Effect of Specimen Size and Aging on Tensile Properties of Sn-Ag-Cu Lead-Free Solders

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1 Materials Transactions, Vol. 49, No. 5 (8) pp to 1179 #8 The Japan Institute of Metals Effect of Specimen Size and Aging on Tensile Properties of Sn-Ag-Cu Lead-Free Solders Ikuo Shohji 1, Tsutomu Osawa 1; *, Takashige Matsuki 2, Yoshiharu Kariya 3, Kiyokazu Yasuda 4 and Tadashi Takemoto 5 1 Graduate School of Engineering, Gunma University, Kiryu , Japan 2 Faculty of Engineering, Gunma University, Kiryu , Japan 3 Faculty of Engineering, Shibaura Institute of Technology, Tokyo , Japan 4 Graduate School of Engineering, Osaka University, Suita , Japan 5 Joining and Welding Research Institute, Osaka University, Ibaraki , Japan Tensile properties of several Sn-Ag-Cu lead-free solders have been investigated by micro size specimens. For as-cast specimens, tensile strength increases with increasing content of Cu and Ag. After aging at 1 C for 168 h, however, tensile strengths are similar among eutectic and hypereutectic alloys. The similar tendency was observed among hypoeutectic alloys. Moreover, negligible change was found in tensile strength between specimens aged at 1 C for 168 h and 54 h in all solders. The strength change with aging corresponds to microstructural change of the solder. In the cases of eutectic and hypoeutectic alloys, as-cast microstructures are composed of coarsened primary Sn phases and finer eutectic phases of Sn and intermetallic compounds. The primary Sn phases and the eutectic phases are homogenized upon aging. In contrast, finer Sn and eutectic phases are formed in as-cast hypereutectic alloys. The finer phases are coarsened upon aging. After aging, homogenization of the Sn phases and the eutectic phases occurred in all solders. [doi:.23/matertrans.mbw75] (Received October 15, 7; Accepted February 19, 8; Published April 25, 8) Keywords: lead-free solder, tin-silver-copper, miniature testing, micro size specimen, tensile properties, microstructure, aging 1. Introduction Some legislative and regulative requirements, such as the Restriction of the Use of Certain Hazardous Substances (RoHS) legislation have driven the development of leadfree electronics. 1) Lead-free soldering has been widely spread in electronic assemblies. Various lead-free alloys have been studied and used for electronic components, packages and assemblies. Understanding mechanical properties such as tensile strength, ductility, fatigue and creep is very important for evaluating the solder joint reliability and necessary for the design of the solder joint. Thus, the characterization of the mechanical properties of lead-free solders is needed. Generally, mechanical properties of solders have been investigated using bulk specimens with larger volume than that of a real solder joint. 2 4) However, it was pointed out that the microstructure of the conventional bulk specimen does not correspond with that of the real solder joint, since the real solder joint is miniaturized and it is difficult to duplicate the solidification conditions of the real joint in the bulk specimen. 5,6) Kariya and co-workers developed a new testing method for solder alloys and joints using micro size specimens having analogous size and microstructures to those of real solder joints. 5 9) They reported that the creep properties of the micro size specimen of a Sn-3 mass%ag-.5 mass%cu solder differ from those of the bulk specimen under low stress. 5) In this study, we aimed to investigate the mechanical properties of several Sn-Ag-Cu lead-free solders by Kariya s method using micro size specimens and to compare them with those of the bulk specimens. Moreover, the effect of aging on their mechanical properties was investigated. *Graduate Student, Gunma University 2. Experimental Five Sn-Ag-Cu lead-free solders and a Sn-37 mass%pb solder were prepared in this study. Table 1 shows the solder type and the solidus and liquidus temperatures for each solder. Micro size specimens with.5 mm diameter and 2 mm gauge length were fabricated by the following procedures. Solder wire was pressed with a divided metal mold to form the specimen shape. The mold with solder wire was heated to 3 C above the liquidus temperature of the solder, and the solder became molten in the mold. After the solder was completely molten, the mold with the molten solder was cooled on a stainless-steel plate. The cooling rate was approximately 5 C/s in this study. More details of the procedure can be found in the previous report. 