- Email: [email protected]
Improvement of bonding strength of a (Pb, Sn)Te–Cu contact manufactured in a low temperature SLID-bonding process
Journal of Alloys and Compounds 613 (2014) 46–54
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Improvement of bonding strength of a (Pb, Sn)Te–Cu contact manufactured in a low temperature SLID-bonding process Tung-Han Chuang a,⇑, Wei-Ting Yeh a, Chien-Hsun Chuang a, Jenn-Dong Hwang b a b
Institute of Materials Science and Engineering, National Taiwan University, 106 Taipei, Taiwan Material & Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31015, Taiwan
a r t i c l e
i n f o
Article history: Received 25 December 2013 Received in revised form 3 June 2014 Accepted 3 June 2014 Available online 12 June 2014 Keywords: Solid–liquid interdiffusion bonding (Pb, Sn)Te Intermetallic compounds Bonding strength
a b s t r a c t A (Pb, Sn)Te thermoelectric (TE) material coated with a Ni barrier layer and a Ag reaction layer was bonded with a Ag-coated Cu electrode at low temperatures in the range of 175–250 °C with an In thin film. The In film reacted initially with the Ag layer to form a double layer of Ag3In and Ag2In intermetallic compounds, which then completely reacted with the Ag layer to form a Ag3In intermetallics layer having a melting point of 690 °C. The resulting module can thus be applied at the optimized operation temperature of (Pb, Sn)Te material (400 °C). The bonding strengths ranged from 6.3 to 7.8 MPa for various bonding temperatures at 30 min, and shear tests revealed that the specimens fractured along the interface between the (Pb, Sn)Te thermoelectric element and the Ni barrier layer. The adhesion of the TE/Ni interface was improved through pre-electroplating a 1 lm Sn film on the surface of the thermoelectric element and heating at 250 °C for 3 min. In this case, the bonding strengths increased to values between 10.9 MPa and 13.2 MPa for a bonding time of 30 min at temperatures ranging from 175 to 250 °C, and in shear tested TE modules, fracture occured in the interior of the TE elements. After high-temperature storage at 400 °C for 200 h, the shear strengths of TE/Cu joints bonded at various temperatures for 30 min decreased to values of 10.5–10.8 MPa, and the fracture occurred in the Ni3Sn4 intermetallics at the (Pb, Sn)Te/Ni interfaces. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction For the manufacturing of thermoelectric (TE) modules, the TE elements are bonded with metallic electrodes. For this purpose, it has been common to use traditional soldering, in which a low melting point solder alloy is melted at the TE/electrode interface and solidifies after the soldering process. Because the bonding temperature of solder alloy is quite low, thermal cracking can be prevented during the bonding process. Certain Bi2Te3 thermoelectric devices have been manufactured through soldering with Cu electrodes [1]. However, soldered TE modules cannot endure temperatures higher than the melting point of solder alloy. Furthermore, Ritzer et al. reported that excessive molten solder may wick up the sides of TE pellets and cause electrical shorts between TE couples [2]. Another conventional method for bonding a TE element and metallic electrode is brazing, which uses a filler metal with a melting point higher than 400 °C. The brazed TE module can thus be applied at much higher temperatures than one manufactured with the soldering method [3]. However, this high ⇑ Corresponding author. E-mail address: [email protected] (T.-H. Chuang). http://dx.doi.org/10.1016/j.jallcom.2014.06.020 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
bonding temperature can result in serious interfacial cracking and the failure of the manufactured TE module during the solidification of the filler metal. In addition, the liquid filler metal can strongly diffuse into the thermoelectric element, leading to the degradation of its TE efficiency. For certain thermoelectric materials such as PbTe, GeTe, CoSb3, Zn4Sb3, silicides, Skutterudites, Si4Ge, and LaTe, the optimized temperatures for application are higher than 400 °C. In this case, soldering is inadequate for the manufacturing of TE modules. On the other hand, brazing has the risks of thermal failure and atomic diffusion. In order to solve this problem for the manufacturing of thermoelectric modules, various methods have been developed for the bonding of medium or high temperature TE elements with metallic electrodes. Weinstein et al. used spark plasma sintering (SPS) to join a medium temperature PbTe element with an Fe electrode [4]. However, the direct bonding of Fe to PbTe is unsatisfactory because there is only diffusion and no intermetallics reaction at the PbTe/Fe interface. Xia et al. further bonded a PbTe material with Ni foil using a rapid hot-press at 600 and 650 °C under a pressure of 40 MPa for various times ranging from 60 to 300 min and investigated the interfacial reactions between PbTe and Ni [5]. For the manufacturing of high temperature CoSb3 modules, spark
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
47
plasma sintering (SPS) was employed by Fan et al. to join a CoSb3 element with Mo electrode by inserting a Ti interlayer [6]. A TiSb intermetallic compound and a composition–gradient alloy layer formed at the Ti/CoSb3 and Ti/Mo interfaces of CoSb3/Ti/Mo joints, respectively, which resulted in a high bonding strength of 58 MPa. Zhao et al. further manufactured a CoSb3/Mo–Cu thermoelectric element using the spark plasma sintering method and its reliability was evaluated through high temperature storage tests [7]. In order to minimize the CTE mismatch between the electrode and CoSb3 material, W–Cu alloy electrode was utilized to fabricate a CoSb3/ W–Cu thermoelectric element [8]. Although spark plasma sintering (SPS) can be used for the fabrication of high temperature CoSb3 modules, the bonding temperatures are higher than 600 °C under pressure of 40 MPa, which will degrade the thermoelectric properties of most medium temperature TE materials such as PbTe, GeTe and Zn4Sb3, and destroy the TE modules due to the thermal stress. An alternate technique is therefore needed for joining the TE elements with metallic electrodes at low temperatures such that the fabricated TE modules can be operated at temperatures higher than that used during the bonding process. Solid–liquid interdiffusion bonding (SLID) is a novel joining technique based on the principle of isothermal solidification and interfacial intermetallics reaction. In general, this technique, as shown in Fig. 1, makes use of a low-melting metallic thin film interlayer (LT) inserted between the high-melting bulk work pieces or metallic layers on certain substrates (HT1 and HT2) that are to be bonded. The LT interlayer, which is molten at low temperatures, reacts rapidly with the HT1 and HT2 bulk materials or layers. After a short time of solid–liquid interfacial reaction, the LT interlayer is exhausted and has completely transformed into intermetallic compounds. The melting point of the newly formed intermetallic phases is much higher than that of the original LT interlayer, so the resulting joints can withstand considerably higher temperatures when the assembled electronic products are operated [9]. Since solid liquid interdiffusion bonding has the merit of a low bonding temperature process and high temperature application, it has been applied in the past few decades to the manufacturing
of microwave packages, high power devices, thick-film resistors, GaAs/Si wafer packages, and even gold jewelry. Recently, diffusion soldering has also been employed for ceramic multichip modules [10,11], MEMS packaging [12], semiconductor packaging [13], hybrid joining [14], and hermetic package sealing [15]. It is known that the optimum operation temperature for a (Pb, Sn)Te material is about 400 °C, which is much higher than the melting point of conventional solder alloys, so a traditional soldering method is inadequate for the assembly of a (Pb, Sn)Te module. Hashimoto et al. reported a liquid phase diffusion bonding method for the manufacturing of a segmented Pb1 xSnxTe TE material with different alloy compositions [16]. They inserted a 50 lm thick sheet of Sn between both Pb1 xSnxTe TE compounds and bonded them at 427 °C for 30 min under 2 MPa pressure, after which they characterized the TE performance. However, for the bonding of a TE element, even the segmented Pb1 xSnxTe element of Hashimoto et al., with a metallic electrode for the production of a TE module, a barrier layer is required to prevent the reaction at the TE/electrode interface. In our previous studies, Bi0.5Sb1.5Te3 and GeTe thermoelectric materials were bonded with Cu electrode using the SLID process at temperatures ranging from 250 °C to 325 °C with the use of an inserted Sn interlayer [17,18]. The present study focuses on the bondability of (Pb, Sn)Te with a Cu electrode at temperatures between 175 °C and 250 °C, much lower than those used previously, using the SLID process and an In interlayer. For this purpose, the interfacial reactions and bonding strengths under various bonding conditions were investigated. In addition, the degradation of the TE/electrode joints after aging at 400 °C was also estimated for the evaluation of long-term applicability.
