Controlling the Microstructures from the Gold-Tin Reaction

Controlling the Microstructures from the Gold-Tin Reaction J. Y. Tsai, C. W. Chang, Y. C. Shieh, Y. C. Hu, and C. R. Kao* Department of Chemical & Materials Engineering National Central University Chungli City, Taiwan (*E-mail: kaocr@hotmail.com Phone/Fax: +886-3-4227382) Abstract The microstructures from the reaction between Au and Sn under different conditions were studied. An Sn/Au/Ni sandwich structure (2.5/3.75/2 µm) was deposited over the Si wafer. The overall composition of the Au and Sn layers corresponded to the Au20Sn binary eutectic (wt.%). When the reaction condition was 290 for 2 min, the microstructure produced was a typical two-phase (Au 5 Sn and AuSn) eutectic microstructure over Ni. In contrast, when the reaction condition was 240 for 2 min, a AuSn/Au 5 Sn/Ni layered microstructure was produced. In both microstructures, a small amount of Ni was dissolved in Au 5 Sn and AuSn. When the AuSn/Au 5 Sn/Ni layered structure was subjected to aging at 240. The AuSn layer gradually exchanged its position with the Au 5 Sn layer, and eventually formed an Au 5 Sn/AuSn/Ni three-layer structure in less than 9 hours. The driving force for Au 5 Sn and AuSn to exchange their positions is for the AuSn phase to seek more Ni. The dominant diffusing species for the AuSn and Au 5 Sn had also been identified to be Au and Sn, respectively. Keywords: Au-Sn, optoelectronic packaging, lead-free solder. 1

Introduction The bonding materials in electronic/optoelectronic packaging serve one or all of the following three major functions: electrical connection, mechanical support, and heat dissipation. According to their melting temperatures, solders for bonding applications in electronic/optoelectronic packages are classified as soft solders and hard solders. Soft solders, such as Sn and In alloys, have low melting temperatures, but exhibit lower yield strengths, which lead to lower creep resistance [1]. Solder creep reduces the reliability of optoelectronic packages since the alignment of devices cannot be maintained overtime. The growth of Sn or In whiskers in soft solders is also known to cause problems in electronic/optoelectronic packages. The growths of Sn and In whiskers have been observed between laser and submount as a result of solder surface migration and electromigration [2-3]. Hard solders, including Au-rich Au-Sn, Au-Si, and Au-Ge alloys, have higher melting temperatures and higher yield strengths. Therefore, they are more resistant to creep. Additionally, whisker growth has never been observed in hard solders. One drawback of the hard solders is their higher melting temperatures. The Au-rich Au-Sn eutectic solder (Au20Sn, wt.%) has a lower melting temperature (278 ) compared to other hard solders, such as Au3.15Si (363 ) and Au12Ge (356 ). This property makes Au20Sn useful for bonding devices that are sensitive to high processing temperature but need good creep resistance, such as GaAs [4-6] or large Si die on alumina [7]. In addition, the high thermal conductivity of Au20Sn (57 W/m ) makes it particularly useful for bonding higher power devices that demand good heat dissipation. The Au-Sn binary system is a complicated equilibrium phase diagram [8]. There are two eutectic compositions, Au20Sn and Au90Sn. The former is widely used for soldering because of its favorable mechanical properties. The latter is not of much interest because it 2

forms brittle phases. The reaction between Au and Sn had been studied before [9-13]. Four of the compounds in the Au-Sn system, Au 5 Sn, AuSn, AuSn 2, and AuSn 4, had been observed in Au-Sn thin film couples of various compositions [9-12]. It was found that Au could diffuse very rapidly into tin-rich matrix [9-12]. It had also been reported that the microstructure of Au20Sn solder on Cu substrate was strongly affected by the amount of Cu dissolution during reflow process [13]. The objective of this study is to study the reaction of Au and Sn that can produce the Au20Sn solder joints on Ni substrate. Specifically, we would like to investigate the possibility of producing Au20Sn solder joints on Ni substrate with different microstructure by changing the bonding condition. Experimental The samples used in this study, illustrated schematically in Fig.1, were formed by depositing an Sn/Au/Ni three-layer structure onto the Si wafer (300 µm thick) through evaporation. The thickness of the Sn, Au, and Ni layer was 2.5 µm, 3.75 µm and 2.0 µm, respectively. This amount of Au and Sn, if uniformly mixed, will produce an alloy with the Au20Sn composition. The samples were reacted for 2 minutes either at 240 or at 290. Then, all the samples were aged at 240 o C for up to 72 hrs. Subsequently, the samples were mounted in epoxy, and metallurgically polished in preparation for characterization. The reaction zone for each sample was examined using a scanning electron microscope (SEM). The compositions of each phase were determined using an electron microprobe (EPMA), operated at 20 kev. In microprobe analysis, the concentration of each element was measured independently, and the total weight percentage of all elements was within 100 ± 1% in each case. The average value from at least three measurements was then reported. 3

