Effects of Minor Fe, Co, and Ni Additions on the Reaction Between SnAgCu Solder and Cu

Effects of Minor Fe, Co, and Ni Additions on the Reaction Between SnAgCu Solder and Cu Y. W. Wang, C. R. Kao* Department of Materials Science and Engineering National Taiwan University Taipei 106, Taiwan Phone/Fax: +886-2-33663745 *Email: crkao@ntu.edu.tw Abstract The reactions between Cu and the Sn2.5Ag0.8Cu solders doped with 0.03 wt.% Fe, Co, or Ni were studied. Reaction conditions included multiple reflows for up to 10 times and solid-state aging at 160 o C for up to 2000 hrs. In the multiple reflow study, Cu 6 Sn 5 was the only reaction product noted for all the different solders used. Reflows using the solder without doping produced a thin, dense layer of Cu 6 Sn 5. Adding Fe, Co, or Ni transformed this microstructure into a much thicker Cu 6 Sn 5 with many small trapped solder regions between the Cu 6 Sn 5 grains. In the solid-state aging study, both Cu 6 Sn 5 and Cu 3 Sn formed, but adding Fe, Co, or Ni produced a much thinner Cu 3 Sn layer. Because the Cu 3 Sn growth had been linked to the formation of micro voids, which in turn increased the potential for a brittle interfacial fracture, thinner Cu 3 Sn layers might translate into better solder joint strength. Keywords: Solder; Soldering; Diffusion; Intermetallics 1. Introduction In the past few years, the SnAgCu family of solders has obtained a wide acceptance as a replacement for the PbSn eutectic solder in electronic applications. At present, relatively fewer activities are undertaken by researchers worldwide to develop another solder family to replace the PbSn eutectic solder. Instead, the current main research thrust in lead-free solder alloy development is on enhancing or fine-tuning the various properties of SnAgCu through adding minor alloying elements. For example, Ni was evaluated for its potential as a minor alloying element to Sn-based lead free solders [1-8], and so were Ge [2, 5], Fe [9], Co [6, 8, 9], Zn [5, 10, 11], Bi [12], Mn [5], Ti [5], Si [5], and Cr [5]. These previous studies point out minor alloy additions can strengthen the solder performance in many different respects. One of the more noteworthy alloying elements is Ni. It was shown Ni addition to Sn3.5Ag (3.5 wt.% Ag, balance Sn) in amounts as minute as 0.1 wt.% could substantially hinder the Cu 3 Sn growth during soldering as well as during the following solid-state aging [1,7]. The Cu 3 Sn growth had been linked to the formation of micro voids, which in turn increased the potential for brittle interfacial fracture [13]. Recently, it was shown drop test performance increased for solders joints with just a small amount of Ni addition (< 1 wt. %) [14]. In this experiment, solder balls with six different compositions were prepared from 99.999% purity elements. In the reaction between 10 mg solder and electroplated Cu substrate. During the preparation of these samples, the reflow time and peak temperature was 90 sec at 235 o C. In addition, some samples were subjected to thermal aging at 160 o C for times as long as 2000 hrs. The objective of this study is to examine whether Fe and Co additions have a similar effect as Ni does. Emphasis is placed on a systematic comparison study on the effect of Fe, Co, and Ni additions. Further, the level of alloy addition is reduced to as low as 0.03 wt. % so the minimum effective addition of each element can be assessed. Solders with simultaneous Fe and Ni addition as well as simultaneous Co and Ni addition are also prepared to examine whether there is any interaction between the alloying elements. 2. Reaction during Reflow Figure 1 shows the backscattered electron micrographs for the samples with different alloy additions that were reflowed for one time. The Cu 6 Sn 5 phase is clearly visible in all cases. At this stage, the Cu 6 Sn 5 phase was the only phase noted at the interface, and Cu 3 Sn was not observed. In the following, Sn2.5Ag0.8Cu will be denoted as SAC, and Sn2.5Ag0.8Cu0.03Fe will be denoted as SAC0.03Fe, etc. Comparing Fig. 1 (a) to Fig. 1 (b-f), one notes adding Fe, Co, or Ni to SAC made the amount of Cu 6 Sn 5 at the interface increase substantially. For the SAC 50

