Diffusion Soldering of a Pd/Ni Couple Using a Sn Thin Film Interlayer

Transcription

1 WELDING RESERCH Diffusion Soldering of a Pd/Ni Couple Using a Sn Thin Film Interlayer preliminary study on the low temperature bonding of a Pd/Ni joint for a hydrogen purifier is presented Y C.-H. CHUNG ND Y.-C. LIN STRCT The applicability of diffusion soldering to manufacturing a Pd cell for hydrogen purification was evaluated in this study. The results indicated that directly inserting a Sn interlayer between the Pd sheet and Ni plate caused the Sn interlayer to be exhausted and Ni 3 Sn 4 and PdSn intermetallic compounds to form after diffusion soldering at 300º to 350ºC for 30 min. In this case, a long crack appeared at the Ni 3 Sn 4 /PdSn interface and the bonding strength was low. dding an g thin film between the Sn coated Pd sheet and Ni plate caused an g 3 Sn intermetallic compound to form between the Ni 3 Sn 4 and PdSn layers and prevented cracking of the Pd/Ni joint. This improvement led to satisfactory bonding strengths of 10.6 to 17.3 MPa. High temperature storage at 400ºC for 100 h did not degrade the bonding interfaces or bonding strengths. n alternative method wherein a multilayer of Ni/Sn/g was coated on the Pd sheet and then diffusion soldered with the Sn coated Ni plate further improved the bonding effect of the Pd/Ni couple. However, the Ni coating may be delaminated from the Pd sheet in this case. Preheating the Nicoated Pd sheet at 450ºC for 30 min to form a Pd Ni solid solution layer effectively enhanced the bonding strengths to values of 18.9 to 24.1 MPa. KEYWORDS Pd/Ni Joint Diffusion Soldering Intermetallic Compounds onding Strength Introduction Manufacturing integrated circuit (IC) and light-emitting diode (LED) chips requires the use of hydrogen with a high purity of 6N to 9N. For example, the purity of hydrogen used in the metal organic chemical-vapor deposition process is above 7N. Even the hydrogen used for fuel cells must have a high purity of 4N to 6N. Therefore, hydrogen purification is an important technique in the IC, LED, and fuel cell industries. Palladium has good selectivity for hydrogen and stable material properties. Therefore, it is often used as the material for hydrogen purification (Ref. 1). The principle of hydrogen purification can be described as in Fig. 1: The hydrogen gas (H 2 ) dissociates on the surface of Pd sheet into H atoms, which diffuse through the Pd sheet and recombine on the other side into purified H 2 gas. During this process, other gases with larger atom sizes are screened out. For hydrogen purification, multiple Pd fine tubes are assembled with an SE 304 stainless steel head to form a Pd cell Fig. 1. During the hydrogen purification process, the Pd cell is operated at a temperature above 350ºC with a hydrogen pressure over 1.7 MPa. For manufacturing a Pd assembly, a key technology is the bonding of Pd fine tubes with a stainless steel head. ecause Pd fine tubes have a diameter of about 2 mm and a thickness of about 60 m, the Pd cell can be deformed if traditional brazing or laser welding approaches are used. In contrast, a soldered Pd cell cannot endure the operation temperatures of a hydrogen purifier due to the lower melting point of the conventional solder alloy. Diffusion soldering, also called solidliquid interdiffusion bonding, is a novel bonding method based on the principle of isothermal solidification and interfacial intermetallics reaction. s shown in Fig. 2, this technique makes use of a low-melting metallic, thin-film interlayer (LT), such as Sn or In, inserted between the high-melting bulk workpieces or metallic layers on certain substrates (HT1 and HT2) to be joined. The LT interlayer, which is molten at low temperatures, reacts rapidly with the HT1 and HT2 bulk materials or layers. fter a short period of interfacial reaction, the LT interlayer is exhausted and has