Results Figure 2 (a) shows the resulting Au20Sn microstructure after reaction at 290 for 2 minutes. The two reaction products formed over Ni are (Au, Ni) 5 Sn and (Au, Ni)Sn according to the EPMA measurements. These two compounds have the Au 5 Sn and AuSn crystal structures respectively, but have small amounts of Ni dissolved in the Au sublattice. From the EPMA measurements (summarized in Table 1), the Ni concentration is 1.1 at.% in (Au, Ni) 5 Sn, and 8.6 at.% in (Au, Ni)Sn. As can be seen in Fig. 2 (a), the reaction products have a two-phase, aggregate type of microstructure. This is quite reasonable because the reaction temperature (290 ) was higher than the Au 5 Sn+AuSn eutectic temperature of 278. This aggregate microstructure here was partially due to the solidified eutectic microstructure. At least part of the Sn and Au layers became molten during the reaction. It is worthy of noting that the (Au, Ni)Sn phase always in direct contact with the Ni layer in all of our 290 samples. In contrast to the aggregate microstructure, (Au, Ni) 5 Sn and (Au, Ni)Sn formed in the reaction at 240 for 2 minutes had a layered morphology as shown in Fig. 2 (b). The compound (Au, Ni) 5 Sn always formed between Ni and (Au, Ni)Sn. This is because the reaction temperature (240 ) was below the eutectic temperature (278 ), and the reaction occurred through the solid-state diffusion of elements. Therefore, the intermetallic richer in Au, (Au, Ni) 5 Sn, took the place where the Au layer was. From the EPMA measurements (Table 1), the concentration of Ni in (Au, Ni) 5 Sn was 1.5 at.%, and in (Au, Ni)Sn 1.0 at.%. The (Au, Ni)Sn phase had a higher Ni concentration in the 290 case then in 240. This is not only because the reaction temperature was higher but also because (Au, Ni)Sn formed right next to Ni in the 290 case. The (Au, Ni) 5 Sn phase had a higher Ni concentration in the 240 case then in 290. This can be explained by the fact that at 4

240 (Au, Ni) 5 Sn formed right next to Ni, and at 290 (Au, Ni) 5 Sn did not form next to Ni. In summary, the phase formed next to Ni always had a higher Ni concentration compared to the same phase that was not in contact with Ni. Next, the results from those samples that had been reacted at 290 for two minutes, followed by aging at 240, are presented. Figure 3 (a) show what happened when the microstructure in Fig. 2 (a) was aged at 240 for 4 hours. It can be seen that the microstructure had coarsened due to aging. From the EPMA measurement, the Ni contents in (Au, Ni) 5 Sn and (Au, Ni)Sn had been enrich as summarized in Table 1. The microstructure that had been aged at 240 for 72 hrs is shown in Fig. 3 (b). As can be seen in Fig. 3 (b), both (Au, Ni) 5 Sn and (Au, Ni)Sn had formed a almost continuous regions, and there was a two-phase region between the (Au, Ni) 5 Sn continuous region and the and the (Au, Ni)Sn continuous region. In addition to (Au, Ni) 5 Sn and (Au, Ni)Sn, there was a Au-Ni-Sn ternary compound between Ni and (Au, Ni)Sn. According to the EPMA line scan shown in Fig. 4, the composition of this compound is Au 20-22 Ni 34-35 Sn 43-44. According to the Au-Ni-Sn isotherm at room temperature [14], reproduced in Fig. 5, and the recent results of Song et al. [15], we propose that the intermetallic compound was the Ni 3 Sn 2 phase with a large amount of Au dissolved. The results for those aged samples that had been reacted at 240 for two minutes first are to be presented next. Figures 6 (a)-(d) show what happened when the microstructure in Fig. 2 (b) was aged at 240 for different amounts of time. As the aging time increased, a second (Au, Ni)Sn layer started to grow between the (Au, Ni) 5 Sn layer and the Ni layer, as shown in Figs. 6 (a) and (b). The second (Au, Ni)Sn layer was much richer in Ni (Table 1). When the aging time reached 9 hrs, as shown in Fig. 6 (c), the first (upper) (Au, Ni)Sn layer had completely disappeared, and the (Au, Ni)Sn layer has completely exchanged its position 5