the interface and its microstructures at the interface from the scallop-type to the aggregatetype. Figure 2 shows the evolution of the microstructures as the number of reflows increased 3, 5, and 10 times. Under all conditions, only Cu 6 Sn 5 was observed as before. Nevertheless, in a previous study [1], it was reported Cu 3 Sn would eventually appear when the reaction time reached 9 hrs at (240 o C). For all the solders used in this study, the amount of Cu 6 Sn 5 at the interface increased with the number of reflows. The amount of the Cu 6 Sn 5 at the interface versus the number of reflows is shown in Fig. 3. Figure 3 also shows the thickness generally followed the following trend for different types of alloy additions: SACCo SACCoNi > SACFe > SACFeNi > SACNi > SAC. Fig. 3: Total amounts of Cu 6 Sn 5 formed near the interface versus the number of reflows. Fig. 1: Backscattered electron micrographs for samples with different alloys additions that were reflowed once. The Cu 6 Sn 5 phase was the only compound observed at the interface. solder, the Cu 6 Sn 5 layer had the classical scallop-type microstructure, consistent with what had been reported in the literature [15-18]. By adding Fe, Co or Ni, the Cu 6 Sn 5 became thicker, and had many voids between the Cu 6 Sn 5 grains. As had been pointed out in our previous study [1], these voids were originally occupied by trapped solder. During sample preparation, the trapped solder in these voids was etched away. This type of microstructure was often referred to as the aggregate-type microstructure. The above observation showed that the type of intermetallic compound formed at the interface was not sensitive to the Fe, Co or Ni additions. The alloy additions only changed the amount of Cu 6 Sn 5 at This study showed doping the SAC with a small amount of Fe, Co, or Ni would not change the type of the reaction product with the Cu substrates. The reaction product was always Cu 6 Sn 5 during a typical reflow. The Cu 3 Sn phase only formed when the reflow time became very long [1] or during the solid-state aging. This differed from the reaction between Ni and the Cu-doped Sn-based solders, which was very sensitive to the Cu concentration [19-22]. Under this situation [19-22], a slight increase in the Cu concentration could switch the reaction product from a Ni 3 Sn 4 -based compound to a Cu 6 Sn 5 - based compound. A detailed rationalization on this sensitivity was present elsewhere [7]. Although adding Fe, Co, or Ni did not change the type of intermetallic compound, the amount of compound at the interface did increase substantially with Fe, Co, or Ni additions, as shown in Fig. 3. It should be pointed out Ni addition to Sn-based solder had also been studied before by other researchers using different Ni concentrations, including 0.07 51

Fig. 2: Evolution of the microstructures for different solders as the number of reflows increased from 3 to 5, and then to 10. [2], 0.05 [3], 0.1 [3], and 0.5 wt. % [3]. Our results in general confirmed their observations. Our new finding was the Ni effect revealed itself even when the Ni concentration was as low as 0.03 wt. %. The growth of Cu6Sn5 shown in Fig. 3 did not follow the parabolic kinetics or the linear kinetics. This was because during reflow there were several concurrent processes. The first was the growth of the compound. The second was the dissolution of the compound into the molten solder. The third was the ripening of the Cu6Sn5 grains, pointed out by Gusak and Tu [23]. The overall kinetics was a weighted average of these processes, and as a result none of them could be clearly be resolved in Fig. 3. 52

