Micro-Scale Solder Joints between Cu-Cu Wires Formed by Nanoparticle Enabled Lead-Free Solder Pastes

Micro-scale solder joints have been formed between copper-copper (Cu-Cu) wires by nanoparticle enabled lead-free solder pastes. Tin-based alloy nanoparticles (Sn-In) have been synthesized by using a surfactant-assisted chemical reduction method. The solder nanoparticles were characterized for size and composition by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Electrical resistivity measurements were conducted for the micro-scale solder joints formed between Cu-Cu wires for evaluation of joint quality and performance. These results were compared to the joints formed using commercial micro-sized lead-free solder pastes. The nanosolder paste is currently capable of melting into small solder balls with a diameter below 500 μm. Additionally, Cu wires with diameters as small as 25 μm have been soldered. Cross-section of the solder balls and solder joints were examined by SEM imaging to characterize their interfacial structure. This investigation shows that nanosolder paste can be used as an effective soldering material to join micro-sized Cu-Cu wires.