with the (Au, Ni) 5 Sn layer. When the reaction time was 72 hrs, as shown in Fig. 6 (d), there was a thin intermetallic compound layer between (Au, Ni)Sn and Ni. According to EPMA line scan shown in Fig. 7, this compound is an Au-Ni-Sn ternary compound. This compound is again probably the Ni 3 Sn 2 phase with a large amount of Au dissolved. The Ni concentration in the (Au, Ni)Sn layer had become very Ni-rich, reaching 20.0 at.%. Discussion The results of this study show that the microstructures of the Au20Sn solder joint can be controlled by controlling the reaction condition. Reaction at 290 for 2 min, the microstructure is the aggregate type, i.e. a mixture of (Au, Ni)Sn and (Au, Ni) 5 Sn. According to the Au-Ni-Sn isotherm [14] (Fig. 5), both AuSn and Au 5 Sn can dissolve appreciable amounts of Ni. At room temperature AuSn can dissolve up to 27 at.% Ni, and Au 5 Sn can dissolve up to 5 at.% Ni. A ternary intermetallic compound often has a lower Gibbs free energy compared to a binary compound of the same structure from the entropy argument. Therefore, both AuSn and Au 5 Sn have the natural tendency to absorb Ni to reach their saturated compositions. Other than coarsening, the aggregate-type of microstructure did not change with aging. The only major difference is the formation of the (Ni, Au) 3 Sn 2 layer between the (Au, Ni)Sn layer and Ni layer. The formation of this new layer is consistent with what the Au-Ni-Sn isotherm (Fig. 5) would predict. Considering a diffusion couple between Ni and the (Au, Ni)Sn phase, there is no tie-line connecting these two phases. Therefore, the Ni/(Au, Ni)Sn interface was not in thermodynamic equilibrium. It follows that there was a driving force for a new phase to form at the (Au, Ni)Sn/Ni interface. According to the isotherm, the (Ni, Au) 3 Sn 2 phase had a tendency to form at the interface. It was due to the fact that there is a tie-line between Ni and (Ni, Au) 3 Sn 2 as well as between (Ni, Au) 3 Sn 2 6

and (Au, Ni)Sn. Reaction at 240 for 2 min, however, produced a AuSn/Au 5 Sn layered structure. The formation of such an interesting structure was, in fact, due to the rapid interdiffusion of Au and Sn. More surprisingly, the system was able to retain the layered structure with aging at 240, even though (Au, Ni)Sn exchanged its position with (Au, Ni) 5 Sn. The driving force for these two compounds to exchange their positions was that (Au, Ni)Sn need to seek Ni. Therefore, (Au, Ni)Sn preferred to have Ni as its immediate neighbor just as in the case of the aggregate structure that (Au, Ni)Sn formed next to Ni. A similar Ni-seeking mechanism had been proposed and widely accepted in the literature for the resettlement of the AuSn 4 phase [16-22]. There are two possible diffusion mechanisms responsible for (Au, Ni)Sn and (Au, Ni) 5 Sn to exchanged their positions, depending on whether Au or Sn is the dominant diffusing species. If Sn is the dominant diffusing species, then Sn atoms from (Au, Ni)Sn diffuse through (Au, Ni) 5 Sn and form a layer of (Au, Ni)Sn at the original interface between (Au, Ni) 5 Sn and Ni. If Au is the dominant diffusing species, then (Au, Ni) 5 Sn near the original (Au, Ni) 5 Sn/Ni interface decompose into (Au, Ni)Sn and Au. The released Au atoms then diffuse through the (Au, Ni) 5 Sn layer and form (Au, Ni) 5 Sn at the (Au, Ni) 5 Sn/(Au, Ni)Sn interface. The Au-Sn reaction for very short reaction time is able to reveal which one is the dominant diffusing species, and it shows that in Au 5 Sn Sn diffuse faster than Au. As shown in Fig. 8, when the reaction time was 30 sec, there were an AuSn 2 layer and an AuSn layer. More importantly, Kirkendall voids at the AuSn/Au interface were clearly visible. The formation of the Kirkendall voids was due to the rapid out-diffusion of Au. This shows that Au is the dominant diffusing species in the AuSn phase. The Au 5 Sn phase started to form beneath the Kirkendall voids after one min at 240, as shown in Fig. 8 (b). The formation 7