Fig. 1. Schematic presentation of the principle of solid liquid interdiffusion bonding.
Fig. 2. The original SLID bonding process for manufacturing (Pb, Sn)Te thermoelectric modules with In interlayers in this study.
2. Experimental P-type (Pb, Sn)Te thermoelectric material with a composition (at.%) of Pb:Sn:Te = 27.2:23.1:49.7 and a porosity of 1.80% was produced with hot-pressed powder metallurgy. The density of this TE material was 7.11 g/cm3, near the theoretical value of 7.24 g/cm3. The TE elements were joined with a Cu electrode using the solid–liquid interdiffusion (SLID) bonding process shown in Fig. 2. For
48
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
the preparation of bonding specimens, the TE material was ground with 4000 Grit SiC paper and then electroplated with a 4 lm thick Ni diffusion barrier layer, and a 10 lm thick layer of Ag acted as the reaction layer for the SLID process. The Cu electrode was electroplated with a 4 lm thick Ag layer and an In thin film with a thickness of 4 lm. The Ni/Ag coated thermoelectric element was assembled with an In/Ag-coated Cu electrode in a vacuum furnace of 5.3 10 4 Pa and subsequently heated at various temperatures between 175 °C and 250 °C for 30 min under a pressure of 3 MPa. Furthermore, to improve the bonding strength of the TE module, a modified SLID bonding method, shown in Fig. 3, was used; in this method, a 1 lm thickness of Sn film was pre-coated on the TE element and pre-heated at 250 °C for 3 min. After the solid–liquid interdiffusion (SLID) bonding processes, the specimens were cross-sectioned, ground with 4,000 Grit SiC paper, and polished with 1 and 0.3 lm Al2O3 powders. The microstructures of the intermetallic compounds that formed at the interfaces were observed via scanning electron microscopy (SEM), and their chemical compositions were analyzed with energy dispersive X-ray spectroscopy (EDX). The shear strengths of various TE/electrode joints were tested with a DAGE 4000 Bond Tester at a speed of 0.3 mm/s. The fractography of the fractured surfaces after bonding strength tests was observed by SEM.
3. Results and discussion Fig. 4 shows the typical microstructure of a (Pb, Sn)Te assembly bonded with a Ag-coated Cu electrode through inserting a Ag reaction layer and a thin film In interlayer using the SLID bonding process. It can be seen that a Ni3Sn4 intermetallics layer with a thickness of about 0.7 lm formed at the interface between the thermoelectric material and the Ni barrier layer. The In thin film inserted between the Ag reaction layer and Ag-coated electrode was completely exhausted during the formation of Ag3In intermetallic compounds. Clear islands of Cu11In7 phase can be observed at the Ag3In/Cu interfaces, indicating that the Ag coating on the Cu electrode was out-reacted. The figure also provides evidence that sound joints without voids can be obtained at the interfaces between TE/Ni/Ag and In/Ag/Cu, in contrast to the report of Bader et al. that voids frequently occurred at the interface of SLID bonded Ni/Sn/Ni and Cu/Sn/Cu specimens. This difference may be explained by the external pressure of 3 MPa employed in this work, which is higher than that used in the work of Bader et al. [19] In addition, it is postulated that fewer voids occur inherently during the intermetallic reaction at the Ag/In/Ag interface than during reactions at Ni/Sn/Ni and Cu/Sn/Cu interfaces. However, Fig. 5 shows that the shear strengths of the SLID bonded TE/electrode joints range from 6.3 to 7.8 Mpa in spite of the satisfactory joint interfaces. The fractography images in Fig. 6 indicate that in
Fig. 4. Typical microstructure of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the original SLID process. (Bonding conditions: 200 °C for 30 min.)
bonding strength tests, the specimens fractured along the interface between the TE element and the Ni barrier layer. This result is attributed to the weak interaction between the (Pb, Sn)Te material and the Ni barrier layer, which led to poor cohesion at their interface. Although Xia et al. reported that both NiTe2 and Ni5Pb2Te3 intermetallic compounds appeared at the PbTe/Ni interface, their bonding temperatures were higher than 600 °C with a bonding pressure of 40 MPa for bonding times ranging from 60 to 300 min [5]. Such bonding conditions can destroy the PbTe elements and is not suitable for the fabrication of a TE module. In order to improve the bonding effect, a modified SLID bonding process was further employed for the manufacturing of (Pb, Sn)Te modules. Fig. 3 shows that the thermoelectric material was precoated with a 1 lm thickness of Sn film and subsequently heated at 250 °C for 3 min under 5.3 10 4 Pa vacuum. It was expected that the Sn film on the surface of the (Pb, Sn)Te would react with Ni layer to promote their interaction and improve the bonding strength of the TE module. The surface morphology of the (Pb, Sn)Te material pre-coated with Sn film is shown in Fig. 7a; this surface transformed into a mixed phase (shown in Fig. 7b) consisting of many dark particles with a composition (at.%) of Pb:Sn: Te = 7.8:57.9:34.3 embedded in the matrix of Pb:Sn:Te = 8.6: 46.3:45.1 after heating at 250 °C for 3 min. It is obvious that the
Fig. 3. Modified SLID bonding process for the manufacturing of (Pb, Sn)Te thermoelectric modules with In interlayers in this study.
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
Fig. 5. Bonding strengths of the (Pb, Sn)Te/Cu joints with In interlayers bonded at various temperatures for 30 min using the original SLID process.
