Controlling the microstructures from the gold-tin reaction

The microstructures from the reaction between Au and Sn under different conditions were studied. A Sn/Au/Ni sandwich structure (2.5/3.752 µm) was deposited over the Si wafer. The overall composition of the Au and Sn layers corresponded to the Au20Sn binary eutectic (wt.%). When the reaction condition was 290°C for 2 min, the microstructure produced was a typical two-phase (Au^sub 5^Sn and AuSn) eutectic microstructure over Ni. In contrast, when the reaction condition was 240°C for 2 min, a AuSn/Au^sub 5^Sn/Ni layered microstructure was produced. In both microstructures, a small amount of Ni was dissolved in Au^sub 5^Sn and AuSn. When the AuSn/Au^sub 5^Sn/Ni layered structure was subjected to aging at 240°C, the AuSn layer gradually exchanged its position with the Au^sub 5^Sn layer and eventually formed an Au^sub 5^Sn/AuSn/Ni three-layer structure in less than 9 h. The driving force for Au^sub 5^Sn and AuSn to exchange their positions is for the AuSn phase to seek more Ni. The dominant diffusing species for the AuSn and Au^sub 5^Sn has also been identified to be Au and Sn, respectively. [PUBLICATION ABSTRACT] Key words: Au-Sn, optoelectronic packaging, lead-free solder