Evaluation on Mechanical Properties of Sn-Bi-Ag Solder and Reliability of the Solder Joint

Transcription

1 [Technical Paper] Evaluation on Mechanical Properties of Sn-Bi-Ag Solder and Reliability of the Solder Joint Hanae Shimokawa*, ***, Tasao Soga*, Koji Serizawa*, Kaoru Katayama**, and Ikuo Shohji*** *Processing Innovation Research Dept., Yokohama Research Laboratory, Hitachi. Ltd. 292 Yoshida-cho, Totsuka-ku, Yokohama , Japan **Global MONOZUKURI Division, Hitachi, Ltd., Information & Telecommunication Systems Company, Hitachi Omori 2nd Bldg., 27-18, 6 chome Minamiohi, Shinagawa-ku, Tokyo , Japan ***Division of Mechanical Science and Technology, Faculty of Science and Technology, Gunma University, Tenjin-cho, Kiryu, Gunma , Japan (Received March 31, 2015; accepted October 13, 2015) Abstract This paper presents low-temperature Pb-free soldering technology using Sn-57Bi-1Ag (mass%). Here, the effects of hightemperature annealing on the mechanical properties of the solder such as tensile strength and elongation are investigated. The experimental results show that during annealing, the sizes of both of Sn and Bi phases coarsen, however the mechanical properties do not deteriorate. The deformation behavior of Sn-57Bi-1Ag is found to be dependent on sliding at grain boundaries between Sn and Bi phases, and this behavior remains consistent even after coarsening. The creep strength of solder joint at high temperature is also studied, and it is found that Sn-57Bi-1Ag exhibits superior creep strength at temperature below approximately 100 C compared to the Sn-37Pb (mass%) solder. The thermal cycling test of Sn-57Bi-1Ag solder joint is also conducted under the condition between 0 C and 90 C. The result shows that the length of crack is shorter than Sn-37Pb in the same conditions, which means Sn-57Bi-1Ag is an effective material for low temperature soldering. Keywords: Sn-Bi-Ag Pb Free Solder, Mechanical Properties, Elongation, Grain Boundary Sliding, Creep Strength 1. Introduction A tin-lead solder such as Sn-37Pb (mass%) was the solder most commonly used in industry to fabricate electronic equipment. In the light of environmental and health concerns, however, a Sn-3Ag-0.5Cu (mass%) solder was developed and has been commonly used already. Since the solidus temperature of Sn-3Ag-0.5Cu is 217 C, it is difficult to replace the Sn-37Pb solder used for step soldering process. It is also difficult to apply Sn-3Ag-0.5Cu for soldering the temperature-sensitive electronic components and substrates. As the eutectic Sn-58Bi (mass%) solder melts at 138 C and this solder is one of the possible candidates for low temperature soldering, the growth of intermetallic compound layer and its mechanical properties have been examined.[1, 2] Several reports showed that the addition of a small amount of third element such as Ag, P and Sb refines the microstructure of eutectic Sn-58Bi solder, improving its ductility.[3 7] Due to the difficulty to control very small amount of P, such as 0.02 mass%,[5] and the health concern by the addition of Sb,[8] Sn-Bi-Ag solder system was studied here. Regarding the amount of Ag, Ueda et al. reported that 1 mass% addition of Ag increases the elongation of eutectic Sn-Bi solder alloy.[3] McCormack et al. reported that 0.25 to 0.5 mass% addition in near the eutectic Sn-Bi solder improves the ductility[4] and Suganuma et al. reported that, for the Sn-Bi eutectic alloy, the addition of Ag should be less than 0.8 mass% to inhibit the formation of large primary Ag 3 Sn.[7] However, Suganuma et al. also reported that there is little influence of Ag content on joining properties of QFP lead frames on a circuit board because of high strength of Sn-57Bi alloy itself.[7] In order to apply low temperature solder to flow soldering process in future, where the solder composition tends to change during operation, the Sn-Bi eutectic solder added relatively high amount of Ag, namely Sn-57Bi-1Ag (mass%) with melting point of 138 C, was selected to study. On Sn-57Bi-1Ag, due to the low melting point, it is con- 46 Copyright The Japan Institute of Electronics Packaging