Te in the (Pb, Sn)Te material migrated predominantly outward and reacted with the pre-coated Sn film to form the Sn–Te rich phases. The electrical resistivity of this (Pb, Sn)Te thermoelectric element after Sn pre-coating and pre-heating was measured to be 1.7 10 4 X cm, close to that of the original (Pb, Sn)Te material (1.9 10 4 X cm). In addition, Orihashi et al. [20] reported that the figure of merit (Z) of (Pb, Sn)Te thermoelectric materials rose and shifted to the high temperature side as the Sn content increased to 50%. It is expected that the pre-treated (Pb, Sn)Te elements using this modified SLID bonding process should not degrade the thermoelectric properties of a (Pb, Sn)Te module. Following this pre-treatment on the surface, the (Pb, Sn)Te thermoelectric material was coated with an additional 4 lm thick Ni
49
barrier layer and a 10 lm thick Ag reaction layer. They were bonded with a Ag-coated Cu electrode using a Ag reaction layer and an In inserted thin film (4 lm thick) heated in a vacuum furnace of 5.3 10 4 Pa at temperatures ranging from 175 °C to 250 °C for various times of 5–120 min under a pressure of 3 MPa. Micrographs of interfacial reactions during the modified SLID process at 175 °C for various times are shown in Fig. 8. It can be observed that a very thin layer of Ni3Sn4 intermetallic compounds (IMCs) formed at the interfaces between the pre-treated (Pb, Sn)Te material and the Ni barrier layer. These Ni3Sn4 intermetallics grew slightly with increased bonding times. Underneath the Ni barrier layers, the pre-coated Ag reaction layers remained after the modified SLID bonding process, while the thin film In interlayer was exhausted and reacted initially into a double layer of Ag3In and Ag2In intermetallic compounds between the Ag reaction layer and Cu electrode. The Ag3In intermetallics grew with the diminution of the Ag2In layer as the bonding times increased. This result is attributed to the interfacial reaction of Ag2In intermetallics and the Ag layer with the increase in bonding time. Fig. 9 shows that the growth of Ni3Sn4 and Ag3In intermetallic compounds became more rapid with increases in bonding temperature. Fig. 10 shows the micrographs of the interfacial reactions during the modified SLID bonding at a high temperature of 250 °C for various times. Those images indicate that the Ag2In intermetallics at the Ag/Cu interface disappeared and that Cu11In7 islands at the Ag3In/Cu interface formed in the initial stage. Since the melting points of Ag3In and Ag2In intermetallic compounds are about 690 °C and 320 °C, respectively, then for the manufacturing of (Pb, Sn)Te modules, which will be applied at an optimized temperature of 400 °C, sufficient bonding temperature and bonding time are required to ensure the formation of a single layer of Ag3In intermetallics, rather than one of Ag2In phase or an Ag3In/Ag2In double layer. The thicknesses (X) of the Ni3Sn4 and Ag3In intermetallics layers formed under various bonding conditions are plotted
Fig. 6. Fractography of the shear tested (Pb, Sn)Te/Cu joints with In interlayers bonded with the original SLID process: (a) TE side, (b) Electrode side.
Fig. 7. Surface of the (Pb, Sn)Te thermoelectric material; (a) after pre-coating with 1 lm Sn film and (b) after subsequent heating at 250 °C for 3 min.
50
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
Fig. 8. Morphology of the intermetallic compounds formed at the interfaces of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process at 175 °C for various times: (a) 5 min, (b) 30 min, (c) 60 min and (d) 90 min.
Fig. 9. Morphology of the intermetallic compounds formed at the interfaces of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process at various temperatures for 10 min: (a) 175 °C, (b) 200 °C, (c) 225 °C and (d) 250 °C.
in Figs. 11a and 12a, respectively, as a function of bonding time (t). For the kinetics analyses, log X versus log t is plotted in Figs. 11b and 12b for the growth of Ni3Sn4 and Ag3In intermetallics, respectively. The growth exponents (n) for Ni3Sn4 intermetallics calculated from the slopes of straight lines in Fig. 11b ranged from 0.16 to 0.18. Similarly, the n values for Ag3In growth calculated from the plots in Fig. 12b ranged from 0.08 to 0.17. It is evidenced that the growth exponents (n) for both Ni3Sn4 and Ag3In
intermetallics deviated from that of a diffusion-controlled reaction (n = 0.5). This deviation can be attributed to the exhaustion of the pre-coated Sn film and the out-reaction of the In interlayer in the early stage of the SLID process. The bonding strengths of the finished (Pb, Sn)Te thermoelectric modules after the modified SLID process at various temperatures for 5–120 min were measured through shear tests and are presented in Fig. 13. The values increased with the bonding times
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
51
Fig. 10. Morphology of the intermetallic compounds formed at the interfaces of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process at 250 °C for various times: (a) 5 min, (b) 30 min, (c) 60 min and (d) 90 min.
Fig. 11. Thicknesses (X) of Ni3Sn4 intermetallics layers as a function of bonding times (t) at the (Pb, Sn)Te/Ni interfaces of the (Pb, Sn)Te/Cu joints with In interlayers after modified SLID bonding at various temperatures with In interlayer: (a) X versus t, (b) log X versus log t.
Fig. 12. Thicknesses (X) of Ag3In intermetallics layers as a function of bonding times (t) at the interfaces between (Pb, Sn)Te/Ni/Ag and Ag-coated Cu electrode after modified SLID bonding at various temperatures with In interlayers: (a) X versus t, (b) log X versus log t.
52
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
Fig. 13. Bonding strengths of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process.
Fig. 16. Degradation of the bonding strengths of the (Pb, Sn)Te/Cu joints with In interlayers bonded at various temperatures for 30 min using the modified SLID process after high temperature storage at 400 °C for 200 h.
Fig. 14. Typical fractography of shear tested (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process: (a) TE side, (b) electrode side.
Fig. 15. Morphology of the intermetallic compounds formed at the interfaces of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process at various temperatures for 30 min and high temperature storage at 400 °C for 200 h: (a) 175 °C, (b) 200 °C, (c) 225 °C and (d) 250 °C.
