Influence of cerium oxide (CeO_2) nanoparticles on the microstructure and hardness of tin-silver-copper (Sn-Ag-Cu) solders on silver (Ag) surface-finished copper (Cu) substrates

摘要

Nano-sized, non-reacting, non-coarsening CeO_2 particles with a density close to that of solder alloy were incorporated into Sn-3.0 wt%Ag-0.5 wt%Cu solder paste. The interfacial microstructure and hardness of Ag surface-finished Cu substrates were investigated, as a function of reaction time, at various temperatures. After the initial reaction, an island-shaped Cu_6Sn_5 intermetallic compound (IMC) layer was clearly observed at the interfaces of the Sn-Ag-Cu based solders/immersion Ag plated Cu substrates. However, after a prolonged reaction, a very thin, firmly adhering Cu_3Sn IMC layer was observed between the Cu_6Sn_5 IMC layer and the substrates. Rod-like Ag_3Sn IMC particles were also clearly observed at the interfaces. At the interfaces of the Sn-Ag-Cu based solder-Ag/Ni metallized Cu substrates, a (Cu, Ni)-Sn IMC layer was found. Rod-like Ag_3Sn and needle-shaped Cu_6Sn_5 IMC particles were also observed on the top surface of the (Cu, Ni)-Sn IMC layer. As the temperature and reaction time increased, so did the thickness of the IMC layers. In the solder ball region of both systems, a fine microstructure of Ag_3Sn, Cu_6Sn_5 IMC particles appeared in the β-Sn matrix. However, the growth behavior of the IMC layers of composite solder doped with CeO_2 nanoparticles was inhibited, due to an accumulation of surface-active CeO_2 nanoparticles at the grain boundary or in the IMC layers. In addition, the composite solder joint doped with CeO_2 nanoparticles had a higher hardness value than the plain Sn-Ag-Cu solder joints, due to a well-controlled fine microstructure and uniformly distributed CeO_2 nanoparticles. After 5 min of reaction on immersion Ag-plated Cu substrates at 250 ℃, the micro-hardness values of the plain Sn-Ag-Cu solder joint and the composite solder joints containing 1 wt% of CeO_2 nanoparticles were approximately 16.6 and 18.6 Hv, respectively. However after 30 min of reaction, the hardness values were approximately 14.4 and 16.6 Hv, while the micro-hardness values of the plain Sn-Ag-Cu solder joints and the composite solder joints on Ag/Ni metallized Cu substrates after 5 min of reaction at 250 ℃ were approximately 15.9 and 17.4 Hv, respectively. After 30 min of reaction, values of approximately 14.4 and 15.5 Hv were recorded.