Utilizing the thermodynamic nanoparticle size effects for low temperature Pb-free solder

Development of a Pb-free Sn nanosolder paste with an initial melting temperature near or below the melting temperature of eutectic Sn–Pb solder (183 °C) has been investigated using the size-dependent melting behavior of small particles. Three to five nanometer Sn nanoparticles were fabricated by sonochemical reduction and observed to melt at temperatures near or below 183 °C. Prototype nanosolder pastes were produced by combining the nanoparticles with flux and were characterized by differential scanning calorimetry (DSC) in terms of their melting, solidification, coalescence, and metal particle loading properties. The results indicate that, although target melting temperatures were achieved, nanoparticle coalescence was limited by low volume loading of the metal, due in part to the capping layer (an organic layer adsorbed on the metal surface during chemical synthesis).

► In this study we developed prototype Sn nanoparticle Pb-free solder pastes.
► Particle size, melting temperature, coalescence, and volume loading were examined.
► DSC results showed melting point depression and nanoparticle coalescence.
► Low metals volume loading inhibited the formation of a solder joint.