were in the range of 8 nm (average size was 4 nm). The particle size distribution and weight percentage of all samples were shown in Fig. 4. In order to verify the structure of Ag nanoparticles manufactured by arc discharge process, the line-analysis was carried out. Figure shows the line analysis result of a -6. mass%ag nanoparticle with a HR-TEM image. The Ag peak was only detected in a certain region with the peak in the nanoparticle, which supports the existence of the phase in the matrix. Accordingly, it is considered that and phases were simultaneously formed during the arc discharge process. From the HR-TEM image, it is considered that the surface was slightly oxidized, but could not be detected from the line analysis or XRD analysis. 3.4 Differential scanning calorimetry (DSC) study Figure 6 shows the DSC results with -3. mass%ag samples with three different heating rates. The measured

3 Preparation and Thermal Analysis of -Ag Nano Solders Fig. 2 XRD analysis of the prepared nanoparticles: -1. mass%ag, -3. mass%ag, -6. mass%ag. Fig. 3 SEM and TEM images of the prepared nanoparticles: SEM image of -1. mass%ag, SEM image of -3. mass%ag, TEM image of -6. mass%ag. onset points (melting starting points) and the second peak points (liquidus temperatures) were almost the same for the three different heating rates. From the DSC analysis result at a heating rate of 1 K/min, it was clearly observed that the main peak was composed of several small peaks. It might be related to the existence of many different sizes of particles, namely as the particle size increased the melting point increased. In addition, during heating, small particles might be coagulated to larger particles. The sequence of coagulation of small particles is of interest, but not so easy to investigate. Additional research is required on the coagulation in the future work. Nevertheless, the second peak is located on the same position regardless of the heating rate. Therefore, when the particle grew to a certain size, the growth rate was considered to become slower, yielding the same liquidus temperature. Figure 7 shows the DSC results of the three samples at the same heating rate of 1 K/min. For comparison the DSC results of bulk samples were shown together. The measured melting temperatures of Ag nanoparticles and bulk alloys were summarized in Table 2. Using the present experimental results, a schematic phase diagram of -Ag alloy nanoparticles at the -rich corner was visualized in Fig. 8. It is clearly found that the melting point was decreased by 67 K from the bulk alloy. Mostly interesting result is that for -3. mass%ag nanoparticle a liquidus point was investigated, which was much higher than the bulk eutectic temperature. Therefore, it was expected that the eutectic composition was shifted for nanoparticles. From the phase diagram, it was likely that the eutectic composition was shifted to the rich corner as suggested in our theoretical study. 13)

4 2148 T. T. Bao, Y. Kim, J. Lee and J.-G. Lee 2 Weight percent (%) Weight percent (%) Weight percent (%) Fig. 4 Size distribution of the prepared nanoparticles: -1. mass%ag, -3. mass%ag, -6. mass%ag. 9 1 Fig. HR-TEM image of a -6. mass%ag nanoparticle, and, line analysis. 4. Conclusions In the present study, we prepared three -Ag nano solders (1., 3. and 6. mass%) by arc discharge process. The samples were analyzed by XRD, SEM, TEM, EDX and DSC. From the thermal analysis, it was found that the congruent melting point of -Ag nanoparticles was decreased to 487 K. In addition, it was expected that the eutectic composition was shifted to the rich corner. ow / µv V Heat fl Ag 1K/min 3K/min K/min Acknowledgements We acknowledges the valuable discussion on DSC analysis with Dr. Geun Woo Lee, KRISS. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (KRF D742) Tempertature, T / K Fig. 6 The DSC curves of -3. mass%ag alloy nanoparticles at different heating rates of 1, 3, and K/min.

5 Preparation and Thermal Analysis of -Ag Nano Solders 2149 µv V Heat flow / Onset Point: K Offset Point: 2K Liquidus Point: K Offset Point: 3.K Liquidus Point: 39K Onset Point: 48.6K Onset Point: K Offset Point: 3K Onset Point: 487.1K Liquidus Point: 2K Offset Point:.2K Onset Point: K Offset Point: 3K Onset Point: 487.1K Offset Point:.2K Liquidus Point: 2K Nano Solders Bulk Temperature, T / K Fig. 7 The DSC analysis results of three different samples: - 1. mass%ag, -3. mass%ag, -6. mass%ag. Temperature, T / K Bulk Nano solder wt% Ag Fig. 8 A schematic illustration of the phase diagram of -Ag system. (solid lines: bulk phase diagram, dashed lines: nano phase diagram) Table 2 DSC peaks analysis results of the prepared nanoparticles and bulk alloys. Sample Heating Rate Onset Point (K) Peak (K) Offset Point (K) Second Peak (K) Nanoparticle 1 K/min Solder 3 K/min mass%ag K/min Bulk Solder 1 K/min ? Nanoparticle 1 K/min Solder 3 K/min mass%ag K/min Bulk Solder 1 K/min Nanoparticle 1 K/min Solder 3 K/min mass%ag K/min Bulk Solder 1 K/min REFERENCES 1) S. Pande, A. K. Sarkar, M. Basu, S. Jana, A. K. Sinha, S. Sarkar, M. Pradhan, S. Saha, A. Pal and T. Pal: Langmuir 24 (8) ) L. Hsiao and J. Duh: J. Elect. Mater. 3 (6) 17. 3) H. Jiang, K. Moon, F. Hua and C. P. Wong: Chem. Mater. 19 (7) ) C. Zou, Y. Gao, B. Yang and Q. Zhai: Mater. Charact. 61 () 474. ) T. Watanabe, H. Itoh and Y. Ishii: Thin Solid Films 39 (1) 44. 6) D. Y. Geng, Z. D. Zhang, W. S. Zhang, P. Z. Si, X. G. Zhao, W. Liu, K. Y. Hu, Z. X. Jin and X. P. Song: Scr. Mater. 48 (3) 93. 7) X. L. Dong, Z. D. Zhang, S. R. Jin and B. K. Kim: J. Appl. Phys. 86 (1999) ) X. L. Dong, Z. D. Zhang, Q. F. Xiao, X. G. Zhao, Y. C. Chuang, S. R. Jin and W. M. Sun: J. Mater. Sci. 33 (1998) ) J. Lee, T. Tanaka, M. Yamamoto and S. Hara: Mater. Trans. 4 (4) ) J. G. Lee, J. Lee, T. Tanaka, H. Mori and K. Penttila: JOM 7 () 6. 11) J. Lee, T. Tanaka, J. G. Lee and H. Mori: Calphad 31 (7). 12) J. Park and J. Lee: Calphad 32 (8) ) J. Lee, J. Park and T. Tanaka: Calphad 33 (9) ) J. G. Lee, J. Lee, T. Tanaka and H. Mori: Nanotechnology (9) ) T. Kim, J. Lee, Y. Kim, J.-M. Kim and Z. Yuan: Mater. Trans. (9) ) H. Xu, Z. Yuan, J. Lee, H. Matsuura and F. Tsukihashi: Colloids Surf. A: Physicochem. Eng. Aspects 39 () 1. 17) H. Ohtani, I. Satoh, M. Miyashita and K. Ishida: Mater. Trans. 42 (1)