Microstructure, wetting characteristics and hardness of tin-bismuth-silver (Sn–Bi–Ag) solders on silver (Ag)-surface finished copper (Cu) substrates

This paper investigates the growth behavior, morphology of intermetallic compound (IMC) layer and hardness of low melting temperature Sn–35Bi–1Ag (wt%) solders on different surface-finished Cu substrates i.e., immersion Ag (ImAg)-plated Cu and Ag/Ni-plated Cu substrates. From SEM observation, a scallop-shaped Cu₆Sn₅ IMC layer was found to adhere at initial reaction stage on Sn–Bi–Ag solders/ImAg-plated Cu substrate system. However, a very thin Cu₃Sn IMC layer was clearly also observed between the Cu₆Sn₅ IMC layer and substrate after a lengthy reaction. On the other hand, in Sn–Bi–Ag solder and Ag/Ni-plated Cu substrate system, a scallop-shaped Ni₃Sn₄ IMC layer was observed at the interface without any Cu₃Sn IMC layer formation. These IMC layers thicknesses were increased with increasing the reaction time and temperature. However, the growth behavior of Ni₃Sn₄ IMC layer was slower than that of Cu₆Sn₅ and Cu₃Sn IMC layers. In solder ball regions, bright contrast Bi and needle-shaped Ag₃Sn phases were clearly observed in the dark contrast β-Sn matrix in both type of solder systems. Furthermore, bright contrast Bi phase size was increased with the reaction time. However, in the Sn–35Bi–1Ag solder and Ag/Ni-plated Cu substrate system, the Bi phase appeared with fine microstructure as compared with the Sn–35Bi–1Ag solder/ImAg-plated Cu substrate system because the Ag layer dissolved into the molten solder and changed the diffusivity and chemical affinity. The hardness values of Sn–Bi–Ag solder joints on ImAg-plated Cu and Ag/Ni-plated Cu substrates after 5 min reaction at 230 °C were about 23.0 and 24.1 HV, respectively—and about 18.9 and 20.4 HV after 30 min reaction at 230 °C, respectively.