6) Tensile test specimens were aged at 1 C for 168 h and 54 h in an oil bath before the tensile test to investigate the effect of aging on the mechanical properties. Microstructural observations were performed for the cross-sections of the gauge regions by electron probe X-ray microanalysis (EPMA). The tensile test was conducted at a strain rate of 1:6 3 s 1 at room temperature. The number of speci- Table 1 Chemical compositions and melting properties of solders studied. Solder type (mass%) Solidus temperature ( C) Liquidus temperature ( C) Sn-2.8Ag-.3Cu Sn-3.8Ag-.7Cu

2 1176 I. Shohji et al. Micro size specimens Bulk specimens 4 µm Fig. 1 Comparison of as-cast micro size specimens in microstructures with conventional bulk specimens 4) (back-scattered electron images). mens for the tensile test was ten for each condition. Vickers hardness measurement was also performed for the crosssection of the grip region of each specimen. A load of.98 N was applied to the specimen for s during the measurement. To compare micro size specimens with conventional bulk specimens, the tensile properties and microstructures of bulk specimens were also investigated. Specimen preparation and microstructural observation were performed following procedures in the previous report. 4) The tensile test using bulk specimens was also conducted at the strain rate of 1:6 3 s 1 at room temperature. For bulk specimens, only specimens without aging treatment were investigated. 3. Results and Discussion 3.1 Microstructures Figure 1 shows microstructures of as-cast micro size specimens and bulk specimens without aging treatment. The bright gray areas were identified as -Sn phases by EPMA mapping analysis in all the specimens. Those phases are primary Sn phases. The dark gray areas were inferred to be eutectic phases consisting of Sn, Ag 3 Sn and Cu 6 Sn 5 on the basis of the results of EPMA mapping analysis in all the solders. For ternary Sn-Ag-Cu alloys, the ternary eutectic composition has been reported to be approximately Sn- 3.5Ag-.75Cu. 12) In this study,, Sn-3.5Ag-.7Cu and were defined to be hypoeutectic, eutectic and hypereutectic compositions, respectively. In micro size specimens, the -Sn phase region becomes larger when the composition changes from hypereutectic to hypoeutectic. -Sn phase areas of 3 mm in size were observed in and. In contrast, -Sn phases were refined and of acicular shape in Sn-4Ag-.9Cu. For Sn-2.8Ag-.3Cu and Sn-3.8Ag-.7Cu, microstructures similar to those of and Sn-4Ag-.9Cu, respectively, were observed. Upon comparing bulk specimens with micro size specimens, it was found that -Sn phases of bulk specimens are much coarser than those of micro size ones. This means the solidification time in the bulk specimen was longer than that in the micro size specimen. The solidification time were estimated to be approximately s and a few second for the bulk and the micro size specimens, respectively. Figure 2 shows microstructures of the micro size specimens after aging. The bright gray, dark gray and black areas were inferred to be -Sn, Ag 3 Sn and Cu 6 Sn 5 on the basis of EPMA mapping analysis results in all the solders. In Sn-3Ag-.5Cu and, refinement of -Sn phases and coarsening of Ag 3 Sn and Cu 6 Sn 5 were observed by EPMA analysis. In particular, coarsening of Cu 6 Sn 5 at grain boundaries was prominently observed. Similar microstructural changes were observed in Sn-2.8Ag-.3Cu. In the cases of hypoeutectic and eutectic Sn-Ag-Cu solders, the refinement of primary -Sn phases and coarsening of intermetallic compounds occur as a result of aging. Figure 3 shows the cross-section of the gauge area in the Sn-3.5Ag-.75Cu micro size specimen after aging at 1 C for 168 h. Grains, which consist of Sn dendrite phases with similar crystal orientation and the eutectic phase of Sn, Ag 3 Sn and Cu 6 Sn 5, are observed in two areas in the gauge section. Similar microstructures have been reported to be observed in the Sn-3Ag-.5Cu micro size specimens. 