completely transformed into intermetallic compounds. The melting point of the newlyformed intermetallic phases is much higher than that of the original LT interlayer, so the resulting joints can withstand considerably higher operation temperatures (Ref. 2). ecause such a thin-film diffusion soldering technique has the merits of a low-temperature bonding process and high-temperature application, it has been applied in the past few decades to the manufacturing C. H. CHUNG (josh604@hotmail.com) is with the Department of Research and Design, Wire Technology Co., Ltd., Taiwan. Y. C. LIN is with the Institute of Materials Science and Engineering, National Taiwan University, Taiwan. 442-s WELDING JOURNL / NOVEMER 2016, VOL. 95

2 WELDING RESERCH Fig. 1 The principle of hydrogen purification through the thin wall of a Pd tube: Hydrogen purification; Pd cell. of microwave packages, high-power devices, thick-film resistors, Gas/Si wafer packages, and even gold jewelry. Recently, diffusion soldering has also been employed for ceramic multichip modules (Refs. 3, 4), microelectro mechanical systems packaging (Ref. 5), semiconductor packaging (Ref. 6), hybrid joining (Ref. 7), and hermetic package sealing (Ref. 8). In our previous studies, i 0.5 Sb 1.5 Te 3, (Pb,Sn)Te, and GeTe thermoelectric materials were bonded with Cu electrodes using a diffusion soldering process with an inserted Sn interlayer (Refs. 9 11). In these cases, satisfactory joints with sufficient bonding strengths were obtained. In addition, thin-film diffusion soldering has also been applied in 3D-IC flip-chip packaging, indicating that diffusion soldered joints have higher reliability than that of traditional soldered bumps (Ref. 12). For the requirement in certain applications, the bonding temperature during the diffusion soldering process has been further reduced through employment of an indium thin-film interlayer (Ref. 13). In this preliminary study for the manufacturing of a Pd cell for hydrogen purification, the diffusion soldering method has been used to join a Pd sheet and Ni plate to evaluate the bondability between Pd or Pd-alloy tubes and a Nicoated stainless steel head. Experimental Fig. 2 Schematic representation of the principle of the diffusion soldering method. C Fig. 4 Microstructure of the Pd/Ni couple diffusion soldered at various temperatures for 30 min using a 3 μmthick Sn interlayer (Case I): 275 C; 300 C; C 325 C; D 350 C. Fig. 3 Schematic representation of the various diffusion soldering processes for the Pd/Ni assembly using a Sn interlayer. D palladium sheet with a thickness of 0.1 mm was joined with a Ni plate with a thickness of 1 mm using various diffusion soldering processes Fig. 3. For the preparation of the bonding specimens in Case I, the Pd sheet was ground with 4000-grit SiC paper, electroplated with a 3- m Sn thin-film interlayer, and then assembled with a Ni plate. In Case II, the 4- m Sn-coated Pd sheet was electroplated with an additional 3- m g layer and then bonded with a 4- m Sn-coated Ni plate. On the other hand, the Pd sheets were precoated with a 6- m Ni layer in Case III and further heated at 450ºC for 30 min in Case IV. The pretreated Pd sheets in Cases III and IV were electroplated with NOVEMER 2016 / WELDING JOURNL 443-s

3 WELDING RESERCH Fig. 5 The thickness (X) of PdSn 4 and Ni 3 Sn 4 intermetallic compounds formed during diffusion soldering of the Pd/Ni couple at various temperatures for 30 min using Sn interlayers (Case I). C D Fig. 6 Microstructure of the Pd/Ni couple diffusion soldered at various temperatures for 30 min using Sn/g/Sn interlayers (Case II): 275 C; 300 C; C 325 C; D 350 C. a double layer of 4- m Sn/3- m g and then assembled respectively with a Sncoated Ni plate similar to that in Case II. Diffusion soldering was conducted in a vacuum furnace of Pa, and the assembly was heated at various temperatures between 275º and 350ºC for 30 min under a pressure of 3 MPa. fter the bonding processes, the specimens were cross sectioned, ground with 4000-grit SiC paper, and polished with 1- and 0.3- m l 2 O 3 powders. The microstructures of the intermetallic compounds that formed at the interfaces were observed with scanning electron 444-s WELDING JOURNL / NOVEMER 2016, VOL. 95