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
53
Fig. 17. Typical fractography of high temperature stored (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process after shear tests: (a) TE side, (b) electrode side.
to maximal values ranging from 10.9 to 13.2 MPa for 30 min of bonding at various temperatures. With the SLID bonding of Bi0.5Sb1.5Te3 material with Cu electrodes using Sn interlayers in our previous study, bonding strengths of 8.0–10.7 MPa were achieved [17], lower than those of the (Pb, Sn)Te/Cu joints with In interlayers. In contrast, the SLID bonding of GeTe elements with Cu electrodes using Sn interlayers led to bonding strengths of 13.9– 19.2 MPa [18], which are much higher than those in the present case of (Pb, Sn)Te/Cu bonding with In interlayers. It is believed that the different bonding strengths of the various TE/electrode joints can be mainly attributed to the mechanical strengths of TE materials. Further increasing the bonding times decreased the bonding strengths, which become stable values between 10.6 and 11.8 MPa at bonding times exceeding 90 min. The decline in bonding strength from its maximal value was correlated to the exhaustion of the pre-coated Sn film at the TE/Ni interface, as shown in Figs. 8–10. However, the formation of the Ni3Sn4 layer was sufficient to achieve the joining effect between the (Pb, Sn)Te thermoelectric material and Ni barrier layer, in spite of the disappearance of the pre-coated Sn film, and resulted in a satisfactory bonding strength. Consequently, this modified SLID process yielded an increase of about 40% in bonding strength over that achieved at the same bonding temperature using the original process. Fig. 14 indicates that the shear-tested TE modules fractured mainly through the interior of the (Pb, Sn)Te thermoelectric material. This improvement in bonding strength can be attributed to the beneficial effect of pre-coating of Sn on the TE surface to enhance the cohesion between the (Pb, Sn)Te material and the Ni barrier layer. TE/Cu joints bonded at various temperatures ranging from 175 to 250 °C for 30 min with maximal bonding strengths were further aged at 400 °C for 200 h. The micrographs in Fig. 15 show that the Ni3Sn4 and Cu11In7 intermetallic compounds grew further, while the Ag3In intermetallics layer remained almost constant. Shear tests for these high-temperature stored TE/Cu joints revealed that the bonding strengths of all specimens dropped to values of 10.5–10.8 MPa, as shown in Fig. 16. In Fig. 17, it can be seen that the fracture path occurred in the Ni3Sn4 intermetallics between the (Pb, Sn)Te material and Ni barrier layer. 4. Conclusions Since the optimized operation temperatures for (Pb, Sn)Te thermoelectric materials fall in the range of 300–500 °C, the traditional soldering method is inadequate for bonding TE elements with metallic electrodes. On the other hand, brazing requires a bonding temperature higher than 450 °C, which can cause thermal stress and increase the risk of failure of TE elements. An alternative method, solid–liquid interdiffusion bonding (SLID), has been
employed through the insertion of a Ag reaction layer and a low melting point In thin film between the (Pb, Sn)Te element and Ag-coated Cu electrode. During the SLID bonding process from 175 to 250 °C, the In thin film reacted rapidly with the Ag layers and was completely exhausted in the formation of a Ag3In intermetallics layer, which had a melting point of 690 °C, which is much higher than the operation temperature of this (Pb, Sn)Te module. The specimens had shear strengths of about 6.3–7.8 MPa for a bonding time of 30 min at various temperatures and fractured along the interface between the TE element and the Ni barrier layer. The adhesion of the TE/Ni interface was improved by the pre-coated 1 lm layer of Sn film on the (Pb, Sn)Te, which led to sound joints in the manufactured TE modules. The modified SLID process increased the bonding strengths to maximal values of 10.9–13.2 MPa for 30 min at temperatures between 175 and 250 °C through the enhancement of the cohesion between the TE material and Ni barrier layer. The fracture occurred in the interior of the TE elements. The bonding strengths declined to constant values between 10.6 and 11.8 MPa at bonding times of over 90 min. According to the study of Orihashi et al. [20], the precoating with Sn thin film and pre-heating using this modified SLID bonding process should not degrade the thermoelectric properties of a (Pb, Sn)Te module. After high-temperature storage tests at 400 °C for 200 h of TE/Cu joints bonded with the modified SLID process at various temperatures for 30 min, the shear strengths dropped to a range of 10.5–10.8 MPa. In this case, the fracture occurred in the Ni3Sn4 intermetallics at the TE/Ni interfaces. Acknowledgements The authors are grateful for the financial support of this study by the National Science Council of Taiwan, under Grant No. 1022221-E-002-057-MY3 and by the Industrial Technology Research Institute, under Grant No. 102-S-A25. References [1] H. Zhang, H.Y. Jing, Y.D. Han, L.Y. Xu, J. Alloys Comp. 576 (2013) 424–431. [2] T.M. Ritzer, P.G. Lau, A.D. Bogard, in: Proc. 16th Int. Conf. Thermoelectrics, Dresden, Germany, 1997, pp. 619–623. [3] R. Zybala, K.T. Wojciechowski, M. Schmidt, Mater. Ceramiczne/Ceram. Mater. 62 (2010) 481–485. [4] M. Weinstein, A.I. Mlavsky, Rev. Sci. Instrum. 33 (1962) 1119–1120. [5] H. Xia, F. Drymiotis, C.L. Chen, A. Wu, G.J. Snyder, J. Mater. Sci. 49 (2014) 1716– 1723. [6] J. Fan, L. Chen, S. Bai, X. Shi, Mater. Lett. 58 (2004) 3876–3878. [7] D.G. Zhao, X.Y. Li, W. Jiang, L.D. Chen, J. Alloys Comp. 477 (2009) 425–431. [8] D.G. Zhao, H. Geng, X. Teng, J. Alloys Comp. 517 (2012) 198–203. [9] D.M. Jacobson, G. Humpston, Solder. Surf. Mt. Technol. 10 (1992) 27–32. [10] T.H. Chuang, H.J. Lin, C.W. Tsao, J. Electron. Mater. 35 (2006) 1566–1570. [11] M.W. Liang, T.E. Hsieh, S.Y. Chang, T.H. Chuang, J. Electron. Mater. 32 (2003) 952–956. [12] W.C. Welch, J. Chae, K. Najafi, IEEE Trans. Adv. Packaging 28 (2005) 643–649.
54
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
[13] R.I. Made, C.L. Gan, L.L. Yan, A. Yu, S.W. Yoon, J.H. Lau, C.K. Lee, J. Electron. Mater. 38 (2009) 365–371. [14] J.F. Li, P.A. Agyakwa, C.W. Johnson, Acta Mater. 58 (2010) 3429–3443. [15] L.L. Yan, C.K. Lee, D.Q. Yu, A.B. Yu, W.K. Choi, J.H. Lau, S.U. Yoon, J. Electron. Mater. 38 (2009) 200–207. [16] M. Hashimoto, I. Shiota, O. Ohashi, Y. Isoda, Y. Imai, Y. Shinohara, I.A. Nishida, in: Proc. 17th Int. Conf. on Thermoelectrics, Nagoya, Japan, 1998, pp. 346–349.