2 Shimokawa et al.: Mechanical Properties of Sn-Bi-Ag Solder and Reliability (2/9) sidered that changes in the microstructure of the solder in high-temperature service will affect the mechanical properties and creep strength of solder joint, but these effects have not been studied enough, especially about the deformation mechanism after microstructure coarsening. In this study, the effects of high-temperature annealing, up to 125 C, on the microstructure and mechanical properties of this Sn-57Bi-1Ag solder were investigated, and the deformation mechanism was discussed in a metallographic point of view. Moreover, although Suganuma et al. reported that heat-exposure below 100 C has no serious degradation on the joint structure with Sn-Pb plated electrodes,[7] the similar phenomenon has not been confirmed yet for Ni/Au plated electrodes. It is important to investigate the joint reliability with Ni/Au plated electrodes because such electrodes are commonly used for lead-free soldering. Under such background, the permissible maximum temperature of the solder joint using the Ni/ Au plated electrode was investigated by the creep test of solder joint as a function of temperature. The reliability of solder joint was evaluated by the thermal cycling test under the permissible maximum temperature obtained. 2. Experimental Procedures 2.1 Tensile test Sn-57Bi-1Ag and Sn-37Pb solders were melted in crucibles on a hot plate and poured into carbon molds heated to 200 C for Sn-57Bi-1Ag, and 250 C for Sn-37Pb. The specimens were then cooled to room temperature. The shape and size of the specimens are shown in Fig. 1. The specimens were aged at room temperature for 10 days after casting. Tensile tests were carried out using an Instron mechanical testing machine (4204) at a cross-head speed of 0.1 mm/min at room temperature to obtain tensile strength and elongation until rupture. The elongation was converted from the amount of displacement of crosshead. To investigate the deformation behavior of the solders, the surfaces of the specimens were polished with 0.25 μm diamond paste and the microstructure was then observed using a scanning electron microscope (SEM), before and after the tensile test. 2.2 Measurement of size of Sn phase The specimens were annealed at 65, 90, and 125 C for Fig. 1 Shape and size of specimen for tensile test. 1,000, 2,500, and 7,500 h in an oven, and photos of the microstructure at three different points in each specimen were taken by SEM. Six lines were drawn in each SEM photo and the width of the Sn and Bi phases crossing the line were measured, as shown in Fig. 2. Such measurements were carried out at about 100 points. The average size of Sn phase was plotted against annealing time. 2.3 Creep test A schematic of the creep test specimen is shown in Fig. 3. The sample was prepared by soldering a Cu lead to a Cu pad on a glass epoxy substrate using solder paste. The Cu lead was coated with Ni (thickness: approximately 2 μm) and then Au (thickness: approximately 0.1 μm). The maximum temperature of reflow soldering was 200 C for Sn-57Bi-1Ag and 220 C for Sn-37Pb. The joint area was 9 mm 2. A load was applied to the Cu lead in an oven at 90 and 125 C, as shown in Fig. 3, and the time until rupture was measured. The load was varied from 0.5 to 3.3 kg. 2.4 Thermal cycling test In order to evaluate the reliability of Sn-57Bi-1Ag solder joint, a ceramic pin grid array (PGA) package with 1,019 pins was soldered to