4 WELDING RESERCH Fig. 7 The thickness (X) of PdSn 4 and Ni 3 Sn 4 intermetallic compounds formed during diffusion soldering of a Pd/Ni couple at various temperatures for 30 min using Sn/g/Sn interlayers (Case II). Fig. 8 Shear strengths of the SLID bonded Pd/Ni couple using Sn/g/Sn interlayers. microscopy using an accelerating voltage of 15 kv, and their chemical compositions were analyzed with energy dispersive x-ray spectroscopy (EDX). The lateral resolution of EDX analysis was about 1 m. The shear strengths of the various Pd/Ni assemblies were tested with a DGE 4000 bond tester according to the IPC-TM650 (TM for die shear strength test). For the testing, the shear height and shear speed were set at 0.1 mm and 0.3 mm/s, respectively. Results and Discussion Figure 4 shows the microstructure of Pd/Ni joints in Case I of Fig. 3 after diffusion soldering at 275º to 350ºC for 30 min. In this case, the bonding between the Pd sheet and Ni plate at 275ºC for 30 min failed due to insufficient reaction at the Pd/Sn/Ni interfaces. For diffusion soldering at temperatures between 300º and 350ºC, it can be seen that the Pd sheet and Ni plate reacted with the Sn interlayer to form an intermetallic layer with a composition (at-%) of Pd : Sn = 52.9 : 47.1, which could be identified as the PdSn intermetallic phase in the Pd-Sn phase diagram (Ref. 14). On the other hand, the Ni plate reacted with the Sn thin film to form a reaction layer, which possessed a composition (at-%) of Ni : Sn = 43.1 : The composition is near the Ni 3 Sn 4 intermetallic phase in the Ni-Sn phase diagram (Ref. 15). The thicknesses (X) of the PdSn intermetallic layers were much greater than those of Ni 3 Sn 4, and those of both intermetallics increased with the bonding temperatures Fig. 5. The apparent activation energies (Q) for the growth of both intermetallics can be estimated from the slopes of the rrhenius plots in Fig. 5 for these intermetallic thicknesses (Ln X) vs. reciprocal bonding temperatures (1/T) that can be calculated as shown in Equation 1: X = 0 exp( Q/RT) (1) The results indicated that the apparent activation energy for PdSn intermetallics growth (40.6 kj/mol) is much higher than that for Ni 3 Sn 4 (26.8 kj/mol), which implied that the interfacial reaction for the growth of PdSn is more sensitive to the bonding temperature than that for the Ni 3 Sn 4 intermetallic compound. Furthermore, Fig. 4 also reveals that the Sn interlayer was completely exhausted under this condition, which prevented the melting of Pd/Ni joints during the hydrogen purification process at temperatures over 350ºC. However, long cracks appeared between the Ni 3 Sn 4 and PdSn intermetallic layers, and the bonding strength was very low. The appearance of cracks at the Ni 3 Sn 4 / PdSn interface is similar to that reported by ader et al. for diffusion soldering of Ni/Ni and Cu/Cu couples using Sn interlayers (Ref. 16). In a study by Chang et al. (Ref. 17), it was verified that the interfacial voids that form between Ni-Sn or Cu- Sn intermetallic compounds can be eliminated by inserting an g 3 Sn intermetallic layer between the diffusion soldered Ni/Ni or Cu/Cu couple with a Sn interlayer. The diffusion soldering process in this study was modified to include a Sn/g/Sn multilayer between the Pd sheet and Ni plate (Case II in Fig. 3). NOVEMER 2016 / WELDING JOURNL 445-s