[17] C.L. Yang, H.J. Lai, J.D. Hwang, T.H. Chuang, J. Mater. Eng. Perform. 22 (2013) 2029–2037. [18] C.L. Yang, H.J. Lai, J.D. Hwang, T.H. Chuang, J. Electron. Mater. 42 (2013) 359– 365. [19] S. Bader, W. Gust, H. Hieber, Acta Metall. Mater. 43 (1995) 329–337. [20] M. Orihashi, Y. Noda, L.-D. Chen, T. Goto, T. Hirai, J. Phys. Chem. Solids 61 (2000) 919–923.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Improvement of bonding strength of a (Pb, Sn)Te–Cu contact manufactured in a low temperature SLID-bonding process Tung-Han Chuang a,⇑, Wei-Ting Yeh a, Chien-Hsun Chuang a, Jenn-Dong Hwang b a b
Institute of Materials Science and Engineering, National Taiwan University, 106 Taipei, Taiwan Material & Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31015, Taiwan
a r t i c l e
i n f o
Article history: Received 25 December 2013 Received in revised form 3 June 2014 Accepted 3 June 2014 Available online 12 June 2014 Keywords: Solid–liquid interdiffusion bonding (Pb, Sn)Te Intermetallic compounds Bonding strength
a b s t r a c t A (Pb, Sn)Te thermoelectric (TE) material coated with a Ni barrier layer and a Ag reaction layer was bonded with a Ag-coated Cu electrode at low temperatures in the range of 175–250 °C with an In thin film. The In film reacted initially with the Ag layer to form a double layer of Ag3In and Ag2In intermetallic compounds, which then completely reacted with the Ag layer to form a Ag3In intermetallics layer having a melting point of 690 °C. The resulting module can thus be applied at the optimized operation temperature of (Pb, Sn)Te material (400 °C). The bonding strengths ranged from 6.3 to 7.8 MPa for various bonding temperatures at 30 min, and shear tests revealed that the specimens fractured along the interface between the (Pb, Sn)Te thermoelectric element and the Ni barrier layer. The adhesion of the TE/Ni interface was improved through pre-electroplating a 1 lm Sn film on the surface of the thermoelectric element and heating at 250 °C for 3 min. In this case, the bonding strengths increased to values between 10.9 MPa and 13.2 MPa for a bonding time of 30 min at temperatures ranging from 175 to 250 °C, and in shear tested TE modules, fracture occured in the interior of the TE elements. After high-temperature storage at 400 °C for 200 h, the shear strengths of TE/Cu joints bonded at various temperatures for 30 min decreased to values of 10.5–10.8 MPa, and the fracture occurred in the Ni3Sn4 intermetallics at the (Pb, Sn)Te/Ni interfaces. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction For the manufacturing of thermoelectric (TE) modules, the TE elements are bonded with metallic electrodes. For this purpose, it has been common to use traditional soldering, in which a low melting point solder alloy is melted at the TE/electrode interface and solidifies after the soldering process. Because the bonding temperature of solder alloy is quite low, thermal cracking can be prevented during the bonding process. Certain Bi2Te3 thermoelectric devices have been manufactured through soldering with Cu electrodes [1]. However, soldered TE modules cannot endure temperatures higher than the melting point of solder alloy. Furthermore, Ritzer et al. reported that excessive molten solder may wick up the sides of TE pellets and cause electrical shorts between TE couples [2]. Another conventional method for bonding a TE element and metallic electrode is brazing, which uses a filler metal with a melting point higher than 400 °C. The brazed TE module can thus be applied at much higher temperatures than one manufactured with the soldering method [3]. However, this high ⇑ Corresponding author. E-mail address: [email protected] (T.-H. Chuang). http://dx.doi.org/10.1016/j.jallcom.2014.06.020 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
bonding temperature can result in serious interfacial cracking and the failure of the manufactured TE module during the solidification of the filler metal. In addition, the liquid filler metal can strongly diffuse into the thermoelectric element, leading to the degradation of its TE efficiency. For certain thermoelectric materials such as PbTe, GeTe, CoSb3, Zn4Sb3, silicides, Skutterudites, Si4Ge, and LaTe, the optimized temperatures for application are higher than 400 °C. In this case, soldering is inadequate for the manufacturing of TE modules. On the other hand, brazing has the risks of thermal failure and atomic diffusion. In order to solve this problem for the manufacturing of thermoelectric modules, various methods have been developed for the bonding of medium or high temperature TE elements with metallic electrodes. Weinstein et al. used spark plasma sintering (SPS) to join a medium temperature PbTe element with an Fe electrode [4]. However, the direct bonding of Fe to PbTe is unsatisfactory because there is only diffusion and no intermetallics reaction at the PbTe/Fe interface. Xia et al. further bonded a PbTe material with Ni foil using a rapid hot-press at 600 and 650 °C under a pressure of 40 MPa for various times ranging from 60 to 300 min and investigated the interfacial reactions between PbTe and Ni [5]. For the manufacturing of high temperature CoSb3 modules, spark
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
47
plasma sintering (SPS) was employed by Fan et al. to join a CoSb3 element with Mo electrode by inserting a Ti interlayer [6]. A TiSb intermetallic compound and a composition–gradient alloy layer formed at the Ti/CoSb3 and Ti/Mo interfaces of CoSb3/Ti/Mo joints, respectively, which resulted in a high bonding strength of 58 MPa. Zhao et al. further manufactured a CoSb3/Mo–Cu thermoelectric element using the spark plasma sintering method and its reliability was evaluated through high temperature storage tests [7]. In order to minimize the CTE mismatch between the electrode and CoSb3 material, W–Cu alloy electrode was utilized to fabricate a CoSb3/ W–Cu thermoelectric element [8]. Although spark plasma sintering (SPS) can be used for the fabrication of high temperature CoSb3 modules, the bonding temperatures are higher than 600 °C under pressure of 40 MPa, which will degrade the thermoelectric properties of most medium temperature TE materials such as PbTe, GeTe and Zn4Sb3, and destroy the TE modules due to the thermal stress. An alternate technique is therefore needed for joining the TE elements with metallic electrodes at low temperatures such that the fabricated TE modules can be operated at temperatures higher than that used during the bonding process. Solid–liquid interdiffusion bonding (SLID) is a novel joining technique based on the principle of isothermal solidification and interfacial intermetallics reaction. In general, this technique, as shown in Fig. 1, makes use of a low-melting metallic thin film interlayer (LT) inserted between the high-melting bulk work pieces or metallic layers on certain substrates (HT1 and HT2) that are to be bonded. The LT interlayer, which is molten at low temperatures, reacts rapidly with the HT1 and HT2 bulk materials or layers. After a short time of solid–liquid interfacial reaction, the LT interlayer is exhausted and has completely transformed into intermetallic compounds. The melting point of the newly formed intermetallic phases is much higher than that of the original LT interlayer, so the resulting joints can withstand considerably higher temperatures when the assembled electronic products are operated [9]. Since solid liquid interdiffusion bonding has the merit of a low bonding temperature process and high temperature application, it has been applied in the past few decades to the manufacturing
of microwave packages, high power devices, thick-film resistors, GaAs/Si wafer packages, and even gold jewelry. Recently, diffusion soldering has also been employed for ceramic multichip modules [10,11], MEMS packaging [12], semiconductor packaging [13], hybrid joining [14], and hermetic package sealing [15]. It is known that the optimum operation temperature for a (Pb, Sn)Te material is about 400 °C, which is much higher than the melting point of conventional solder alloys, so a traditional soldering method is inadequate for the assembly of a (Pb, Sn)Te module. Hashimoto et al. reported a liquid phase diffusion bonding method for the manufacturing of a segmented Pb1 xSnxTe TE material with different alloy compositions [16]. They inserted a 50 lm thick sheet of Sn between both Pb1 xSnxTe TE compounds and bonded them at 427 °C for 30 min under 2 MPa pressure, after which they characterized the TE performance. However, for the bonding of a TE element, even the segmented Pb1 xSnxTe element of Hashimoto et al., with a metallic electrode for the production of a TE module, a barrier layer is required to prevent the reaction at the TE/electrode interface. In our previous studies, Bi0.5Sb1.5Te3 and GeTe thermoelectric materials were bonded with Cu electrode using the SLID process at temperatures ranging from 250 °C to 325 °C with the use of an inserted Sn interlayer [17,18]. The present study focuses on the bondability of (Pb, Sn)Te with a Cu electrode at temperatures between 175 °C and 250 °C, much lower than those used previously, using the SLID process and an In interlayer. For this purpose, the interfacial reactions and bonding strengths under various bonding conditions were investigated. In addition, the degradation of the TE/electrode joints after aging at 400 °C was also estimated for the evaluation of long-term applicability.