6 9) Since the formation of recrystallization grains by aging was not observed in Fig. 3, the refinement of primary -Sn phases seems to be caused by homogenization of the primary -Sn phases and the eutectic phases upon aging. In contrast, microstructure coarsening upon aging was observed in (refer to Figs. 1 and 2). Similar microstructural change was observed in Sn-3.8Ag-.7Cu. Although slight growth of intermetallic compounds was observed in the specimens aged at 1 C for 54 h compared with those aged for 168 h, analogous microstructures were observed regardless of aging time in all solders. Therefore, the effect of aging time on the microstructure is negligible

3 Effect of Specimen Size and Aging on Tensile Properties of Sn-Ag-Cu Lead-Free Solders C, 168h 1 C, 54h Fig. 2 Microstructural changes of micro size specimens after aging treatment (back-scattered electron images). µ m Fig. 3 Cross-section of gauge region in micro size specimen after aging at 1 C for 168 h (an optical microscope image). under the aging conditions investigated. Since the microstructural change has been almost completed after aging at 1 C for 168 h in this study, further study using aged specimens in a shorter aging time is needed to investigate more details of the microstructural change upon aging. 3.2 Comparison of tensile properties of micro size specimens and conventional bulk specimens Figure 4 shows the tensile properties of micro size specimens and conventional bulk specimens. The tensile strengths of micro size specimens, except Sn-2.8Ag-.3Cu, are higher than those of bulk specimens. The same tendency can be recognized in Sn-2.8Ag-.3Cu considering the dispersion of data. Micro size specimens show larger dispersion of tensile strength data than do bulk specimens. Since the crystal system of Sn is body-centered tetragonal, mechanical properties strongly depend on crystallographic orientation. It has been reported that a few grains were observed in the gauge section in the micro size specimen of, while the polycrystalline state was found in the bulk specimen. 6,7) Since the tensile properties of lead-free solders evaluated using micro size specimens depend on the crystallographic orientation of Sn-matrix grains, the dispersion of tensile strength evaluated with micro size specimens seems to be greater than that evaluated with bulk specimens. Therefore, the sample number should be approximately ten when we perform tensile tests using micro size specimens of lead-free solders. The elongation of the micro size specimen is less than that of the bulk specimen. Elongation evaluated with the micro size specimen seems to correspond to that of the real solder joint. 3.3 Effect of aging on tensile properties of micro size specimens Figure 5 shows the effect of aging on the tensile properties of micro size specimens. Tensile strength decreases upon aging in the solders investigated, whereas only a slight difference is observed between specimens aged for 168 h and 54 h. This is caused by microstractural change, as described above. That is, the microstructures of solders change negligibly in an aging time range from 168 h to 54 h, as shown in Fig. 2, and thus analogous tensile strength was obtained. A similar tendency was also observed in the Vickers hardness measurement results shown in Fig. 6. Elongation improves upon aging treatment in all the solders, although the dispersion of data is relatively large. Elongation increases slightly with increasing aging time in eutectic and hypereutectic specimens. On the contrary, the elongation of hypoeutectic specimens was reduced when aging time was changed from 168 h to 54 h. A similar tendency was observed in the solder. In hypoeutectic Sn-Ag-Cu and, aging at 1 C for 54 h is probably excessive aging.

