Solder paste with polymerizing flux for lead-free solder alloy

Environmental concerns are driving the development of lead-free solder alloys and VOC-free soldering fluxes. The primary problem is that lead-free solder alloys are generally worse in wettability than Sn-Pb eutectic solder alloys. This paper describes a new solder paste that improves the wettability of lead-free solder alloys and provides the VOC-free flux without the volatile solvent; requirement of the flux cleaning. This solder paste is comprised lead-free Sn-Ag-Cu solder alloys and polymerizing flux that consists of liquid epoxy resins, polyfunctional carboxylic acids, and polyfunctional alcohols. The polyfunctional carboxylic acids and the alcohols of the flux coincidentally react with the liquid epoxy resins and are activated during the reflow process. The relationship of wettability and the reaction temperature of the flux were examined. From the results obtained through our experiments, it was found that some solder pastes showed good wettability in a non-reductive atmosphere and low density of the void dispersion in the solder layer. During the reflow, the flux is polymerized without VOC. Even though this solder paste is applied without post-cleaning, the flux-residue is only epoxy resin and it does not prevent adhesion between printed wiring boards and molding compounds. The insulation reliability test was for copper ionic migration conducted under high temperature(85 degreesC) and high humidity(85%). Results showed that ionic migration was not observed until after 168 hours.