the hole of a glass epoxy substrate, as shown in Fig. 4. The PGA was 42 mm 42 mm 2.34 mm in size, and the temperature of the solder bath was 215 C. The pin was made of Kovar plated with Ni (thickness: approximately 2 μm) followed by Au (thickness: approximately 0.1 μm). The pin pitch was 1.27 mm and the thickness of the substrate was 4 mm. After soldering the PGA package to the substrate, the thermal cycling test were carried out on this specimen in air at between 0 C and Fig. 2 Schematic representation of measurement of size of Sn phase. Fig. 3 Schematic of creep test set up. 47

3 Fig. 4 Shape of the specimen for reliability test. Table 1 Material properties used. Material Sn-57Bi-1Ag Sn-37Pb PGA Substrate Temperature Young s modulus Poisson ratio CTE Yield stress Work-hardening coefficient ( C) (kgf/mm 2 ) (1/ C) (kgf/mm 2 ) (kgf/mm 2 ) -50 3, , , , , , , , , , , , C, with a hold time at each temperature of 30 min and heating-up and cooling-down period of about 10 min. Along the diagonal line of PGA package, the cross-sectional microstructure of the soldered joints and the length of cracks were then observed by SEM. In parallel, a 2D plane model of the solder joint was established for finite element analysis and the amplitude of equivalent strain occurred at the solder joint along the diagonal line was calculated. Table 1 shows the material properties used in the modeling. The principal stress in x direction was also obtained. The results were compared with that of Sn-37Pb solder joint. 3. Results and Discussion 3.1 Mechanical properties and deformation behavior The tensile strength and elongation of Sn-57Bi-1Ag and Sn-37Pb are shown in Fig. 5. The tensile strength of Sn-57Bi-1Ag is higher than that of Sn-37Pb, however the elongation of both solders is almost the same. As Bi is con- Fig. 5 Tensile strength and elongation of Sn-57Bi-1Ag and Sn-37Pb. sidered to be generally brittle, the origin of the high elongation of the Sn-57Bi-1Ag solder was investigated. Figure 6 shows the microstructures of the surfaces of Sn-57Bi-1Ag and Sn-37Pb specimens before and after the tensile test. The initial microstructure of Sn-57Bi-1Ag is fine lamellar structure. The light phases in Fig. 6(a) are Bi phases, and the gray regions are the Sn-rich phases. Ag is dispersed as Ag 3 Sn at the boundary of the Sn and Bi phases. As there is a little Ag in the Sn-57Bi-1Ag solder, Ag 3 Sn is not clearly observed in Fig. 6. Primary crystals of Sn in the initial 48

4 Shimokawa et al.: Mechanical Properties of Sn-Bi-Ag Solder and Reliability (4/9) μm μm μm Fig. 6 Microstructures of Sn-57Bi-1Ag and Sn-37Pb before and after tensile test. microstructure of Sn-57Bi-1Ag are also observed, considered to have crystallized in localized areas of higher Sn concentration. In these primary crystals of Sn, Bi is finely precipitated. Figures 6(c) and (d) show the microstructure changes from Figs. 6(a) and (b), respectively, following the tensile test. The direction of tensile distortion is shown in Fig. 6. These figures show that sliding occurs at grain boundaries between Sn and Bi phases. Figure 6(e) shows the microstructure of another point on the same specimen after tensile test. This figure also shows that sliding occurs at grain boundaries between Sn and Bi phases. Primary crystals of Sn, however, exhibit little deformation under tensile strain, due to the precipitation hardening of Sn by smaller Bi particles. These results indicate that elongation will decrease when the amount of Sn in Sn-57Bi-1Ag is increased because the proportion of precipitation-hardened primary crystal increases. Therefore, the relatively high elongation of Sn-57Bi-1Ag is attributable to sliding at grain boundaries in the area of fine lamellar microstructure and the contribution of the sliding at