| dc.description.abstract | The good mechanical property, high thermal conductivity and high electrical conductivity of alloy make it widely used in optoelectronic packaging. Among of all hard solders, Au20Sn (wt.%) solder has the lowest melting point and good high strength and therefore is useful for devices sensitive to high processing temperature. Thin film Au20Sn solder layer not only can spread heat from the bonded device to the substrate quickly, but also can reduce the misalignment of z-position in passive alignment of fiber. It has been reported microstructure of solder may influence the reliability of solder. In this study, the microstructures of the eutectic Au20Sn (wt.%) solder developed on the Cu and Ni substrates were studied. The Sn/Au/Ni sandwich structure (2.5/3.75/2 mm) and the Sn/Au/Ni sandwich structure (1.83/2.74/5.8 mm) were deposited on Si wafers first. The overall composition of the Au and Sn layers corresponded to the Au20Sn binary eutectic. The microstructures of the Au20Sn solder on the Cu and Ni substrates could be controlled by using different bonding conditions. When the bonding condition was 290oC for 2 min, the microstructure of Au20Sn/Cu and Au20Sn/Ni was a two-phase (Au5Sn and AuSn) eutectic microstructure. When the bonding condition was 240oC for 2 minutes, the AuSn/Au5Sn/Cu and AuSn/Au5Sn/Ni layered microstructure formed. The major difference between Au20Sn/Ni and Au20Sn/Cu is that (Au, Ni)Sn preferred to form next to Ni and (Au, Cu)5Sn preferred to form next to Cu due to the different solubility of Ni and Cu in AuSn and Au5Sn. It is because a ternary intermetallic compound often has a lower Gibbs free energy compared to a binary compound of the same structure from the entropy argument.
After bonding, the Au20Sn/Cu and Au20Sn/Ni diffusion couples were subjected to aging at 240oC. In the Au20Sn/Ni system, the AuSn layer gradually exchanged its position with the Au5Sn layer, and eventually formed an Au5Sn/AuSn/Ni three-layer structure in less than 9 hours. The driving force for Au5Sn and AuSn to exchange their positions is for the AuSn phase to seek more Ni. From the result of short time reaction, the diffusion mechanism for the exchange of AuSn and Au5Sn is the diffusion of Sn through Au5Sn. The thermal stability of Au20Sn/Ni was better than that of Au20Sn/Cu. Moreover, less Ni was consumed compared to that of Cu. This indicates that Ni is a more effective diffusion barrier material for the Au20Sn solder. | en_US |