Preparation and characterization of nano-solder paste with high nanoparticle loading and their thermal and printing properties

Abstract

A lead-free nano-solder paste has been successfully prepared by dispersing tin-silver (Sn–Ag) nanoparticles within a flux using planetary mixing, with nanoparticle loading up to 50 wt%. An aqueous chemical reduction method was used to synthesize the Sn–Ag alloy nanoparticles. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were used to image the morphology of the nanoparticles and the size was estimated to be an average of 19.6 ± 3.6 nm. Composition of the Sn–Ag nanoparticles was measured using energy dispersive x-ray spectroscopy (EDS) and their crystal structures were analyzed via x-ray diffraction (XRD). Differential scanning calorimetry (DSC) measurements of the Sn–Ag nanoparticles, containing approximately 4 wt% Ag, showed an onset of melting at 219.8 °C and an endothermic peak at 222.5 °C. These measured values for the nanoparticles are slightly lower as compared to the corresponding bulk material. The synthesized Sn–Ag nanoparticles were dispersed within a flux using planetary mixing, forming a uniform nanosolder paste, and as high as 50 wt% nanoparticle loading was achieved. Multiple heating cycles of the Sn–Ag nanosolder paste via DSC showed 1.6 °C increase in the onset of melting between the first and fourth cycles. Thermogravimetric analysis (TGA) of the 50 wt% nanosolder paste showed a 76% retention of initial weight up to 250 °C and 56% up to 500 °C, indicating residual material remaining on solder materials after heating. The nanosolder paste was screen-printed and reflowed on Cu substrates in an inert environment. The cross-section of the interface was observed via FE-SEM for intermetallic compound (IMC) formation between the reflowed nanosolder paste and substrate, confirming that a metallurgical bonding has been achieved. Both Cu3Sn and Cu6Sn5 were observed in the interfacial layer.