5 CHUNG SUPP NOV 2016_Layout 1 10/14/16 9:50 M Page 446 WELDING RESERCH Fig. 9 Microstructure of the Ni coated Pd/Ni couple diffusion sol dered at 300 C for 30 min using Sn/g/Sn interlayers (Case III). Fig. 10 Diffusion profile for the Ni coated Pd sheet after pre heating at 450 C for 30 min. C D Fig. 11 Microstructure of the Pd Ni coated Pd/Ni couples diffusion soldered at various temperatures for 30 min using Sn/g/Sn interlay ers (Case IV): 275 C; 300 C; C 325 C; D 350 C. Figure 6 shows that this improvement prevented the cracking shown in Fig. 4, and a sound interface was achieved in the Pd/Ni joint. In addi- tion to the PdSn and Ni3Sn4 intermetallic compounds that formed, a reaction layer with a composition (at-%) of g : Sn = 76.5 : 23.5, being consis- 446-s WELDING JOURNL / NOVEMER 2016, VOL. 95 tent with the g3sn intermetallic phase in the g-sn phase diagram (Ref. 18), appeared between the Ni3Sn4 and PdSn. The compositions of Ni3Sn4

6 WELDING RESERCH Fig. 12 The thickness (X) of Ni 3 Sn 4 intermetallic compounds formed during diffusion soldering of Pd Ni solid solution coated Pd/Ni couples at various temperatures for 30 min using Sn/g/Sn interlayers (Case IV). C D Fig. 13 Diffusion profiles of the Ni element in the Pd Ni solid solution covered Pd/Ni couples corresponding to the micrographs in Fig. 11 (Case IV): 275 C; 300 C; C 325 C; D 350 C. and PdSn intermetallics in this bonding case are similar to those in Case I. The apparent activation energy for the growth of PdSn and Ni 3 Sn 4 intermetallic compounds as calculated from the measurements of intermetallic thicknesses in Fig. 7 and their rrhenius plots in Fig. 7 were 44.3 and 36.9 kj/mol, respectively. It was found that the apparent activation energies for the growth of both intermetallics are slightly higher than those obtained in Case I. The discrepancy can be attributed to the extra reaction of a Sn interlayer with the g film in competition with those of Pd/Sn and Ni/Sn interfacial reactions. Shear tests indicated that satisfactory bonding strengths ranging from 10.6 to 17.3 MPa can be achieved with diffusion soldering at temperatures NOVEMER 2016 / WELDING JOURNL 447-s

7 WELDING RESERCH Fig. 14 Microstructure of the diffusion soldered Pd Ni coated Pd/Ni couple (Case IV) bonded at 300 C for 30 min after high temperature storage at 400 C for 500 h. between 270º and 325ºC for 30 min Fig. 8. The shear strength of these diffusion soldered Pd/Ni joints using such a Sn/g/Sn interlayer decreased from the maximal value of 17.3 to 12.2 MPa when the bonding temperature was increased from 325º to 350ºC, which can be attributed to the high thermal stress during diffusion soldering at 350ºC. To further improve the bonding effect of the diffusion soldering of Pd/Ni joints, the Pd sheet was electroplated with a multilayer of 6- m Ni film, 4- m Sn, and 3- m g. The pretreated Pd sheet was diffusion soldered with a 4- m Sn-coated Ni plate at 300ºC for 30 min (Case III in Fig. 3). Figure 9 shows sound interfaces without any voids or cracks in the Pd/Ni joints sandwiched with a multilayer structure of Pd/Ni/Ni 3 Sn 4 / g 3 Sn/Ni 3 Sn 4 /Ni. The perfect diffusion soldered interfaces resulted in a satisfactory bonding strength of 22.6 MPa. However, delamination of electroplated Ni film from the Pd surface occurred quite often during the subsequent electroplating processes of Sn and g layers on the Ni-coated Pd sheet. To solve this problem, the Ni-coated Pd sheet was preheated at 450ºC for 30 min and then electroplated with Sn and g layers before being diffusion soldered with the Sn-coated Ni plate (Case IV in Fig. 3). ccording to a Ni-Pd phase diagram reported by Nash et al., Ni and Pd form a continuous fcc solid solution with a minimum in both the liquidus and solidus boundaries (Ref. 19). The diffusion profile of the Ni element in the Pd matrix in Fig. 10 indicates that a Pd-Ni solid solution zone with a thickness of about 5 m appears between the Pd sheet and Ni layer. The micrographs of such Pd-Ni solid solution covered Pd/Ni joints using a Sn/g/Sn