the grain boundary of the fine Bi inside primary crystal of Sn to the deformation is negligible. From the report of mechanical properties of the Sn-57Bi eutectic solder,[9] it was shown that the dominant factor of creep deformation at 75 C is sliding at grain boundary. Although the evaluation method is different from our test, the deformation behavior would not be changed so much by the addition of 1 mass% Ag. Figures 6(f) and (g) show the microstructures of Sn- 37Pb before and after the tensile test. The microstructure of Sn-37Pb is consisted of two phases; the lighter areas are associated with Pb phases, whereas the darker areas are μm Fig. 7 Microstructures of Sn-57Bi-1Ag (a) Initial. (b) After annealing at 90 C for 2,500 h. Sn phases. Since sliding is not clearly observed at the boundary between Sn and Pb phases, the deformation in Sn-37Pb seems to occur as a result of sliding within Sn and Pb phases. Kitamura et al. reported that boundary sliding also occurs at the interface between colonies in the initial deformation stage.[10] The relatively high elongation of the Sn-57Bi-1Ag solder suggests that this solder will be useful in service. However, its low melting point (138 C) means that the Sn-57Bi- 1Ag solder will be particularly sensitive to high-temperatures. High-temperature may induce microstructure coarsening, with the likely result that the mechanical properties will change because Sn-57Bi-1Ag is deformed by sliding at fine grain boundaries in the area of lamellar microstructure. Therefore, the microstructure and mechanical properties of the Sn-57Bi-1Ag solder were examined as a function of annealing temperature. Figure 7 shows the microstructural change of the same 49

5 μ Fig. 8 Change of size of Sn phase with annealing for Sn-57Bi- 1Ag solder. μm Fig. 9 Change in tensile strength and elongation with size of Sn phase. Fig. 10 Microstructures of Sn-57Bi-1Ag annealed at 90 C for 7,500 h and change after tensile test. (a) Before tensile test (b) After tensile test (same point with (a)) (c) Another point after tensile test μm area after high-temperature annealing. Figure 7(a) is the initial microstructure, and (b) is after annealing at 90 C for 2,500 h. Coarsening of both Sn and Bi phases is observed. The sizes of the phases were measured as described in Fig. 2 for each annealing condition, with the results shown in Fig. 8. Sn phase can be seen to coarsen with increasing temperature and time. Tensile tests were carried out for each annealing condition in order to examine the mechanical properties of coarsened Sn-57Bi-1Ag. The change in tensile strength and elongation according to the average size of the Sn phase is shown in Fig. 9. As the size of the Sn phase becomes approximately 2 μm, the tensile strength increased while elongation decreased. When the size of Sn phases becomes larger than approximately 4 μm, elongation can be seen to increase. In order to determine the reason that Sn-57Bi-1Ag coarsened by high-temperature annealing has the same elongation as unannealed Sn-57Bi-1Ag, which has a fine microstructure, the deformation behavior of annealed Sn-57Bi-1Ag was examined. Figure 10 shows the microstructures of Sn-57Bi-1Ag annealed at 90 C for 7,500 h, by tensile test. Deformation still occurs at grain boundaries between Sn and Bi phases, the same as in unannealed Sn-57Bi-1Ag, and the Sn primary crystal still does not deform, due to precipitationhardening by Bi. However, sliding lines are observed very locally in the Bi phase shown in Fig. 10(c), because the solid solution of Sn to Bi is very limited. The cause of elongation becoming lower at a Sn grain size of approximately 2 μm compared to the initially fine structure is not clear. However it is considered that rapid cooling such as that in soldering processes does not allow the solder to solidify along the line of Sn-Bi equilibrium, and excess Sn initially soluted in the Bi phase may precipitate during Sn coarsening, according to a report on the effects of annealing on Sn-Bi eutectic solder by Hu et al.