WELDING RESEARCH Diffusion Soldering of a Pd/Ni Couple Using a Sn ThinFilm Interlayer A preliminary study on the lowtemperature bonding of a Pd/Ni joint 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 a hydrogen purifier is presented 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. 1A: The hydrogen gas (H2) dissociates on the surface of Pd sheet into H atoms, which diffuse through the Pd sheet and recombine on the other side into purified H2 gas. During this process, other gases with larger atom sizes are screened out. For hydrogen purification, multiple Pd fine tubes are assembled with an SAE 304 stainless steel head to form a Pd cell — Fig. 1B. 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 442-s WELDING JOURNAL / NOVEMBER 2016, VOL. 95 is the bonding of Pd fine tubes with a stainless steel head. Because 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. As 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. After 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). Because 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 BY C.-H. CHUANG AND Y.-C. LIN ABSTRACT 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 Ni3Sn4 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 Ni3Sn4/PdSn interface and the bonding strength was low. Adding an Ag thin film between the Sncoated Pd sheet and Ni plate caused an Ag3Sn intermetallic compound to form between the Ni3Sn4 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. Hightemperature storage at 400ºC for 100 h did not degrade the bonding interfaces or bonding strengths. An alternative method wherein a multilayer of Ni/Sn/Ag was coated on the Pd sheet and then diffusion soldered with the Sncoated 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 PdNi 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 • Bonding Strength C.H. CHUANG (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.
Welding Journal | November 2016
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