Influence of Noble Metal Coating on Wettability of Copper Substrate by Sn-Ag Eutectic Solder

Abstract

The influence of thin coatings (70 nm) of Au, Ag and Pd on the wettability of a Cu substrate by a Sn-3.5Ag eutectic solder was investigated using the contact angle measuring system combined with meniscograph tester. The wettability of the Cu substrate with Sn-3.5Ag was improved by the Au and Ag coatings, while reduced by a Pd coating, especially in terms of the contact angle. Namely, the contact angles were 29—30° on the Au coated-Cu substrate, 34—35° on the Ag coated-Cu substrate, 49—52° on the Pd coated-Cu substrate and 42—45° on the uncoated Cu substrate. On the wetted area, each coating layer dissolved into the solder and the Cu-Sn intermetallic compound was formed at the solder/substrate interface as well as on the uncoated Cu substrate, but the coating metal layers remained on the unwetted area. The difference in wettability of the noble metal coating on the Cu substrate was found to be attributed to the difference in the substrate-flux interfacial tensions (γsf).