[11] It has been also reported that, in the case of the Sn-Bi eutectic solder, the solid solution hardening of Bi into Sn-rich solid solution causes the increase of the hardness at high temperature, such as 100 C, by Miyazawa et al.[12] These microstructural changes are expected to occur also in the case of Sn-57Bi-1Ag, resulting in lowering the elongation at a Sn grain size of approximately 2 μm. These results were obtained from tensile tests at cross-head speed of 0.1 mm/ min, in which the strain rate was estimated to be about s -1. However, at higher test speed of 500 mm/ min, where the strain rate is about s -1, Sn-57Bi- 50

6 Shimokawa et al.: Mechanical Properties of Sn-Bi-Ag Solder and Reliability (6/9) Fig. 11 The relationship of elongation and test speed. 1Ag becomes brittle. Figure 11 shows the elongation of Sn-57Bi-1Ag and Sn-37Pb at a cross-head speed of 0.1 and 500 mm/min. In the case of the Sn-57Bi-1Ag specimen tested at 500 mm/min, elongation becomes significantly lower than that at 0.1 mm/min, whereas in the case of the Sn-37Pb specimen tested at 500 mm/min, elongation becomes lower than that at 0.1 mm/min, but the difference of the elongation between two cross head speed is small. From these results, Sn-57Bi-1Ag is considered to be essentially brittle, however, it has a useful property that grain boundary sliding is likely to occur at low strain rates at room temperature. This property is probably attributable to its low melting point, 138 C, giving the solder a high homologous temperature at room temperature. This indicates that Sn-57Bi-1Ag would be useful in applications where low strain rates prevail, such as by thermomechanical fatigue, yet would not be suitable for joints subjected to high strain rates, such as by rapid heating and cooling or impact. 3.2 Creep properties of solder joint Figure 12 shows the rupture time for solder joint under shear stress. The rupture time for Sn-57Bi-1Ag is shorter than that for Sn-37Pb at 125 C, yet longer than Sn-37Pb at 90 C. Rupture occurred inside both the Sn-57Bi-1Ag and Sn-37Pb solder layer, as shown in Fig. 12(b). Figure 13 shows the dependence of temperature on rupture time at shear stresses of around 1.6 MPa. In this evaluation, only two temperature conditions were conducted. However, from the assumption that the properties change almost proportionally in the range between 90 C and 125 C as both temperatures are above half absolute temperature of those melting points, the dotted lines were described in Fig. 13. By taking the intersection of these lines, creep strength of the solder joint of Sn-57Bi-1Ag and Sn-37Pb are almost the same at approximately 100 C, and at below approximately 100 C, the creep strength of Sn-57Bi-1Ag is superior to that of Sn-37Pb, indicating that the permissible maximum temperature of the Sn-57Bi-1Ag solder joint using Ni/Au plated electrodes is approximately 100 C. Sn-57Bi-1Agat125 Sn-57Bi-1Agat 90 Sn-37Pbat125 Sn-37Pbat90 μ Fig. 12 Rupture time by creep test. Fig. 13 Effect of temperature on rupture time by shear stress. 1.E+05 1.E+03 1.E+04 1.E+02 Above 100 C, the Sn-57Bi-1Ag solder becomes very soft; the hardness at 21 C is 20 HV, yet the hardness at 100 C lowers about 75% to 5 HV by the measurement of Sn-57Bi- 1Ag solder ingot. Until now, the creep strength of the Sn-Bi eutectic solder has been often studied and compared with Sn-37Pb. From the report of Mei et al.,[13, 14] at 65 C, the steady state shear strain rate of Sn-58Bi solder is smaller than that of Sn-37Pb, and from the report of Jin et al.,[15] at 100 C, the creep resistance of Sn-37Pb solder is inferior to that of the Sn-57Bi solder. Taking into consideration of these results on creep tests on the Sn-Bi eutectic solder together, Sn-57Bi-1Ag solder seems to be able to use up to the temperature, approximately 100 C. 3.3 Reliability of solder joint Because the Sn-57Bi-1Ag solder joint using Ni/Au plated 51

