Solid/solid interfacial reactions between Sn–0.7 wt% Cu and Ni–7 wt% V

Sn–0.7 wt% Cu alloy is an important Pb-free solder, and Ni–7 wt% V is the major diffusion barrier layer material of flip chip technology. Reactions at the Sn–0.7 wt% Cu/Ni–7 wt% V interface are examined at 160, 180, and 210 °C. Only the Cu₆Sn₅ phase is formed in the Sn–0.7 wt% Cu/Ni–7 wt% V couple reacted at 160 and 180 °C; however, in addition to the Cu₆Sn₅ and Ni₃Sn₄ phases, a quaternary Q phase is formed in the Sn–0.7 wt% Cu/Ni–7 wt% V couple reacted at 210 °C. The Q phase is a mixture of nanocrystalline Ni₃Sn₄ phase and an amorphous phase. With longer reaction time at 210 °C in the Ni–V/Q/Sn–Cu couple where the Q phase is in direct contact with solder, the Ni₃Sn₄ phase nucleates inside the preformed Q phase, and the alternating layer phenomenon Ni–V/Q/Ni₃Sn₄/Q/Ni₃Sn₄/Cu₆Sn₅/Sn–Cu is observed. The interesting solid state amorphization and alternating layer phenomena at 210 °C are primarily caused by the fact that Sn and Cu are fast diffusing species, while V is relatively immobile.