Microstructure, thermal analysis and hardness of a Sn–Ag–Cu–1 wt% nano-TiO 2 composite solder on flexible ball grid array substrates

Abstract

Sn–Ag–Cu composite solders reinforced with nano-sized, nonreacting, noncoarsening 1 wt% TiO 2 particles were prepared by mechanically dispersing TiO 2 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 Cu 6Sn 5 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% TiO 2 nano-particles. In addition, the intermetallic compound layer thicknesses increased substantially with the number of reflow cycles. In the solder ball region, Ag 3Sn, Cu 6Sn 5 and AuSn 4 IMC particles were found to be uniformly distributed in the β-Sn matrix. However, after the addition of TiO 2 nano-particles, Ag 3Sn, AuSn 4 and Cu 6Sn 5 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% TiO 2 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 TiO 2 nano-particles which gave a second phase dispersion strengthening mechanism.