Wettability and Rheological Behavior of Low Ag Lead-Free SAC/Graphene and Cobalt-Graphene Nanocomposite Solder Paste

The impacts of dopant nanoparticles, graphene nanosheets (GNSs) and cobalt decorated-graphene nanosheets (CoGNSs), were studied in relation to the wettability and rheological behavior in low-Ag lead-free SAC0307 (Sn–0.3Ag–0.7Cu) solder paste. The solidification range of the solders was evaluated using differential scanning calorimetry. Phase identification in the solder bulk and interface of the solder and copper substrate was carried out by X-ray diffraction and energy-dispersive X-ray spectroscopy. Spreading properties and reactive wetting behavior along with the rheological properties of the solders were also studied. Results showed that the addition of both nanoparticles did not considerably change the melting point of SAC0307 solder. During the reflow soldering, GNSs floated away and separated from the solder bulk because of their low density and inappropriate wetting by the molten tin because of the nonmetallic nature of GNSs. However, CoGNS nanoparticles resulted in better wettability and fluidity of the solder because of the modified density of CoGNS particles and better coherency of the metallic cobalt/tin interface as well as the increased active absorption surface on the copper substrate. The fluidity of solder including CoGNSs was significantly higher compared to the solder with GNSs in the solidification range. Additionally, CoGNSs played an important role in the interfacial reactions by forming the refined (Co,Cu) 6 Sn 5 compound with a flat morphology, which promoted reactive wetting.