7 25 μm Fig. 14 Crack occurred at Sn-57Bi-1Ag solder joint (0~90 C, 1,000 cycles). electrodes has excellent creep properties below approximately 100 C comparing with Sn-37Pb as mentioned above, the thermal cycling test was conducted between 0 and 90 C, for 1,000 cycles to evaluate the reliability of the solder joint. Figure 14 shows the crack occurred at the Sn-57Bi-1Ag solder joint after 1,000 cycles. The crack propagates in the solder layer, not at the interface between the solder and the Ni layer. The length of the crack at the each pin along the diagonal line of PGA package is shown in Fig. 15, revealing that the length of crack of Sn-57Bi-1Ag solder is shorter than that of Sn-37Pb solder. In order to clarify the relationship between the life of solder joint and the strain occurred at the solder joint, the elasto-plastic finite element analysis was conducted and the amplitude of equivalent strain at the solder joint along the diagonal line of PGA package was obtained. Figure 16 shows the amplitude of equivalent strain of the Sn-57Bi- 1Ag and Sn-37Pb solder joints. It shows the amplitude of equivalent strain of Sn-57Bi-1Ag solder joint is smaller than that of Sn-37Pb solder joint, due to the smaller Young s modulus of Sn-37Pb than Sn-57Bi-1Ag in the considerable range between 0 and 90 C. From the above FEM analysis, the principal stress in x direction at the interface between solder and Ni was also obtained, as shown in Fig. 17. The x direction principal stress of Sn-57Bi-1Ag solder joint at the No. 15 pin, at most external side of PGA package, is about 1.7 times of the Sn-37Pb solder joint. This result means that it is important for the Sn-57Bi-1Ag solder joint to choose appropriate materials of pin to avoid peeling at the interface, because of relatively high stress at the interface than the conventional solder joint. As respects the life of solder joint, we decided that the life of the solder joint is the cycle when the crack became 300 μm long, the half length of the depth, 600 μm, of the hole. Figure 18 shows the relationship of the life and the μ Fig. 15 Comparison of length of crack between Sn-57Bi-1Ag and Sn-37Pb (0~90 C, 1,000 cycles). Amplitude of equivalent strain Sn-37Pb Sn-57Bi-1Ag Number of pin from center Fig. 16 Comparison of the amplitude of equivalent strain between Sn-57Bi-1Ag and Sn-37Pb (0~90 C). Fig. 17 Comparison of x-direction principal stress between Sn-57Bi-1Ag and Sn-37Pb (0~90 C). Sn-57Bi-1Ag Sn-37Pb Fig. 18 Fatigue life of Sn-57Bi-1Ag and Sn-37Pb (0~90 C). amplitude of the equivalent strain. This result indicates that if the strain occurred in the solder joint is same, the life of Sn-37Pb solder joint may be longer. However, due to 52

8 Shimokawa et al.: Mechanical Properties of Sn-Bi-Ag Solder and Reliability (8/9) the mechanical properties of Sn-57Bi-1Ag, the strain occurred in the solder joint is smaller than Sn-37Pb and finally the life of Sn-57Bi-1Ag solder joint could be longer than that of Sn-37Pb. Therefore, the Sn-57Bi-1Ag solder is effective for soldering the electronic component and substrate with low temperature resistance, in moderate environment conditions like under approximately 100 C. 4. Conclusions In this study, the effects of high-temperature annealing, up to 125 C, on the microstructure and mechanical properties of the Sn-57Bi-1Ag solder were investigated, including an investigation of the creep strength of solder joint as a function of temperature and time. The reliability of solder joint was evaluated by the thermal cycling test under the permissible maximum temperature obtained. The results can be summarized as follows: 1. The deformation behavior of Sn-57Bi-1Ag depends on sliding at grain boundaries between Sn and Bi phases in the area of lamellar structure, and this behavior remains consistent even after coarsening. 