Fig. 1. Schematic presentation of the principle of solid liquid interdiffusion bonding.
Fig. 2. The original SLID bonding process for manufacturing (Pb, Sn)Te thermoelectric modules with In interlayers in this study.
2. Experimental P-type (Pb, Sn)Te thermoelectric material with a composition (at.%) of Pb:Sn:Te = 27.2:23.1:49.7 and a porosity of 1.80% was produced with hot-pressed powder metallurgy. The density of this TE material was 7.11 g/cm3, near the theoretical value of 7.24 g/cm3. The TE elements were joined with a Cu electrode using the solid–liquid interdiffusion (SLID) bonding process shown in Fig. 2. For
48
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
the preparation of bonding specimens, the TE material was ground with 4000 Grit SiC paper and then electroplated with a 4 lm thick Ni diffusion barrier layer, and a 10 lm thick layer of Ag acted as the reaction layer for the SLID process. The Cu electrode was electroplated with a 4 lm thick Ag layer and an In thin film with a thickness of 4 lm. The Ni/Ag coated thermoelectric element was assembled with an In/Ag-coated Cu electrode in a vacuum furnace of 5.3 10 4 Pa and subsequently heated at various temperatures between 175 °C and 250 °C for 30 min under a pressure of 3 MPa. Furthermore, to improve the bonding strength of the TE module, a modified SLID bonding method, shown in Fig. 3, was used; in this method, a 1 lm thickness of Sn film was pre-coated on the TE element and pre-heated at 250 °C for 3 min. After the solid–liquid interdiffusion (SLID) bonding processes, the specimens were cross-sectioned, ground with 4,000 Grit SiC paper, and polished with 1 and 0.3 lm Al2O3 powders. The microstructures of the intermetallic compounds that formed at the interfaces were observed via scanning electron microscopy (SEM), and their chemical compositions were analyzed with energy dispersive X-ray spectroscopy (EDX). The shear strengths of various TE/electrode joints were tested with a DAGE 4000 Bond Tester at a speed of 0.3 mm/s. The fractography of the fractured surfaces after bonding strength tests was observed by SEM.
3. Results and discussion Fig. 4 shows the typical microstructure of a (Pb, Sn)Te assembly bonded with a Ag-coated Cu electrode through inserting a Ag reaction layer and a thin film In interlayer using the SLID bonding process. It can be seen that a Ni3Sn4 intermetallics layer with a thickness of about 0.7 lm formed at the interface between the thermoelectric material and the Ni barrier layer. The In thin film inserted between the Ag reaction layer and Ag-coated electrode was completely exhausted during the formation of Ag3In intermetallic compounds. Clear islands of Cu11In7 phase can be observed at the Ag3In/Cu interfaces, indicating that the Ag coating on the Cu electrode was out-reacted. The figure also provides evidence that sound joints without voids can be obtained at the interfaces between TE/Ni/Ag and In/Ag/Cu, in contrast to the report of Bader et al. that voids frequently occurred at the interface of SLID bonded Ni/Sn/Ni and Cu/Sn/Cu specimens. This difference may be explained by the external pressure of 3 MPa employed in this work, which is higher than that used in the work of Bader et al. [19] In addition, it is postulated that fewer voids occur inherently during the intermetallic reaction at the Ag/In/Ag interface than during reactions at Ni/Sn/Ni and Cu/Sn/Cu interfaces. However, Fig. 5 shows that the shear strengths of the SLID bonded TE/electrode joints range from 6.3 to 7.8 Mpa in spite of the satisfactory joint interfaces. The fractography images in Fig. 6 indicate that in
Fig. 4. Typical microstructure of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the original SLID process. (Bonding conditions: 200 °C for 30 min.)
bonding strength tests, the specimens fractured along the interface between the TE element and the Ni barrier layer. This result is attributed to the weak interaction between the (Pb, Sn)Te material and the Ni barrier layer, which led to poor cohesion at their interface. Although Xia et al. reported that both NiTe2 and Ni5Pb2Te3 intermetallic compounds appeared at the PbTe/Ni interface, their bonding temperatures were higher than 600 °C with a bonding pressure of 40 MPa for bonding times ranging from 60 to 300 min [5]. Such bonding conditions can destroy the PbTe elements and is not suitable for the fabrication of a TE module. In order to improve the bonding effect, a modified SLID bonding process was further employed for the manufacturing of (Pb, Sn)Te modules. Fig. 3 shows that the thermoelectric material was precoated with a 1 lm thickness of Sn film and subsequently heated at 250 °C for 3 min under 5.3 10 4 Pa vacuum. It was expected that the Sn film on the surface of the (Pb, Sn)Te would react with Ni layer to promote their interaction and improve the bonding strength of the TE module. The surface morphology of the (Pb, Sn)Te material pre-coated with Sn film is shown in Fig. 7a; this surface transformed into a mixed phase (shown in Fig. 7b) consisting of many dark particles with a composition (at.%) of Pb:Sn: Te = 7.8:57.9:34.3 embedded in the matrix of Pb:Sn:Te = 8.6: 46.3:45.1 after heating at 250 °C for 3 min. It is obvious that the
Fig. 3. Modified SLID bonding process for the manufacturing of (Pb, Sn)Te thermoelectric modules with In interlayers in this study.
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
Fig. 5. Bonding strengths of the (Pb, Sn)Te/Cu joints with In interlayers bonded at various temperatures for 30 min using the original SLID process.