Fig. 4: Solidification microstructures with different cooling rates for Sn2.5Ag0.1Ni (a)-(c) and Sn3Ag0.7Cu (d)-(f). The Sn-rich corner of the Sn-Cu-Ni isotherm was shown in Fig. 5 [7]. 3. Effect of Cooling Rate In order to understand why a slight addition of Fe, Co, or Ni was able to increase the amount of Cu6Sn5 substantially, solidification experiments with different cooling rates were carried out by using Sn2.5Ag0.1Ni and Sn3Ag0.7Cu solders. As shown in Fig. 4 (a)-(c), different cooling rates produced different amounts (Cu1-xNix)6Sn5 near the interface, with slower cooling rates producing more (Cu1xNix)6Sn5. In all cooing rates used, (Cu1-xNix)6Sn5 always exhibited the aggregate-type microstructure. These two observations suggest that a major part of the (Cu1-xNix)6Sn5 actually formed during the solidification of the solder. Also shown in Fig. 4 (a)-(c) were the measured x values for (Cu1-xNix)6Sn5. Those (Cu1-xNix)6Sn5 near the solder side had a higher x value, and those near the Cu side had a smaller x. It was proposed that those (Cu1-xNix)6Sn5 near the Cu side formed during the reaction of Cu with the molten solder, and those near the solder side formed during the solidification of the molten solder. This evidence in compositional difference also supports the suggestion that (Cu1xNix)6Sn5 formed in different stages of the reflow process. Without the Ni addition, as shown in Fig 4 (d)-(f), a slower cooling rate also produced more Cu6Sn5. Nevertheless, all these cooling rates produced Cu6Sn5 with the classical scallop-type microstructure. This scallop-type microstructure indicated that these Cu6Sn5 formed directly from the reaction between Cu and the molten solder, and not during the solidification of the solder. Fig. 5: Sn-rich corner of the Sn-Cu-Ni isotherm at 240 oc [7]. This isotherm shows that, with a slight Ni addition, the solder alloy moved from the liquid phase field to the liquid+cu6sn5 two-phase field. It was very likely, that with this Ni addition, the solder alloy also entered the primary solidification phase field of Cu6Sn5. With the alloy now in the primary solidification phase field of Cu6Sn5, a substantial amount of this compound would then form when the temperature reached the liquidus surface. This was the possible reason for the observation that Cu6Sn5 increased with alloy additions, in Figs. 1 and 2. 53

assembly. In all cases, two intermetallic compounds, the Cu6Sn5-based phase and the Cu3Sn-based phase, formed. These two compounds both displayed a dense layered structure. The voids formed during the reflow (Fig. 1) disappeared after 500 hrs of aging. Even though the concentration of the addition elements was very low (0.03 wt. %), 4. Reaction during Aging In addition to the reaction during reflow, the reaction between solid solders and Cu at 160oC is also studied. Figure 6 shows the interface after 500, 1000, and 2000 hrs of reaction for solders with different alloy additions. Before the aging, the solder joints were reflowed once during Fig. 6: Backscattered electron micrographs for samples with different alloy additions that were aged at 160oC for 500, 1000, and 2000 hrs. Before aging, the solder joints were reflowed once during assembly. 54

some of the addition elements did get incorporated into Cu6Sn5 and Cu3Sn. The amounts of these incorporated elements are shown in Table 1 for solder joints that had been added at 160oC for 2000 hrs. As these concentrations were determined using an electron microprobe, which had an accuracy of about one percent, the data in Table 1 only serves to qualitatively illustrate the existence of these elements in Cu6Sn5 and Cu3Sn. although the growth kinetics did not follow the parabolic kinetics very well. The growth kinetics for Cu3Sn, shown in Fig. 7 (b), had a better fit with the