interlayer are shown in Fig. 11. In this case, a sandwich structure of intermetallic compounds Ni 3 Sn 4 /g 3 Sn/Ni 3 Sn 4 formed between the Ni-coated Pd sheet and Ni plate. The apparent activation energy for the intermetallics growth of the upper Ni 3 Sn 4 (Ni plate side) and lower Ni 3 Sn 4 (Pd sheet side) in Fig. 11, as calculated from the measurements of intermetallic thicknesses in Fig. 12 and their rrhenius plots in Fig. 12, were 29.6 and 50.2 kj/mol, respectively. The apparent activation energy for the growth of the upper Ni 3 Sn 4 (Ni plate side) is near that for the Ni 3 Sn 4 growth in Case I (26.8 kj/mol). However, the growth of lower Ni 3 Sn 4 (Pd sheet side) showed a much higher apparent activation energy than that in Case I. s compared to the diffusion profile in Fig. 10, the Ni concentration in front of the Pd-Ni solid solution zone decreased gradually with the increase of bonding temperatures, as revealed in Fig. 13, which implies that the precoating Ni film on the Pd sheet was consumed during diffusion soldering. However, the thickness of the Pd-Ni solid solution zone changed slightly, which indicates most of the consuming Ni atoms reacted with the Sn atoms in the g 3 Sn intermetallic layer, leading to the growth of the Ni 3 Sn 4 layer (Pd sheet side). ecause the diffusion source of the Ni atoms from the Pd-Ni solid solution zone for the growth of lower Ni 3 Sn 4 (Pd sheet side) is much less than that from the pure Ni plate for the growth of upper Ni 3 Sn 4 (Ni plate side), the higher apparent activation energy for the Ni 3 Sn 4 growth on the Pd sheet side (50.2 kj/mol) in comparison to that on the Ni plate side (29.6 kj/mol) could be resulted in. The bonding strengths of these Pd-Ni solid solution covered Pd/Ni joints (Case IV), as shown in Fig. 8, ranged from 18.9 to 24.1 MPa, which was higher than those bonded in Case II. For the evaluation of reliability, the Pd-Ni solid solution covered Pd/Ni joint diffusion soldered at 300ºC for 30 min was aged at 400ºC for 500 h. During this period of aging, the upper Ni 3 Sn 4 intermetallic layer grew from about 2.6 to 7.8 m Fig. 14. On the other hand, the g 3 Sn intermetallic layers grew from 4.3 to 12.4 m, accompanied by the consumption of the lower layer of Ni 3 Sn 4, which was reduced from 1.7 to 0.2 m. It was also found that the bonding strengths did not obviously decay after this high-temperature storage test, which is optimistic for the longterm durability of the Pd/Ni joints during the operation of hydrogen purifiers at elevated temperatures above 350ºC. However, a leakage test with a hydrogen pressure of more than 1.7 MPa and further reliability tests are needed to ensure the real applicability of the diffusion soldered Pd cell for hydrogen purification. Conclusions Pd sheet and Ni plate can be bond- 448-s WELDING JOURNL / NOVEMER 2016, VOL. 95

8 WELDING RESERCH ed with the diffusion soldering method. The results indicated that directly electroplating a 3- m Sn interlayer on the Pd sheet and diffusion soldering with the Ni plate completely exhausted the Sn film and caused Ni 3 Sn 4 and PdSn intermetallic compounds to form, respectively. Unfortunately, a long crack appeared at the Ni 3 Sn 4 /PdSn interface, resulting in a very low bonding strength. Inserting a 3- m g layer between the Pd sheet and Ni plate, both coated with 4- m Sn, caused an additional g 3 Sn intermetallic compound to form between the Ni 3 Sn 4 and PdSn intermetallic layers and prevented cracking of the Pd/Ni joint. Satisfactory bonding strengths ranging from 10.6 to 17.3 MPa were achieved with diffusion soldering at temperatures between 275º and 350ºC for 30 min using such Sn/g/Sn interlayers. The bonding effect of Pd/Ni joints can be further improved by precoating a 6- m Ni film on the Pd sheet and preheating the coated Pd sheet at 450ºC for 30 min to form a Pd-Ni solid solution zone on the Pd