Te in the (Pb, Sn)Te material migrated predominantly outward and reacted with the pre-coated Sn film to form the Sn–Te rich phases. The electrical resistivity of this (Pb, Sn)Te thermoelectric element after Sn pre-coating and pre-heating was measured to be 1.7 10 4 X cm, close to that of the original (Pb, Sn)Te material (1.9 10 4 X cm). In addition, Orihashi et al. [20] reported that the figure of merit (Z) of (Pb, Sn)Te thermoelectric materials rose and shifted to the high temperature side as the Sn content increased to 50%. It is expected that the pre-treated (Pb, Sn)Te elements using this modified SLID bonding process should not degrade the thermoelectric properties of a (Pb, Sn)Te module. Following this pre-treatment on the surface, the (Pb, Sn)Te thermoelectric material was coated with an additional 4 lm thick Ni
49
barrier layer and a 10 lm thick Ag reaction layer. They were bonded with a Ag-coated Cu electrode using a Ag reaction layer and an In inserted thin film (4 lm thick) heated in a vacuum furnace of 5.3 10 4 Pa at temperatures ranging from 175 °C to 250 °C for various times of 5–120 min under a pressure of 3 MPa. Micrographs of interfacial reactions during the modified SLID process at 175 °C for various times are shown in Fig. 8. It can be observed that a very thin layer of Ni3Sn4 intermetallic compounds (IMCs) formed at the interfaces between the pre-treated (Pb, Sn)Te material and the Ni barrier layer. These Ni3Sn4 intermetallics grew slightly with increased bonding times. Underneath the Ni barrier layers, the pre-coated Ag reaction layers remained after the modified SLID bonding process, while the thin film In interlayer was exhausted and reacted initially into a double layer of Ag3In and Ag2In intermetallic compounds between the Ag reaction layer and Cu electrode. The Ag3In intermetallics grew with the diminution of the Ag2In layer as the bonding times increased. This result is attributed to the interfacial reaction of Ag2In intermetallics and the Ag layer with the increase in bonding time. Fig. 9 shows that the growth of Ni3Sn4 and Ag3In intermetallic compounds became more rapid with increases in bonding temperature. Fig. 10 shows the micrographs of the interfacial reactions during the modified SLID bonding at a high temperature of 250 °C for various times. Those images indicate that the Ag2In intermetallics at the Ag/Cu interface disappeared and that Cu11In7 islands at the Ag3In/Cu interface formed in the initial stage. Since the melting points of Ag3In and Ag2In intermetallic compounds are about 690 °C and 320 °C, respectively, then for the manufacturing of (Pb, Sn)Te modules, which will be applied at an optimized temperature of 400 °C, sufficient bonding temperature and bonding time are required to ensure the formation of a single layer of Ag3In intermetallics, rather than one of Ag2In phase or an Ag3In/Ag2In double layer. The thicknesses (X) of the Ni3Sn4 and Ag3In intermetallics layers formed under various bonding conditions are plotted
Fig. 6. Fractography of the shear tested (Pb, Sn)Te/Cu joints with In interlayers bonded with the original SLID process: (a) TE side, (b) Electrode side.
Fig. 7. Surface of the (Pb, Sn)Te thermoelectric material; (a) after pre-coating with 1 lm Sn film and (b) after subsequent heating at 250 °C for 3 min.
50
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
Fig. 8. Morphology of the intermetallic compounds formed at the interfaces of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process at 175 °C for various times: (a) 5 min, (b) 30 min, (c) 60 min and (d) 90 min.
Fig. 9. Morphology of the intermetallic compounds formed at the interfaces of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process at various temperatures for 10 min: (a) 175 °C, (b) 200 °C, (c) 225 °C and (d) 250 °C.
in Figs. 11a and 12a, respectively, as a function of bonding time (t). For the kinetics analyses, log X versus log t is plotted in Figs. 11b and 12b for the growth of Ni3Sn4 and Ag3In intermetallics, respectively. The growth exponents (n) for Ni3Sn4 intermetallics calculated from the slopes of straight lines in Fig. 11b ranged from 0.16 to 0.18. Similarly, the n values for Ag3In growth calculated from the plots in Fig. 12b ranged from 0.08 to 0.17. It is evidenced that the growth exponents (n) for both Ni3Sn4 and Ag3In
intermetallics deviated from that of a diffusion-controlled reaction (n = 0.5). This deviation can be attributed to the exhaustion of the pre-coated Sn film and the out-reaction of the In interlayer in the early stage of the SLID process. The bonding strengths of the finished (Pb, Sn)Te thermoelectric modules after the modified SLID process at various temperatures for 5–120 min were measured through shear tests and are presented in Fig. 13. The values increased with the bonding times
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
51
Fig. 10. Morphology of the intermetallic compounds formed at the interfaces of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process at 250 °C for various times: (a) 5 min, (b) 30 min, (c) 60 min and (d) 90 min.
Fig. 11. Thicknesses (X) of Ni3Sn4 intermetallics layers as a function of bonding times (t) at the (Pb, Sn)Te/Ni interfaces of the (Pb, Sn)Te/Cu joints with In interlayers after modified SLID bonding at various temperatures with In interlayer: (a) X versus t, (b) log X versus log t.
Fig. 12. Thicknesses (X) of Ag3In intermetallics layers as a function of bonding times (t) at the interfaces between (Pb, Sn)Te/Ni/Ag and Ag-coated Cu electrode after modified SLID bonding at various temperatures with In interlayers: (a) X versus t, (b) log X versus log t.
52
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
Fig. 13. Bonding strengths of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process.
Fig. 16. Degradation of the bonding strengths of the (Pb, Sn)Te/Cu joints with In interlayers bonded at various temperatures for 30 min using the modified SLID process after high temperature storage at 400 °C for 200 h.
Fig. 14. Typical fractography of shear tested (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process: (a) TE side, (b) electrode side.
Fig. 15. Morphology of the intermetallic compounds formed at the interfaces of the (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process at various temperatures for 30 min and high temperature storage at 400 °C for 200 h: (a) 175 °C, (b) 200 °C, (c) 225 °C and (d) 250 °C.