parabolic kinetics. The alloy additions had different effects between Cu6Sn5 and Cu3Sn. Any alloy addition to SAC, increased the Cu6Sn5 thickness substantially compared to SAC. Different additions had a different thickening effect on Cu6Sn5, and the thickness generally followed the following trend for different types: SACCo > SACCoNi SACFe > SACFeNi > SACNi > SAC. This trend was similar to the thickening effect for multiple reflow shown in Fig. 2, although not exactly in the same order. Figure 7 (a) shows during the solid-state aging at 160oC. The parabolic kinetics did not describe the growth of Cu6Sn5 very well. However, the growth of Cu3Sn did follow the parabolic kinetics closely, as shown in Fig. 7 (b). This was because during the solid-state aging, the Cu6Sn5 layer first had to consolidate itself from the microstructures shown in Fig. 1 into a dense layered structure shown in Fig. 6. This consolidation kinetics did not necessarily have to be parabolic. The growth of Cu3Sn, however, involved the reaction between Cu6Sn5 and Cu, which was controlled by diffusion, and thus the parabolic kinetics was able to describe the Cu3Sn growth. Figure 7 (b) shows simultaneously adding 0.03 wt.% of Fe and Ni was the most effective in reducing the Cu3Sn thickness. The Cu3Sn thickness was only one-third the SAC solder. Because the Cu3Sn growth had been linked to the formation of micro voids, which in turn increased the potential for brittle interfacial fracture [13], a thinner Cu3Sn might translate into better solder joint strength. More studies on the strength of solder joints with and without Fe, Co, or Ni additions are required to fully confirm this promising suggestion. The key observation of this study was adding Fe, Co, or Ni at an amount as small as 0.03 wt.% was able to reduce the Cu3Sn thickness substantially, as shown in Fig. 7 (b). This effect was effective even after aging at 160oC for 2000 hrs, which surpassed the typical industrial high temperature storage requirement of 150oC for 1000 hrs. The Ni effect had been reported before by us [1], and by others [2-6], but the previous minimum Ni concentration reported was 0.05 wt. % [3]. In the literature, Fe [9] and Co [6-9] had also been reported to reduce the Cu3Sn thickness. Nevertheless, the previous amount of Fe or Co additions (0.2 wt. %) was much higher than that used in this study (0.03 wt. %). The reason Fe, Co, and Ni are effective in reducing the Cu3Sn thickness is unclear at this moment. Several theories have been proposed, including thermodynamic Table 1: Amounts of alloy elements in Cu6Sn5 and Cu3Sn after aging at 160oC for 2000 hrs. (unit: at.%) Alloys SAC0.03Fe SAC0.03Co SAC0.03Ni SAC0.03Fe0.03Ni SAC0.03Co0.03Ni Fe 0 Cu3Sn Co Ni Fe 0.1 0.7 0.1 0.5 Cu6Sn5 Co Ni 0.2 1.2 0.8 0.4 0.1 0.3 0.9 0.4 0.4 Figure 6 also shows the thicknesses of both Cu6Sn5 and Cu3Sn increased with the aging time. The most striking feature in Fig. 6 is that adding Fe, Co, or Ni substantially reduced the Cu3Sn to Cu6Sn5 thickness ratio. The thickness data are plotted in Fig. 7 (a) for Cu6Sn5, and in Fig. 7 (b) for Cu3Sn. Fig. 7: Layer thickness versus the aging time at 160oC for (a) Cu6Sn5 and (b) Cu3Sn. As shown in Fig. 7 (a), for all the cases, Cu6Sn5 grew thicker with the aging time, 55

arguments [24] and kinetic arguments [4, 7, 14]. It is highly likely that adding Fe, Co, or Ni somehow reduced the interdiffusion coefficient of Cu3Sn relative to Cu6Sn5. It is widely known a phase with a higher interdiffusion coefficient will grow faster at the expense of its neighboring phase that has a lower interdiffusion coefficient. The mechanism explaining how adding Fe, Co, or Ni can reduce the Cu3Sn interdiffusion coefficient is still lacking. More studies are needed to clarify this point. 2. 3. 4. 