Development Of The Water-based Liquid Flux In Lead-free Electronic Assemblies

The use of the water-based liquid flux is gradually increasing in lead-free electronicsassemblies, owing to its charactristics of eco-friendly solvent, good safety and low price.However, thus far there is very little research about the water-based liquid flux. In this thesisstudy, the effect of activators, surfactants and cosolvents on the performance of water-basedliquid flux was investigated. The flux related problems occurred during the lead-free wavesoldering process were systematically analyzed. Further, the influence of preheat on the wavesoldering quality and soldering processing defects （including solder beads） was also studied.At first, the effect of the composition, formula, content of activators on activity andelectric reliability of the water-based liquid flux was estimated by conducting the spreadingtest using Sn-0.7Cu solder, the copper corrosion test and surface insulation resistance （SIR）test. The results show that:（1） The optimal content of the organic carboxylic acid activators is3.0-4.0wt%, with which the flux exhibits a high activity and the SIR values of the solderedassemblies are very high up to those achieved by using no-clean flux (≥1.0×10¹⁰), and thismay meet the requirement of no-clean soldering flux;（2） The optimal mixing ratio of succinicacid, glutaric acid, adipic acid and maleic acid is2:5:5:0. Both succinic acid and glutaricacid have significant effect on the spreading ability of the solder, while adipic acid and maleicacid don’t;（3） Trolamine could reduce the activity of the water-based liquid flux, howeverthe excessive use of trolamine would increase the caustic nature of soldering residues.The water-based liquid flux with trolamine content of0.2wt%shows better performance interms of the high electric reliability of soldered assemblies and the decreasing caustic natureof the soldering residue.Furthermore, measurements of the surface tension and wetting balance of thewater-based liquid flux were performed to study the effect of surfactants and cosolvents onthe activity and solderability of the flux. The results show that:（1） The surface tension of thewater-based liquid flux could be reduced to30mN/m after adding the surfactants. SurfactantOP-10has a higher surface activity than surfactant CTAB at room temperature, while CTABis more surface active than OP-10at80℃. Surfactants without halogen have no influenceon the activity of the flux when their concentration in the flux is higher than critical micell concentration （CMC）;（2） The cosolvent has significant influence on the surface tension of thewater-based flux, and cosolvents with low surface tension can improve the spraying of thewater-based flux. It has also been shown that the cosolvent with a high boiling point couldexhibit a good protective effect on the prepared pads and make the maximum wetting force（Fmax） increased.Moreover, the soldering test results of using the the water-based flux show that preheat isa key step in the process. Preheat has an obvious effect on soldering defects, and too highpreheating could lead to an increase of soldering defects. The residual water droplets afterpreheat is the actual cause for the formation of solder beads, thus an appropriate preheat couldeffectively reduce the solder beading problem. In addition, increasing the dwelling time ofpreheat is a better choice to solve the problem when the water-based flux is preheated. Also,the narrow wave could improve the soldering quality when using appropriate preheat.