Lead-free solder paste for ultrafine solder powder and preparation method thereof

The invention relates to a lead-free solder paste for ultrafine solder powder and a preparation method thereof. The lead-free solder paste is taken as a fluxing material for an SMT (Surface Mount Technology) and belongs to the technical field of soldering materials. The lead-free solder paste comprises the following components by weight percent: 11.0%-13.0% of vacuum degassed paste fluxing agent and the balance of spherical alloy solder powder which is subjected to surface treatment and has a grain size of 15 microns to 25 microns, wherein a mixture of ethylene glycol monobutyl ether, halogen-free quaternary ammonium salt and polyglycol 600 with the weight ratio of (7.5-8.5):1:1, is taken as a surface treating agent in the alloy solder powder, and a weight ratio of the alloy solder powder to the surface treating agent is (88.5-87.0):(1.0-2.0). A paste fluxing agent prepared by using the preparation method provided by the invention has the advantages of few air bubbles and excellent soldering performance and storage stability. Meanwhile, the speed of reaction between the alloy solder powder and the paste fluxing agent is reduced, and the stability and the soldering performance of the ultrafine lead-free solder paste are well promoted.