5. Conclusions 1. In the study of multiple reflow using the SAC, SAC0.03Fe, SAC0.03Co, SAC0.03Ni, SAC0.03Fe0.03Ni, and SAC0.03Co0.03Ni solder over Cu substrate, Cu6Sn5 was the only reaction product for all the different solders used. 2. Reflows using the solder without doping produced a thin, dense layer of Cu6Sn5. Adding Fe, Co, or Ni transformed the microstructure into a much thicker Cu6Sn5 with many small trapped solder regions between the grains. 3. The amount of Cu6Sn5 formed at the interface increased with the number of reflows. 4. In solid-state aging study, both Cu6Sn5 and Cu3Sn formed, but adding Fe, Co, or Ni produced a much thinner Cu3Sn layer in all cases in this study. The growth of Cu3Sn followed the parabolic kinetics. 5. Because the Cu3Sn growth had been linked to the formation of micro voids, which in turn increased the potential for brittle interfacial fracture, a thinner Cu3Sn might translate into a better solder joint strength. 6. Simultaneously adding 0.03 wt.% of Fe and Ni was the most effective in reducing the Cu3Sn thickness. The Cu3Sn thickness was only one-third Sn2.5Ag0.8Cu. 7. Adding Fe, Co, or Ni in an amount as small as 0.03 wt.% was effective in reducing the Cu3Sn thickness at 160oC for at least 2000 hrs. 5. 6. 7. 8. 9. 10. 11. Acknowledgement This work was supported by the National Science Council of R.O.C. through grant NSC95-2221-E-002-443-MY3. The EPMA analysis performed by Ms. S. Y. Tsai with National Tsing Hua University is also acknowledged. 12. References 1. J. Y. Tsai, Y. C. Hu, C. M. Tsai and C. R. Kao, A study on the reaction between Cu and Sn3.5Ag solder doped with small 13. 56 amounts of Ni, J. Electron. Mater., Vol. 32, No. 11, pp. 1203-1208, 2003. C. M. Chung and K. L. Lin, Effect of microelements addition on the interfacial reaction between Sn-Ag-Cu solders and the Cu substrate, J. Electron. Mater., Vol. 32, No. 12, pp. 1426-1431, 2003. H. Nishikawa, J. Y. Piao and T. Takemoto, Interfacial reaction between Sn-0.7Cu (-Ni) solder and Cu substrate, J. Electron. Mater., Vol. 35, No. 5, pp. 1127-1132, 2006. H. Yu, V. Vuorinen and J. Kivilahti, Effect of Ni on the formation of Cu6Sn5 and Cu3Sn intermetallics., in: Proc. 2006 Electron. Comp. Technol. Conf., IEEE., pp. 12041209, 2006. I. E. Anderson and J. L. Harringa, Suppression of void coalescence in thermal aging of tin-silver-copper-x solder joints, J. Electron. Mater., Vol. 35, No. 1, pp. 94-106, 2006. F. Gao, T. Takemoto, H. Nishikawa and A. Komatsu, Microstructure and mechanical properties evolution of intermetallics between Cu and Sn-3.5Ag solder doped by Ni-Co additives, J. Electron. Mater., Vol. 35, No. 5, pp. 905-911, 2006. C. E. Ho, S. C. Yang and C. R. Kao, Interfacial reaction issues for lead-free electronic solders, J. Mater. Sci. Mater. Electron., Vol. 18, pp. 155-174, 2007. F. Gao, H. Nishikawa and T. Takemoto, Additive effect of Kirkendall void formation in Sn-3.5Ag solder joints on common substrates, J. Electron. Mater., Vol. 37, No. 1, pp. 45-50, 2008. I. E. Anderson and J. L. Harringa, Elevated temperature aging of solder joints based on Sn-Ag-Cu: Effects on joint microstructure and shear strength, J. Electron. Mater., Vol. 33, No. 12, pp. 1485-1496, 2004. S. K. Kang, D. Leonard, D. Y. Shih, L. Gignac, D. W. Henderson, S. Cho and J. Yu, Interfacial reactions of Sn-Ag-Cu solders modified by minor Zn alloying addition, J. Electron. Mater., Vol. 35, No. 3, pp. 479485, 2006. S. C. Yang, C. E. Ho, C. W. Chang and C. R. Kao, Strong Zn concentration effect on the soldering reactions between Sn-based solders and Cu, J. Mater. Res. Vol. 21, No. 10, pp. 2436-2439, 2006. M. He and V. L. Acoff, Effect of Bi on the interfacial reaction between Sn-3.7Ag-xBi solders and Cu, J. Electron. Mater., Vol. 37 No. 3, pp. 288-299, 2008. K. Zeng, R. Stierman, T. C. Chiu, D. Edwards, K. Ano and K. N. Tu, Kirkendall void formation in eutectic SnPb solder joints on bare Cu and its effect on joint

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