2. The creep strength of Sn-57Bi-1Ag is superior to Sn- 37Pb solder at temperatures below approximately 100 C when soldered to a lead plated with Ni and Au. 3. As the results of thermal cycling test under the condition between 0 C and 90 C, the length of crack in Sn-57Bi-1Ag is shorter than that in Sn-37Pb, which means Sn-57Bi-1Ag is effective material for low temperature soldering. References [1] P. T. Vianco, A. C. Kilgo, and R. Grant, Intermetallic Compound Layer Growth by Solid State Reactions between 58Bi-42Sn Solder and Copper, J. Electron. Mater., Vol. 24, No. 10, pp , [2] C. H. Raeder, L. E. Felton, D. B. Knorr, G. B. Schmeelk, and D. Lee, Microstructural Evolution and Mechanical Properties of Sn-Bi Based Solders, IEEE/CHMT International Electronics Manufacturing Technology Symposium, pp , [3] H. Ueda, M. Ochiai, and Y. Yamagishi, Thermal Fatigue Behavior of Lead-free Solder of Tin-Bismuth System, Proc. 2nd Symposium on Microjoining and Assembly Technology in Electronics, pp , [4] M. McCormack, H. S. Chen, G. W. Kammlott, and S. Jin, Significantly Improved Mechanical Properties of Bi-Sn Solder Alloys by Ag-Doping, Journal of Electronic Materials, Vol. 26, No. 8, pp , [5] Y. Nakahara, J. Matsunaga, and R. Ninomiya, Effect of Trace Elements on the Mechanical Properties of Sn-Bi Solder, Proc. 6th Symposium on Microjoining and Assembly Technology in Electronics, pp , [6] S. Sakuyama, T. Akamatsu, K. Uenishi, and T. Sato, Effects of a Third Element on Microstructure and Mechanical Properties of Eutectic Sn-Bi Solder, Transactions of The Japan Institute of Electronics Packaging, Vol. 2, No. 1, pp , [7] K. Suganuma, T. Sakai, K. Kim, Y. Takagi, J. Sugimoto, and M. Ueshima, Thermal and Mechanical Stability of Soldering QFP With Sn-Bi-Ag Lead- Free Alloy, IEEE Transactions of Electronics Packaging Manufacturing, Vol. 25, No. 4, pp , [8] M. Okamoto, T. Nakatsuka, O. Ikeda, K. Serizawa, and H. Shimokawa, Lead-Free Soldering Technologies Meet the Restriction of the Use of Certain Hazardous Substances in Regions Including EU, Hitachihyoron, Vol. 88, No. 12, pp , [9] Y. Kariya, Mechanical Properties, Journal of Japan Welding Society, Vol. 76, No. 2, pp , [10] T. Kitamura, S. Kikuchi, and M. Koiwa, Nonuniform Deformation and Dynamic Recrystallization of Ascast Pb-Sn Eutectic Alloys, Journal of the Society of Materials Science Japan, Vol. 40, No. 448, pp , [11] J. Hu, H. Tanaka, O. Munenaga, T. Taguchi, and T. Narita, Study of Microstructure and Hardness of Sn-Bi Eutectic Alloy at Aging Treatment, Proc. 5th Symposium on Microjoining and Assembly Technology in Electronics, pp , [12] Y. Miyazawa and T. Ariga, Influences of Aging Treatment on Microstructure and Hardness of Sn- (Ag, Bi, Zn)Eutectic Solder Alloys, Materials Transactions, Vol. 42, No. 5, pp , [13] Z. Mei, H. V. Plas, J. Gleason, and J. Baker, Low- Temperature Solders, Proceeding of the Electronic Materials and Processing Symposium, pp , [14] Z. Mei and J. W. Morris, Jr., Characterization of Eutectic Sn-Bi Solder Joints, Journal of Electronic Materials, Vol. 21, No. 6, pp , [15] S. Jin and M. McCormack, Dispersoid Additions to 53

9 a Pb-Free Solder for Suppresion of Microstructural Coarsening, Journal of Electronic Materials, Vol. 23, No. 8, pp , Hanae Shimokawa Tasao Soga Koji Serizawa* Kaoru Katayama Ikuo Shohji *Koji Serizawa Current address: Senju Metal Industry Co., Ltd 54