In situ heating transmission electron microscopy observation of nanoeutectic lamellar structure in Sn-Ag-Cu alloy on Au under-bump metallization.
We investigated the microstructural evolution of Sn(96.4)Ag(2.8)Cu(0.8) solder through in situ heating transmission electron microscopy observations. As-soldered bump consisted of seven layers, containing the nanoeutectic lamella structure of AuSn and Au₅Sn phases, and the polygonal grains of AuSn₂ and AuSn₄, on Au-plated Cu bond pads. Here, we found that there are two nanoeutectic lamellar layers with lamella spacing of 40 and 250 nm. By in situ heating above 140°C, the nanoeutectic lamella of AuSn and Au₅Sn was decomposed with structural degradation by sphering and coarsening processes of the lamellar interface. At the third layer neighboring to the lamella layer, on the other hand, Au₅Sn particles with a zig-zag shape in AuSn matrix became spherical and were finally dissipated in order to minimize the interface energy between two phases. In the other layers except both lamella layers, polycrystal grains of AuSn₂ and AuSn₄ grew by normal grain growth during in situ heating. The high interface energy of nanoeutectic lamella and polygonal nanograins, which are formed by rapid solidification, acted as a principal driving force on the microstructural change during the in situ heating.
Advanced Analysis Center, Korea Institute of Science & Technology, Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea.
Jong-Hyun
JH
Seo
Sang-Won
SW
Yoon
Kyou-Hyun
KH
Kim
Hye-Jung
HJ
Chang
Kon-Bae
KB
Lee
Tae-Yeon
TY
Seong
Eric
E
Fleury
Jae-Pyoung
JP
Ahn