Phase stabilities of Pb-Sn and Bi-Ni alloys under current stressing: An ab initio-aided CALPHAD study

• Main Authors: Chao-kueiYeh, 葉朝貴
• Other Authors: Shih-kang Lin
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/76531762825255042092

碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 101 === The effects of electric currents on metallic materials, such as the Joule heating and electromigration, crucially affect the reliability of electronic products. With the developments of the advanced packaging methods, e.g. the 3D IC and flip-chip packaging, the current densities applied in modern devices are continuously increased. Thus, the impacts of these electric current-induced effects are more and more pronounced and crucial to the reliability of electronic products. In the study, we choose two systems, Pb-Sn binary alloys and Bi-Ni binary couples, synthesize what is the mechanism of the Sn supersaturation in Pb-rich matrix of Pb-Sn binary system and the non-polarity effect of sandwich reaction couples of Bi-Ni binary system under current stressing. the phase stability of Pb-Sn and Bi-Ni alloys under current stressing are theoretically investigated by using the ab initio-aided CALPHAD approach. For Pb-Sn binary system, we found that the phase equilibria of the Pb-Sn alloys would be changed by electric current stressing, when the current density is higher than the critical value around 3 x104 A/cm2. As the result, for a Pb-Sn alloy, which is composed of the Pb-rich FCC and the Sn-rich BCT phases, under current stressing with a sufficient high current density, the solubility of Sn in the FCC phase would increase, while that of Pb in the BCT phase would decrease. These would lead to the supersaturation of Sn in the FCC phase and precipitation of the FCC phase from the BCT phase under current stressing. For Bi-Ni binary system, we found that the phase stabilities of the Bi-Ni phases would be changed by electric currents. As the only intermetallic compound formed at the Ni/Bi couples is the NiBi3 phase, the meta-stabilities between Ni, NiBi3, and Bi phases were calculated. The chemical potential gradient of the dominant diffusion specie, Bi, in the NiBi3 reaction layer, i.e. the driving force for the NiBi3 layer growth, was increased under current stressing. Therefore, the “non-polarity effect” is likely caused by the larger chemical potential gradients under current stressing, which is independent of the direction of electron flows. We hope that this study opens a door to the fundamental understanding on the electric current effect upon interfacial reactions and the phase stability change, which may lead to further researches on Pb-free solder joints with electric currents applied.