Behavior of eutectic Sn–Bi powder in Cu nanoparticle joints during the thermal treatment

Ni-coated SiC chips and Cu substrates were joined by a paste of Cu nanoparticles and eutectic Sn–Bi powders as an alternative joint to conventional solders. The first thermal treatment of the joint was performed to melt eutectic Sn–Bi powders, while the second thermal treatment was conducted at 623 K for 5 min to eliminate the organic molecules around the Cu nanoparticles. The influence of the first thermal treatment conditions (423–473 K for 2–20 min) on the joint structure and strength was investigated. During the first thermal treatment conducted at 423 K for 20 min, or at 473 K for 2 min, Sn–Bi did not migrate to the nanoparticle area. During the second thermal treatment, Bi liquefied, migrated out of the powder, and segregated at the joint interfaces. This resulted in the formation of brittle interfaces and consequently a low bonding strength. In contrast, when the first thermal treatment was performed at 448 K for 20 min or 473 K for more than 5 min, liquefied Sn–Bi migrated out of the powder and segregated at the joint interfaces, where Bi subsequently reacted with the Ni layer at the interface to form Bi_xNi_y intermetallic compounds, which prevented the formation of a brittle interface and thus improved the bonding strength.