Effect of the size of carbon nanotubes (CNTs) on the microstructure and mechanical strength of CNTs-doped composite Sn0.3Ag0.7Cu-CNTs solder

Carbon nanotubes (CNTs) with three different diameter ranges (10-20, 40-60, and 60-100 nm) were doped into tin-silver-copper (SAC) solder, to study the performance of the composite SAC-CNTs solder materials - as well as the effect of the size of the CNTs. It was found that all the CNTs-doped composite solder samples displayed refined microstructure, inhibited interfacial intermetallic compound (IMC) growth, and reinforced mechanical strength - while the melting point of the composite solder was close to that of the pristine solder. The reinforcement in mechanical strength was due to the doped CNTs pinned at the solder grain boundaries, which acted as second-phase particles that refined the microstructure and increased the dislocation density. The adsorbed CNTs destroyed the integrity of the interfacial IMCs, leading to reduced growth rate. Among these composite solders, CNTs with a diameter of 40-60 nm provided superior performance in refining the microstructure, lowering the IMC growth rate by 30.9% - and reinforcing the ball shear strength by 15.3% and the hardness by 16.1%. This size effect on the performance of composite solders was due to the various surface energy values for CNTs - that led to the agglomeration and adsorption of CNTs in the solder matrix and interfacial IMCs.