Effect of Purging Gas on Wetting Behavior of Sn-3.5Ag Lead-Free Solder on Nickel-Coated Aluminum Substrate

The wetting characteristics of Sn-3.5Ag lead-free solder alloy on nickel-coated aluminum substrates in air (ambient), nitrogen, and argon atmospheres were investigated. The contact angles for the solder alloy obtained under air and argon atmospheres were in the range of 36°-38°. With nitrogen atmosphere the contact angle was found to be significantly lower at about 26°. Solder solidifying in air exhibited needle-shaped tin-rich dendrites surrounded by a eutectic matrix. The amount of tin dendrites decreased in argon atmosphere. However, the morphology of tin dendrites transformed from needle-shaped to nearly non-dendritic shape as the soldering atmosphere was changed from air to nitrogen. The interfacial microstructures revealed the presence of Ni3Sn and Ni3Sn4 IMCs at the interface. The enhanced wettability observed under nitrogen atmosphere is attributed to the higher thermal conductivity of nitrogen gas and the formation of higher amount of Ni3Sn IMCs at the interface compared to air and argon atmospheres.