Effect of Au Coating on the Wettability of Cu Substrate by Sn-Ag Eutectic Solder

The influence of thin coating of Au (70 nm) on the wettability of the Cu substrate by Sn-Ag eutectic solder (Sn-3.5Ag) was investigated, using a meniscograph testing machine equipped with a contact angle measuring system. Wettability of Cu by liquid Sn-3.5Ag was improved by Au coating, especially in contact angles. Namely, the contact angle was reduced to 29°∼30° on the Au-coated Cu from 42°∼45° on the uncoated Cu. On the Au-coated Cu, Au layers remained on unwetted areas, while it dissolved into solder on wetted areas, followed by the formation of a Cu-Sn intermetallic compound as seen on the uncoated Cu. The improvement in the wettability by Au coating on the Cu substrate would be caused by the difference of substrate-flux interfacial tensions (γsf).On the other hand, the wettability of the Au substrate by liquid Sn-3.5Ag with the contact angle of 60°∼64° was inferior to that of the Au-coated Cu. At the solder-Au substrate interface, Au-Sn intermetallic compounds were formed, which were not formed on the Au-coated Cu substrate. Consequently, the difference of the wettability between Au and Au-coated Cu is attributed to the difference of substrate-liquid solder interfacial tensions (γsl).These results suggest that it is necessary for the Au layer to be thin so as not to form Au-Sn intermetallic compound at the solder-substrate interface for the improvement in the wettability of the Cu substrate.