Effect of CNTs and Ni coated CNTs on the mechanical performance of Sn57.6Bi0.4Ag BGA solder joints

In this study, Sn57.6Bi0.4Ag solder joints with different content of CNTs and Ni-CNTs were investigated. Aims were to make a clear description on the effect of Ni coat and to find the best doping amount to improve the mechanical properties of solder joints. Melting point measuring, micromorphology analyzing, ball shear testing, as well as fracture surface detecting were carried out. To make a fair comparison, only the weight of CNTs was counted in the doping amount, excluding the weight of Ni. According to experimental results, the melting point of Sn57.6Bi0.4Ag solder alloy was hardly influenced by small doping amount of CNTs and Ni-CNTs. Besides, the mechanical properties of Sn57.6Bi0.4Ag solder joints could be improved by the addition of CNTs and Ni-CNTs. Solder joints with 0.05 wt% CNTs or 0.07 wt% Ni-CNTs showed the best performance respectively. With the addition of CNT and Ni-CNTs, the fractured surface of solder joints became rougher, showing better bonding structure. Though both CNTs and Ni-CNTs have the capability to improve the mechanical performance of solder joints, Ni-CNTs worked much better. Ni coat was prove to have significant effect on inhibiting the aggregation of CNTs that could induce cracks and wetting problems and even deteriorate the strength of solder joints.