12. - 14. 10. 2010, Olomouc, Czech Republic, EU ANALYTICAL ELECTRON MICROSCOPY OF LEAD-FREE NANOPOWDER SOLDERS Jiří BURŠÍK a, Jiří SOPOUŠEK b, Jakub ZÁLEŠÁK b, Vilma BURŠÍKOVÁ c a Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, 616 62 Brno, Czech Republic, [email protected] b Department of Chemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic, [email protected], [email protected] c Department of Physical Electronics, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic, [email protected] Abstract During the last decade, the EU legislative regulations enforced lead-free solders and hence initiated an extensive search for the best replacement of lead-containing solders. Parallel to new binary and ternary bulk solders, metal nanoparticles are also considered as potential candidates for solder materials. It is known that physical, electric and thermodynamic properties of nanoobjects are significantly different from those of the bulk materials. The oxidation, high reactivity of the surfaces and aggregation are frequent problems of nanotechnology applications. The nanoparticles of pure metals and alloys exhibit the depression of the melting point compared to bulk material, hence they are able to aggregate and to form firm interlayer joints at low temperatures. Exploiting this effect can save energy, work and materials. In this work we study the quality of bulk materials produced of Ag nanopowders annealed at various temperatures. Ag nanoparticles were prepared by the chemical wet synthesis and studied in a transmission electron microscope. Sandwich structures were prepared of Cu disks and a layer of Ag nanopowder. The sandwiches were annealed in the range 200−350 °C, metallographic cross sections were prepared from annealed samples and the microstructure and quality of joints was studied by analytical electron microscopy and depth sensing indentation technique. Keywords: solder, nanopowder, nanoindentation 1. INTRODUCTION Since 2006, when regulations enforcing lead-free solders were implemented into the EU legislation, there is an ongoing strong drive to find the best lead-free alternatives. Those presently used have often reliability problems caused by worse mechanical properties, higher tendency to oxidation, higher occurrence of undesirable intermetallic phases and higher melting temperature. They are generally either more expensive (e.g. Sn−Ag based solders) or their use leads to higher technology expenses. After some years of research in this field it turns out that there is no single replacement for the lead containing alloys, which would cover all technical applications. Melting temperature of a (prospective) solder is of the primary concern for both economical and technological reasons. The melting temperature of classical Sn−Pb solder is as low as 183 °C. Several eutectic lead-free solder systems attempt to approach this point (e.g. Sn−Cu: 227 °C, Sn−Ag−Cu: 217.5 °C, Sn−Ag−Bi−In: 210 °C, Sn−Zn−Bi: 193 °C). The melting point of bulk silver (as an example of low-toxic material) is 962 °C, which is far too high even in a category of high-temperature solders (defined by Tm≥230 °C and limited usually by about 350 °C due to polymer materials in substrates used in electronic industry). 12. - 14. 10. 2010, Olomouc, Czech Republic, EU The general effect of lowering melting point of powders with decreasing particle size can be utilized to subscribe to the solution of the high melting point problem of presently used soldering materials [1−6]. For this work, we have synthesized a silver nanopowder, as a potential low toxic constituent of novel solders. We have prepared model joints and studied their microstructure and local mechanical properties. 2. EXPERIMENTAL Silver nanopowder was prepared by a chemical wet synthesis from chemicals of high purity and its characteristics were studied using a Philips CM12 STEM transmission electron microscope (TEM). Copper (99.9 wt%, metal base) disks 5.5 mm in diameter were punched from a sheet 0.127 mm thick. Then they were etched in acid ethanol (0.5 ml 98 wt% H2SO4 and 100 ml 95 wt% ethanol) at 60 °C and finally washed in 60 °C ethanol to remove surface oxides. A layer of silver nanopowder suspension in toluene was put on one of the Cu disks and covered by another one. The sandwich structures were annealed at temperatures 200, 250, 300 and 350 °C for 1.5 hour in the furnace LAC1200. Metallographic cross-sections were prepared from annealed samples and studied using a JEOL JSM 6460 scanning electron microscope (SEM) with Oxford Instruments INCA Energy analyser (EDX). Microhardness and other mechanical properties of sintered Ag layers were measured by depth sensing indentation technique using a Fischerscope H100 tester. 3. RESULTS AND DISCUSSION Ag nanoparticles prepared by wet synthesis were deposited on holey carbon film supported by Cu grid and studied in the TEM. The results are in Figure 1. It is seen that the size distribution of particles is markedly bimodal: the majority of particles have diameter around 20 nm, other particles are substantially larger with diameter around 100−200 nm. Fig. 1. TEM micrographs of Ag nanoparticles prepared by wet synthesis. 12. - 14. 10. 2010, Olomouc, Czech Republic, EU The results of SEM observations are summarized in Figure 2. The samples are named in accordance with the annealing temperature as S200, S250, S300 and S350. It was found, that the microstructure of Ag layer in S200 shows fine grains and pores, whereas the higher annealing temperatures in S250, S300 and S350 produce smooth Ag layer without fine features and with decreasing amount of pores. The Cu−Ag interface is prone to formation of Cu oxide layer, which was also shown in our previous work on similar samples with commercial Ag nanopowder . The SEM resolution does not allow imaging of the oxide layer in S200 (Figure 2a), however the EDX analyses reveal slightly increased oxygen level at the interface. S250 micrograph (Figure 2b) shows a hint of oxide layer; samples S300 and S350 have a well-developed Cu2O layer about 1.5 µm and 2.5µm thick, respectively (see Figure 2c,d). Fig. 2. SEM micrographs of Ag layer and Cu−Ag interface in samples S200 (a), S250 (b), S300 (c), S350 (d). Depth sensing indentation tests were performed on cross sections to characterize local mechanical properties of sintered Ag layers. Loading-unloading curves in Figure 3 clearly show the gradually improving quality of Ag layers with increasing temperature of annealing. Table 1 sums up the mean values of indentation hardness and Young modulus of the Ag layers. The comparison of these values with the results 12. - 14. 10. 2010, Olomouc, Czech Republic, EU obtained previously on samples prepared of commercial Ag nanopowders  shows better mechanical properties of the recently produced Ag layers. Fig. 3. The loading-unloading curves of depth-sensing indentation tests of Ag layers. Table 1. Mechanical properties of Ag layers produced at various annealing temperatures. sample HIT [MPa] Y [GPa] S200 S250 S300 S350 810 ± 30 1070 ± 70 1080 ± 150 1700 ± 200 30 ± 2 54 ± 2 40 ± 4 40 ± 2 ACKNOWLEDGEMENT This research is supported by the Czech Science Foundation (Project 106/09/0700). REFERENCES  McCLUSKEY, F.P. et al. Microel. Reliability, 2006, vol. 46, p. 1910.  YOUN, J.I., HA, W., KIM, Y. J. Adv. Mater. 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