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
53
Fig. 17. Typical fractography of high temperature stored (Pb, Sn)Te/Cu joints with In interlayers bonded with the modified SLID process after shear tests: (a) TE side, (b) electrode side.
to maximal values ranging from 10.9 to 13.2 MPa for 30 min of bonding at various temperatures. With the SLID bonding of Bi0.5Sb1.5Te3 material with Cu electrodes using Sn interlayers in our previous study, bonding strengths of 8.0–10.7 MPa were achieved [17], lower than those of the (Pb, Sn)Te/Cu joints with In interlayers. In contrast, the SLID bonding of GeTe elements with Cu electrodes using Sn interlayers led to bonding strengths of 13.9– 19.2 MPa [18], which are much higher than those in the present case of (Pb, Sn)Te/Cu bonding with In interlayers. It is believed that the different bonding strengths of the various TE/electrode joints can be mainly attributed to the mechanical strengths of TE materials. Further increasing the bonding times decreased the bonding strengths, which become stable values between 10.6 and 11.8 MPa at bonding times exceeding 90 min. The decline in bonding strength from its maximal value was correlated to the exhaustion of the pre-coated Sn film at the TE/Ni interface, as shown in Figs. 8–10. However, the formation of the Ni3Sn4 layer was sufficient to achieve the joining effect between the (Pb, Sn)Te thermoelectric material and Ni barrier layer, in spite of the disappearance of the pre-coated Sn film, and resulted in a satisfactory bonding strength. Consequently, this modified SLID process yielded an increase of about 40% in bonding strength over that achieved at the same bonding temperature using the original process. Fig. 14 indicates that the shear-tested TE modules fractured mainly through the interior of the (Pb, Sn)Te thermoelectric material. This improvement in bonding strength can be attributed to the beneficial effect of pre-coating of Sn on the TE surface to enhance the cohesion between the (Pb, Sn)Te material and the Ni barrier layer. TE/Cu joints bonded at various temperatures ranging from 175 to 250 °C for 30 min with maximal bonding strengths were further aged at 400 °C for 200 h. The micrographs in Fig. 15 show that the Ni3Sn4 and Cu11In7 intermetallic compounds grew further, while the Ag3In intermetallics layer remained almost constant. Shear tests for these high-temperature stored TE/Cu joints revealed that the bonding strengths of all specimens dropped to values of 10.5–10.8 MPa, as shown in Fig. 16. In Fig. 17, it can be seen that the fracture path occurred in the Ni3Sn4 intermetallics between the (Pb, Sn)Te material and Ni barrier layer. 4. Conclusions Since the optimized operation temperatures for (Pb, Sn)Te thermoelectric materials fall in the range of 300–500 °C, the traditional soldering method is inadequate for bonding TE elements with metallic electrodes. On the other hand, brazing requires a bonding temperature higher than 450 °C, which can cause thermal stress and increase the risk of failure of TE elements. An alternative method, solid–liquid interdiffusion bonding (SLID), has been
employed through the insertion of a Ag reaction layer and a low melting point In thin film between the (Pb, Sn)Te element and Ag-coated Cu electrode. During the SLID bonding process from 175 to 250 °C, the In thin film reacted rapidly with the Ag layers and was completely exhausted in the formation of a Ag3In intermetallics layer, which had a melting point of 690 °C, which is much higher than the operation temperature of this (Pb, Sn)Te module. The specimens had shear strengths of about 6.3–7.8 MPa for a bonding time of 30 min at various temperatures and fractured along the interface between the TE element and the Ni barrier layer. The adhesion of the TE/Ni interface was improved by the pre-coated 1 lm layer of Sn film on the (Pb, Sn)Te, which led to sound joints in the manufactured TE modules. The modified SLID process increased the bonding strengths to maximal values of 10.9–13.2 MPa for 30 min at temperatures between 175 and 250 °C through the enhancement of the cohesion between the TE material and Ni barrier layer. The fracture occurred in the interior of the TE elements. The bonding strengths declined to constant values between 10.6 and 11.8 MPa at bonding times of over 90 min. According to the study of Orihashi et al. [20], the precoating with Sn thin film and pre-heating using this modified SLID bonding process should not degrade the thermoelectric properties of a (Pb, Sn)Te module. After high-temperature storage tests at 400 °C for 200 h of TE/Cu joints bonded with the modified SLID process at various temperatures for 30 min, the shear strengths dropped to a range of 10.5–10.8 MPa. In this case, the fracture occurred in the Ni3Sn4 intermetallics at the TE/Ni interfaces. Acknowledgements The authors are grateful for the financial support of this study by the National Science Council of Taiwan, under Grant No. 1022221-E-002-057-MY3 and by the Industrial Technology Research Institute, under Grant No. 102-S-A25. References [1] H. Zhang, H.Y. Jing, Y.D. Han, L.Y. Xu, J. Alloys Comp. 576 (2013) 424–431. [2] T.M. Ritzer, P.G. Lau, A.D. Bogard, in: Proc. 16th Int. Conf. Thermoelectrics, Dresden, Germany, 1997, pp. 619–623. [3] R. Zybala, K.T. Wojciechowski, M. Schmidt, Mater. Ceramiczne/Ceram. Mater. 62 (2010) 481–485. [4] M. Weinstein, A.I. Mlavsky, Rev. Sci. Instrum. 33 (1962) 1119–1120. [5] H. Xia, F. Drymiotis, C.L. Chen, A. Wu, G.J. Snyder, J. Mater. Sci. 49 (2014) 1716– 1723. [6] J. Fan, L. Chen, S. Bai, X. Shi, Mater. Lett. 58 (2004) 3876–3878. [7] D.G. Zhao, X.Y. Li, W. Jiang, L.D. Chen, J. Alloys Comp. 477 (2009) 425–431. [8] D.G. Zhao, H. Geng, X. Teng, J. Alloys Comp. 517 (2012) 198–203. [9] D.M. Jacobson, G. Humpston, Solder. Surf. Mt. Technol. 10 (1992) 27–32. [10] T.H. Chuang, H.J. Lin, C.W. Tsao, J. Electron. Mater. 35 (2006) 1566–1570. [11] M.W. Liang, T.E. Hsieh, S.Y. Chang, T.H. Chuang, J. Electron. Mater. 32 (2003) 952–956. [12] W.C. Welch, J. Chae, K. Najafi, IEEE Trans. Adv. Packaging 28 (2005) 643–649.
54
T.-H. Chuang et al. / Journal of Alloys and Compounds 613 (2014) 46–54
[13] R.I. Made, C.L. Gan, L.L. Yan, A. Yu, S.W. Yoon, J.H. Lau, C.K. Lee, J. Electron. Mater. 38 (2009) 365–371. [14] J.F. Li, P.A. Agyakwa, C.W. Johnson, Acta Mater. 58 (2010) 3429–3443. [15] L.L. Yan, C.K. Lee, D.Q. Yu, A.B. Yu, W.K. Choi, J.H. Lau, S.U. Yoon, J. Electron. Mater. 38 (2009) 200–207. [16] M. Hashimoto, I. Shiota, O. Ohashi, Y. Isoda, Y. Imai, Y. Shinohara, I.A. Nishida, in: Proc. 17th Int. Conf. on Thermoelectrics, Nagoya, Japan, 1998, pp. 346–349.
[17] C.L. Yang, H.J. Lai, J.D. Hwang, T.H. Chuang, J. Mater. Eng. Perform. 22 (2013) 2029–2037. [18] C.L. Yang, H.J. Lai, J.D. Hwang, T.H. Chuang, J. Electron. Mater. 42 (2013) 359– 365. [19] S. Bader, W. Gust, H. Hieber, Acta Metall. Mater. 43 (1995) 329–337. [20] M. Orihashi, Y. Noda, L.-D. Chen, T. Goto, T. Hirai, J. Phys. Chem. Solids 61 (2000) 919–923.