Thermal and Wetting Properties of Water-Soluble Fluxes and Solder Pastes

The thermal and wetting properties of paste fluxes for use in water-soluble solder pastes largely determine solder paste performance during reflow. The thermal properties, wetting behavior, and reflow performance of a halogenated paste flux and solder paste (Flux A/Paste A) and a novel zero halogen paste flux and solder paste (Flux B/Paste B) were characterized and compared in air and nitrogen environments. In wetting balance tests with Sn96.5Ag3.0Cu0.5 (SAC305) solder at 250 °C, Flux A produced satisfactory wetting behavior in nitrogen, while Flux B produced excellent wetting in nitrogen with a high and stable wetting force that exceeded that of Flux A. In an air environment, Flux A exhibited a large and rapid decrease in wetting force, whereas Flux B achieved a higher wetting force than Flux A that remained relatively high over the duration of the wetting balance test. Thermogravimetric analysis (TGA) of Flux A showed significant degradation in air relative to N₂, while Flux B showed almost identical weight loss behavior in air and N₂. The TGA data for Paste A and Paste B were similar to that of the fluxes. The excellent thermal and wetting properties of Flux B/Paste B and the generally poorer properties of Flux A/Paste A were validated with reflow experiments that were designed to test the fluxing capability and resistance of the fluxes and pastes to air degradation.