of Au 5 Sn involved the decomposition of AuSn 2 and the diffusion of Sn toward the residual Au layer. The fact that the Kirkendall voids situated between Au 5 Sn and AuSn indicates that Sn is the dominant diffusion species in Au 5 Sn. As the reaction time increased, Sn continued diffusing through Au 5 Sn to react with Au until Au was consumed completely, as shown in Fig. 8 (c). In Fig. 8 (c), the location of the Kirkendall voids was still between AuSn and Au 5 Sn. Hence, the mechanism responsible for the exchange of (Au, Ni)Sn and (Au, Ni) 5 Sn is the diffusion of Sn through Au 5 Sn. Conclusion The microstructure of the Au20Sn solder joints can be controlled by reacting Au and Sn at different conditions. When the reaction condition was 290 for two min, the microstructure was a typical eutectic Au20Sn structure, which was a mixture of Au 5 Sn and AuSn. When the reaction condition was 240 for two min, the microstructure was a AuSn/Au 5 Sn/Ni layered structure. When this AuSn/Au 5 Sn/Ni three-layer structure was subjected to aging at 240 for a few hours. The AuSn layer gradually exchanged its position with the Au 5 Sn layer, eventually forming an Au 5 Sn/AuSn/Ni three-layer structure in less than 9 hours. The driving force for Au 5 Sn and AuSn to exchange their positions is for the AuSn phase to seek more Ni. From the location of the Kirkendall voids, Au is the dominant diffusing species in the AuSn phase, and Sn is the dominant diffusing species in Au 5 Sn. The mechanism for Au 5 Sn and AuSn to exchange their positions is that Sn from (Au, Ni)Sn diffuses through (Au, Ni) 5 Sn and formed a layer of (Au, Ni)Sn at the original interface between (Au, Ni) 5 Sn and Ni. 8

Acknowledgment. This work was supported by the National Science Council of R.O.C. through grants NSC-92-2216-E-008-006 and NSC-92-2214-E-008-003. 9

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Table 1: The Ni concentration in (Au, Ni) 5 Sn and (Au, Ni)Sn after different processing conditions. 2 min at 290 2 min at 290 2 min. at 290 2 min at 240 2 min at 240 2 min at 240 + + 72 hr at 240 + + 4 hr at 240 4 hr at 240 72 hr at 240 (Au, Ni)Sn 8.6 at.% 14.5 at.% 20.0 at.% 1.0 at.% 1.0 at.% 1 20.0 at.% 11.0 at.% 2 (Au, Ni) 5 Sn 1.1 at.% 3.0 at.% 0.8 at.% 1.5 at.% 3.0 at.% 0.9 at.% 1. The Ni concentration for those (Au, Ni)Sn regions over the (Au, Ni) 5 Sn layer. 2. The Ni concentration for the (Au, Ni)Sn layer formed between the (Au, Ni) 5 Sn layer and the Ni layer. 12

Figure Captions Fig. 1 Schematic drawing showing the structure and dimensions of the samples used in this study. Fig. 2 Microstructures after reaction for 2 min at (a) 290, and (b) 240. The reaction products were (Au, Ni) 5 Sn and (Au, Ni)Sn in both cases, but had different morphology. An aggregate-type microstructure formed at 290, and a layered microstructure formed at 240. Fig. 3 Backscatter electron micrographs showing the microstructures that had been reacted at 290 for two minutes, followed by aging at 240 for (a) 4 hours, and (b) 72 hours. Fig. 4 EPMA line-scan across the reaction zone for the sample shown in Fig. 3 (b). The intermetallic compound between (Au, Ni)Sn and Ni was an Au-Ni-Sn ternary compound with the composition Au 20-22 Ni 34-35 Sn 43-44. Fig. 5 The Au-Ni-Sn ternary isotherm at room temperature. This isotherm was determined by Anh ck et al. [14]. Fig. 6 Backscatter electron micrographs showing the microstructure that had been reacted at 240 for two minutes, followed by aging at 240 for (a) 1 hour, (b) 4 hours, (c) 9 hours, and (d) 72 hour. Fig. 7 EPMA line-scan across the reaction zone for the sample shown in Fig. 6 (d). The intermetallic compound between (Au, Ni)Sn and Ni was an Au-Ni-Sn ternary compound. 13

Fig. 8 Backscatter electron micrographs showing the formation of intermetallic compounds in the reaction between Sn and Au at 240 for (a) 30 sec, (b) 1 min, and (c) 2 min. 14