Microstructure, thermal analysis and hardness of a Sn–Ag–Cu–1 wt% nano-TiO2 composite solder on flexible ball grid array substrates

Sn–Ag–Cu composite solders reinforced with nano-sized, nonreacting, noncoarsening 1 wt% TiO2 particles were prepared by mechanically dispersing TiO2 nano-particles into Sn–Ag–Cu solder powder and the interfacial morphology of the solder and flexible BGA substrates were characterized metallographically. At their interfaces, different types of scallop-shaped intermetallic compound layers such as Cu6Sn5 for a Ag metallized Cu pad and Sn–Cu–Ni for a Au/Ni and Ni metallized Cu pad, were found in plain Sn–Ag–Cu solder joints and solder joints containing 1 wt% TiO2 nano-particles. In addition, the intermetallic compound layer thicknesses increased substantially with the number of reflow cycles. In the solder ball region, Ag3Sn, Cu6Sn5 and AuSn4 IMC particles were found to be uniformly distributed in the β-Sn matrix. However, after the addition of TiO2 nano-particles, Ag3Sn, AuSn4 and Cu6Sn5 IMC particles appeared with a fine microstructure and retarded the growth rate of IMC layers at their interfaces. The Sn–Ag–Cu solder joints containing 1 wt% TiO2 nano-particles consistently displayed a higher hardness than that of the plain Sn–Ag–Cu solder joints as a function of the number of reflow cycles due to the well-controlled fine microstructure and homogeneous distribution of TiO2 nano-particles which gave a second phase dispersion strengthening mechanism.