WELDING RESEARCH Fig. 1 — The principle of hydrogen purification through the thin wall of a Pd tube: A — Hydrogen purification; B — Pd cell. NOVEMBER 2016 / WELDING JOURNAL 443-s 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 (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, Bi0.5Sb1.5 Te3, (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 A 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 Ag 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 Fig. 2 — Schematic representation of the principle of the diffusion soldering method. Fig. 3 — Schematic representation of the various diffusion soldering processes for the Pd/Ni assembly using a Sn interlayer. Fig. 4 — Microstructure of the Pd/Ni couple diffusion soldered at various temperatures for 30 min using a 3μmthick Sn interlayer (Case I): A — 275°C; B — 300°C; C — 325°C; D — 350°C. A A C B B D
Welding Journal | November 2016
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