4 1178 I. Shohji et al. Elongation (%) Tensile strength, σ /MPa Sn-2.8Ag-.3Cu Sn-2.8Ag-.3Cu Micro size Bulk Sn-3.8Ag-.7Cu Micro size Bulk On comparing with lead-free solders, the tensile strengths of and show similar values to that of before and after aging, respectively, although the elongation of lead-free solders is only a half that of. For as-cast lead-free specimens, the tensile strength increases with increasing Ag and Cu content in the solder. This tendency was not observed in bulk specimens (refer to Fig. 4). This means that the tensile strength of the Sn-Ag-Cu micro size specimen is sensitive to the addition of slight amounts of Ag and Cu. However, the effect does not hold (a) (b) Sn-3.8Ag-.7Cu Fig. 4 Comparison of micro size specimens in tensile properties with conventional bulk specimens (a) Tensile strength, (b) Elongation. Elongation (%) Tensile strength, σ / MPa Sn-2.8Ag-.3Cu Sn-2.8Ag-.3Cu 1 C, 168h 1 C, 54h 1 C, 168h 1 C, 54h Sn-3.8Ag-.7Cu Sn-3.8Ag-.7Cu Fig. 5 Effect of aging on tensile properties of micro size specimens (a) Tensile strength, (b) Elongation. after aging. Although the tensile strengths of hypoeutectic specimens are lower than those of eutectic and hypereutectic specimens, the difference among hypoeutectic specimens is negligible. Moreover, there is little difference in tensile strength among eutectic and hypereutectic specimens after aging. As shown in Fig. 2, analogous microstructures formed in all specimens after aging. Thus, although a difference appeared in tensile strength between hypoeutectic specimens and eutectic and hypereutectic specimens, similar tensile strengths were obtained regardless of Ag and Cu content among hypoeutectic specimens and among eutectic and (a) (b)

5 Effect of Specimen Size and Aging on Tensile Properties of Sn-Ag-Cu Lead-Free Solders 1179 Vickers hardness (HV) C, 168h 1 C, 54h specimens did not hold. Although the tensile strengths of hypoeutectic specimens are lower than those of eutectic and hypereutectic specimens, similar tensile strengths were obtained among hypoeutectic specimens and among eutectic and hypereutectic specimens, respectively. (4) The effects of aging on the tensile properties and the microstructure of the lead-free solder are negligible in the aging time range from 168 h to 54 h at 1 C. (5) Since the tensile properties evaluated using micro size specimens almost correspond to those of real solder joints, this evaluation method can be used to elucidate mechanical properties of real solder joints in order to evaluate the mechanical reliability of solder joints. Sn-2.8Ag-.3Cu Sn-3.8Ag-.7Cu Acknowledgement This study was conducted as one of the research activities of the Solder Research Committee, Japan Welding Engineering Society. The authors express their hearty thanks to the committee members for helpful assistance. Fig. 6 Vickers hardness measurement results for micro size specimens. REFERENCES hypereutectic specimens after aging. The tensile properties evaluated using micro size specimens in this study probably correspond to those of real solder joints. 4. Conclusions The tensile strengths of several Sn-Ag-Cu lead-free solders were investigated using micro size specimens and compared with those of bulk specimens. Moreover, the effects of aging on the tensile properties and microstructures of the solders were investigated. The results obtained are summarized as follows. (1) The tensile strength of as-cast Sn-Ag-Cu lead-free solders increases with increasing content of Ag and Cu. (2) Compared with bulk specimens, the tensile strength of a micro size specimen is sensitive to the addition of slight amounts of Ag and Cu. (3) After aging at 1 C for 168 h, the tensile strength was reduced and the ranking of the tensile strength in as-cast 1) S. Ganesan and M. Pecht: Lead-free Electronics (Wiley-Interscience publication, 6). 2) I. Shohji, T. Yoshida, T. Takahashi and S. Hioki: Mater. Sci. Eng. A 366 (4) ) I. Shohji, T. Yoshida, T. Takahashi and S. Hioki: J. Mater. Sci.: Materials in Electronics 15 (4) ) I. Shohji, K. Yasuda and T. Takemoto: Mater. Trans. 46 (5) ) Y. Kariya, T. Asai, T. Suga and M. Otsuka: Proc. th Symposium on Microjoining and Assembly Technology in Electronics, (The Japan Welding Society, 4) pp ) Y. Kariya, T. Niimi, T. Suga and M. Otsuka: Mater. Trans. 46 (5) ) Y. Kariya, T. Asai, T. Suga and M. Otsuka: TMS Letter 1 (4) ) Y. Kariya: Journal of Japan Institute of Electronics Packaging 9 (6) ) Y. Kariya and T. Suga: Fatigue Fract. Engng. Mater. Struct. 3 (7) ) M. E. Loomans and M. E. Fine: J. El. Mater. 31A () ) K. W. Moon, W. J. Boettinger, U. R. Kattner, F. S. Biancaniello and C. A. Handwerker: J. El. Mater. 29 () ) I. Ohnuma, X. J. Liu, H. Ohtani and K. Ishida: J. El. Mater. 28 (1999)