Au-Ni plated Cu pads were reflow soldered by using lead free solder balls. The microstructure and strength of the as-reflowed solder joints were investigated. For solder joints using Cu-free Sn-Pb and Sn-Ag solder balls, a Ni 3 Sn 4 reaction layer was formed on the boundary between solder and pads. On the other hand, a Cu-Sn based (Cu, Ni) 6 Sn 5 reaction layer (η ) was formed in solder joints using Cu-containing solder balls. The growth rate for an η reaction layer during heat exposure at 423 K was much slower than that for a Ni 3 Sn 4 reaction layer. This suppression of an η reaction layer growth can be attributed to the fact that the Cu in solder balls was mostly removed during the formation of the η layer. By ball shear test, cold bump pull and hot bump pull tests, mechanical properties of the obtained BGA joints were investigated. Fracture loads and crack propagation path changed by changing the mechanical tests, the BGA joints using Cu containing Sn-Ag-Cu solder or low P type Ni plating revealed better mechanical properties. We established the mismatch of the boundaries between reaction layers and the P-enriched Ni-P layer, which was caused by the chained voids formed due to the Kirkendall effect, led to low joint strength.

Epitaxial thin films of a half-metallic ferromagnet Sr2FeMoO6 have been grown on (001) SrTiO3, and MgO substrates by magnetron sputtering in Ar+H2 mixture gas. Their structural, magnetic, magneto-optical and transport properties at room temperature were investigated and compared. Large difference has been observed especially in the magnetic and magneto-optical properties between thin films on SrTiO3 and MgO. The films on SrTiO3 exhibited stronger out-of-plane magnetic anisotropy and larger complex polar Kerr effect with a rotation θk up to -0.32° at 1.6 eV and RT. The observed difference in the properties of the films on the two substrates can be interpreted in terms of the structural disorder.

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We fabricated Cu core Sn-Ag solder balls by plating pure Sn and Ag on Cu balls and clarified that Sn/Ag plating began to melt at a rather low temperature, the eutectic temperature of Sn-Ag-Cu. This early melting at the eutectic temperature was ascribed to the diffusion of Cu and Ag into the Sn plating during the heating process. We investigated the solderability of the BGA joint with the Ni/Au coated Cu pad to compare it with that of the commercial Sn-Ag and Sn-Ag-Cu balls. After reflow soldering, we observed a eutectic microstructure composed of β-Sn, Ag 3 Sn, and Cu 6 Sn 5 phases in the solder, and a η -(Au, Cu, Ni) 6 Sn 5 reaction layer was formed at the interface between the solder and the Cu pad. The BGA joint using Cu core solder balls could prevent the degradation of joint strength during aging at 423 K because of the slower growth rate of the η -(Au, Cu, Ni) 6 Sn 5 reaction layer formed at the solder-pad interface. Furthermore, we were able to fabricate Cu-cored, multicomponent Sn-Ag-Bi balls by sequentially coating binary Sn-Ag and Sn-Bi solders onto Cu balls. The coated balls also exhibited almost the same melting and soldering behaviors as those of the previously alloyed Sn-2Ag-0.75Cu-3Bi solders.