surface. The Pd-Ni solid solution covered Pd/Ni joints using a Sn/g/Sn interlayer revealed higher shear strengths of 18.9 to 24.1 MPa. High-temperature storage at 400ºC for 500 h did not degrade the bonding interface or bonding strength. cknowledgments This study was sponsored by the Small usiness Innovation Research (SIR) program of the Ministry of Economic ffairs (MOE), Taiwan, under Grant No. 1Z , and the industrial and academic cooperation program between Wire Technology Co. and National Taiwan University, financially supported by the Ministry of Science and Technology under Grant No. MOST E CC2. References 1. Lin, Y. C., Chuang, C. H., Tsai, H. H., and Chuang, T. H. ug. 22, 23, Low temperature bonding of Pd/Ni assembly for hydrogen purifier. Proc. 3rd Int. Conf. on Mining, Material and Metallurgical Eng. udapest, Hungary. 2. Jacobson, D. M., and Humpston, G Diffusion soldering. Solder. Surf. Mt. Tech. 10: Chuang, T. H., Lin, H. J., and Tsao, C. W Intermetallic compounds formed during diffusion soldering u/cu/l 2 O 3 and Cu/Ti/Si with Sn/In interlayer. J. Electron. Mater. 35: Liang, M. W., Hsieh, T. E., Chang, S. Y., and Chuang, T. H Thin-film reactions during diffusion soldering of Cu/Ti/Si and u/cu/l 2 O 3 with Sn interlayers. J. Electron. Mater. 32: Welch, W. C., Chae, J., and Najafi, K Transfer of metal MEMS packages using a wafer-level solder transfer technique. IEEE Trans. dv. Packag. 28: Made, R. I., Gan, C. L., Yan, L. L., Yu,., Yoon, S. W., Lau, J. H., and Lee, C. K Study of low-temperature thermocompression bonding in g-in solder for packaging applications. J. Electron. Mater. 38: Li, J. F., gyakwa, P.., and Johnson, C. W Kinetics of g 3 Sn growth in g-sn-g system during transient liquid phase soldering process. cta Metall. Mater. 58: Yan, L. L., Lee, C. K., Yu, D. Q., Yu,.., Choi, W. K., Lau, J. H., and Yoon, S. U hermetic seal using composite thin-film In/Sn solder as an intermediate layer and its interdiffusion reaction with Cu. J. Electron. Mater. 38: Yang, C. L., Lai, H. J., Hwang, J. D., and Chuang, T. H Diffusion soldering of i 0.5 Sb 1.5 Te 3 thermoelectric material with Cu electrode. J. Mater. Eng. Perform. 22: Chuang, T. H., Lin, H. J., Chuang, C. H., Yeh, W. T., Hwang, J. D., and Chu, H. S Solid liquid interdiffusion bonding of (Pb,Sn)Te thermoelectric modules with Cu electrodes using a thin-film Sn interlayer. J. Electron. Mater. 43: Yang, C. L., Lai, H. J., Hwang, J. D., and Chuang, T. H Diffusion soldering of Pb-doped GeTe thermoelectric modules with Cu electrodes using a thin-film Sn interlayer. J. Electron. Mater. 42: Chang, J. Y., Cheng, R. S., Kao, K. S., Chang, T. C., and Chuang, T. H Reliable microjoints formed by solid liquid interdiffusion (SLID) bonding within a chipstacking architecture. IEEE Trans. Compon. Packag. Manufact. Technolog. 6: Chuang, T. H., Yeh, W. T., Chuang, C. H., and Hwang, J. D Improvement of bonding strength of a (Pb,Sn)Te- Cu contact manufactured in a low temperature SLID-bonding process. J. lloy Compd. 613: Massalski, T.., ed The Pd- Sn system (drawn from Elliott, R. P.). inary lloy Phase Diagrams, 4, p Metals Park, Ohio: merican Society for Metals. 15. Massalski, T.., ed The Ni-Sn system (drawn from Nash, P., and Nash,.). inary lloy Phase Diagrams, 4, p Metals Park, Ohio: merican Society for Metals. 16. ader, S., Gust, W., and Hieber, H Rapid formation of intermetallic compounds interdiffusion in the Cu-Sn and Ni-Sn systems. cta Metall. Mater. 43: Chang, J. Y., Chang, T. C., Chuang, T. H., and Lee, C. Y Dual-phase intermetallic interconnections structure and method of fabricating the same. US Patent, No. 8,742, Massalski, T.., ed The g- Sn system (drawn from Hansen, M.). inary lloy Phase Diagrams, 1, p. 71. Metals Park, Ohio: merican Society for Metals. 19. Nash,., and Nash, P The Ni- Pd (nickel-palladium) system. ull. lloy Phase Diagrams 5(5): NOVEMER 2016 / WELDING JOURNL 449-s